coroutines: Wrap co_await in a target expr where needed [PR95050]

[thirdparty/gcc.git] / gcc / cp / coroutines.cc

/* coroutine-specific state, expansions and tests.

   Copyright (C) 2018-2020 Free Software Foundation, Inc.

 Contributed by Iain Sandoe <iain@sandoe.co.uk> under contract to Facebook.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "target.h"
#include "cp-tree.h"
#include "stringpool.h"
#include "stmt.h"
#include "stor-layout.h"
#include "tree-iterator.h"
#include "tree.h"
#include "gcc-rich-location.h"
#include "hash-map.h"

static bool coro_promise_type_found_p (tree, location_t);

/* GCC C++ coroutines implementation.

  The user authors a function that becomes a coroutine (lazily) by
  making use of any of the co_await, co_yield or co_return keywords.

  Unlike a regular function, where the activation record is placed on the
  stack, and is destroyed on function exit, a coroutine has some state that
  persists between calls - the coroutine frame (analogous to a stack frame).

  We transform the user's function into three pieces:
  1. A so-called ramp function, that establishes the coroutine frame and
     begins execution of the coroutine.
  2. An actor function that contains the state machine corresponding to the
     user's suspend/resume structure.
  3. A stub function that calls the actor function in 'destroy' mode.

  The actor function is executed:
   * from "resume point 0" by the ramp.
   * from resume point N ( > 0 ) for handle.resume() calls.
   * from the destroy stub for destroy point N for handle.destroy() calls.

  The functions in this file carry out the necessary analysis of, and
  transforms to, the AST to perform this.

  The C++ coroutine design makes use of some helper functions that are
  authored in a so-called "promise" class provided by the user.

  At parse time (or post substitution) the type of the coroutine promise
  will be determined.  At that point, we can look up the required promise
  class methods and issue diagnostics if they are missing or incorrect.  To
  avoid repeating these actions at code-gen time, we make use of temporary
  'proxy' variables for the coroutine handle and the promise - which will
  eventually be instantiated in the coroutine frame.

  Each of the keywords will expand to a code sequence (although co_yield is
  just syntactic sugar for a co_await).

  We defer the analysis and transformation until template expansion is
  complete so that we have complete types at that time.  */


/* The state that we collect during parsing (and template expansion) for
   a coroutine.  */

struct GTY((for_user)) coroutine_info
{
  tree function_decl; /* The original function decl.  */
  tree promise_type; /* The cached promise type for this function.  */
  tree handle_type;  /* The cached coroutine handle for this function.  */
  tree self_h_proxy; /* A handle instance that is used as the proxy for the
                        one that will eventually be allocated in the coroutine
                        frame.  */
  tree promise_proxy; /* Likewise, a proxy promise instance.  */
  location_t first_coro_keyword; /* The location of the keyword that made this
                                    function into a coroutine.  */
  /* Flags to avoid repeated errors for per-function issues.  */
  bool coro_ret_type_error_emitted;
  bool coro_promise_error_emitted;
};

struct coroutine_info_hasher : ggc_ptr_hash<coroutine_info>
{
  typedef tree compare_type; /* We only compare the function decl.  */
  static inline hashval_t hash (coroutine_info *);
  static inline hashval_t hash (const compare_type &);
  static inline bool equal (coroutine_info *, coroutine_info *);
  static inline bool equal (coroutine_info *, const compare_type &);
};

/* This table holds all the collected coroutine state for coroutines in
   the current translation unit.  */

static GTY (()) hash_table<coroutine_info_hasher> *coroutine_info_table;

/* We will initialise state lazily.  */
static bool coro_initialized = false;

/* Return a hash value for the entry pointed to by INFO.
   The compare type is a tree, but the only trees we are going use are
   function decls.  We use the DECL_UID as the hash value since that is
   stable across PCH.  */

hashval_t
coroutine_info_hasher::hash (coroutine_info *info)
{
  return DECL_UID (info->function_decl);
}

/* Return a hash value for the compare value COMP.  */

hashval_t
coroutine_info_hasher::hash (const compare_type& comp)
{
  return DECL_UID (comp);
}

/* Return true if the entries pointed to by LHS and RHS are for the
   same coroutine.  */

bool
coroutine_info_hasher::equal (coroutine_info *lhs, coroutine_info *rhs)
{
  return lhs->function_decl == rhs->function_decl;
}

bool
coroutine_info_hasher::equal (coroutine_info *lhs, const compare_type& rhs)
{
  return lhs->function_decl == rhs;
}

/* Get the existing coroutine_info for FN_DECL, or insert a new one if the
   entry does not yet exist.  */

coroutine_info *
get_or_insert_coroutine_info (tree fn_decl)
{
  gcc_checking_assert (coroutine_info_table != NULL);

  coroutine_info **slot = coroutine_info_table->find_slot_with_hash
    (fn_decl, coroutine_info_hasher::hash (fn_decl), INSERT);

  if (*slot == NULL)
    {
      *slot = new (ggc_cleared_alloc<coroutine_info> ()) coroutine_info ();
      (*slot)->function_decl = fn_decl;
    }

  return *slot;
}

/* Get the existing coroutine_info for FN_DECL, fail if it doesn't exist.  */

coroutine_info *
get_coroutine_info (tree fn_decl)
{
  if (coroutine_info_table == NULL)
    return NULL;

  coroutine_info **slot = coroutine_info_table->find_slot_with_hash
    (fn_decl, coroutine_info_hasher::hash (fn_decl), NO_INSERT);
  if (slot)
    return *slot;
  return NULL;
}

/* We will lazily create all the identifiers that are used by coroutines
   on the first attempt to lookup the traits.  */

/* Identifiers that are used by all coroutines.  */

static GTY(()) tree coro_traits_identifier;
static GTY(()) tree coro_handle_identifier;
static GTY(()) tree coro_promise_type_identifier;

/* Required promise method name identifiers.  */

static GTY(()) tree coro_await_transform_identifier;
static GTY(()) tree coro_initial_suspend_identifier;
static GTY(()) tree coro_final_suspend_identifier;
static GTY(()) tree coro_return_void_identifier;
static GTY(()) tree coro_return_value_identifier;
static GTY(()) tree coro_yield_value_identifier;
static GTY(()) tree coro_resume_identifier;
static GTY(()) tree coro_address_identifier;
static GTY(()) tree coro_from_address_identifier;
static GTY(()) tree coro_get_return_object_identifier;
static GTY(()) tree coro_gro_on_allocation_fail_identifier;
static GTY(()) tree coro_unhandled_exception_identifier;

/* Awaitable methods.  */

static GTY(()) tree coro_await_ready_identifier;
static GTY(()) tree coro_await_suspend_identifier;
static GTY(()) tree coro_await_resume_identifier;

/* Create the identifiers used by the coroutines library interfaces.  */

static void
coro_init_identifiers ()
{
  coro_traits_identifier = get_identifier ("coroutine_traits");
  coro_handle_identifier = get_identifier ("coroutine_handle");
  coro_promise_type_identifier = get_identifier ("promise_type");

  coro_await_transform_identifier = get_identifier ("await_transform");
  coro_initial_suspend_identifier = get_identifier ("initial_suspend");
  coro_final_suspend_identifier = get_identifier ("final_suspend");
  coro_return_void_identifier = get_identifier ("return_void");
  coro_return_value_identifier = get_identifier ("return_value");
  coro_yield_value_identifier = get_identifier ("yield_value");
  coro_resume_identifier = get_identifier ("resume");
  coro_address_identifier = get_identifier ("address");
  coro_from_address_identifier = get_identifier ("from_address");
  coro_get_return_object_identifier = get_identifier ("get_return_object");
  coro_gro_on_allocation_fail_identifier =
    get_identifier ("get_return_object_on_allocation_failure");
  coro_unhandled_exception_identifier = get_identifier ("unhandled_exception");

  coro_await_ready_identifier = get_identifier ("await_ready");
  coro_await_suspend_identifier = get_identifier ("await_suspend");
  coro_await_resume_identifier = get_identifier ("await_resume");
}

/* Trees we only need to set up once.  */

static GTY(()) tree coro_traits_templ;
static GTY(()) tree coro_handle_templ;
static GTY(()) tree void_coro_handle_type;

/* ================= Parse, Semantics and Type checking ================= */

/* This initial set of routines are helper for the parsing and template
   expansion phases.

   At the completion of this, we will have completed trees for each of the
   keywords, but making use of proxy variables for the self-handle and the
   promise class instance.  */

/* [coroutine.traits]
   Lookup the coroutine_traits template decl.  */

static tree
find_coro_traits_template_decl (location_t kw)
{
  /* If we are missing fundmental information, such as the traits, (or the
     declaration found is not a type template), then don't emit an error for
     every keyword in a TU, just do it once.  */
  static bool traits_error_emitted = false;

  tree traits_decl = lookup_qualified_name (std_node, coro_traits_identifier,
                                            0,
                                            /*complain=*/!traits_error_emitted);
  if (traits_decl == error_mark_node
      || !DECL_TYPE_TEMPLATE_P (traits_decl))
    {
      if (!traits_error_emitted)
        {
          gcc_rich_location richloc (kw);
          error_at (&richloc, "coroutines require a traits template; cannot"
                    " find %<%E::%E%>", std_node, coro_traits_identifier);
          inform (&richloc, "perhaps %<#include <coroutine>%> is missing");
          traits_error_emitted = true;
        }
      return NULL_TREE;
    }
  else
    return traits_decl;
}

/*  Instantiate Coroutine traits for the function signature.  */

static tree
instantiate_coro_traits (tree fndecl, location_t kw)
{
  /* [coroutine.traits.primary]
     So now build up a type list for the template <typename _R, typename...>.
     The types are the function's arg types and _R is the function return
     type.  */

  tree functyp = TREE_TYPE (fndecl);
  tree arg = DECL_ARGUMENTS (fndecl);
  bool lambda_p = LAMBDA_FUNCTION_P (fndecl);
  tree arg_node = TYPE_ARG_TYPES (functyp);
  tree argtypes = make_tree_vec (list_length (arg_node)-1);
  unsigned p = 0;

  while (arg_node != NULL_TREE && !VOID_TYPE_P (TREE_VALUE (arg_node)))
    {
      /* See PR94807, as to why we must exclude lambda here.  */
      if (is_this_parameter (arg) && !lambda_p)
        {
          /* We pass a reference to *this to the param preview.  */
          tree ct = TREE_TYPE (TREE_TYPE (arg));
          TREE_VEC_ELT (argtypes, p++) = cp_build_reference_type (ct, false);
        }
      else
        TREE_VEC_ELT (argtypes, p++) = TREE_VALUE (arg_node);

      arg_node = TREE_CHAIN (arg_node);
      arg = DECL_CHAIN (arg);
    }

  tree argtypepack = cxx_make_type (TYPE_ARGUMENT_PACK);
  SET_ARGUMENT_PACK_ARGS (argtypepack, argtypes);

  tree targ = make_tree_vec (2);
  TREE_VEC_ELT (targ, 0) = TREE_TYPE (functyp);
  TREE_VEC_ELT (targ, 1) = argtypepack;

  tree traits_class
    = lookup_template_class (coro_traits_templ, targ,
                             /*in_decl=*/NULL_TREE, /*context=*/NULL_TREE,
                             /*entering scope=*/false, tf_warning_or_error);

  if (traits_class == error_mark_node)
    {
      error_at (kw, "cannot instantiate %<coroutine traits%>");
      return NULL_TREE;
    }

  return traits_class;
}

/* [coroutine.handle] */

static tree
find_coro_handle_template_decl (location_t kw)
{
  /* As for the coroutine traits, this error is per TU, so only emit
    it once.  */
  static bool coro_handle_error_emitted = false;
  tree handle_decl = lookup_qualified_name (std_node, coro_handle_identifier,
                                            0, !coro_handle_error_emitted);
  if (handle_decl == error_mark_node
      || !DECL_CLASS_TEMPLATE_P (handle_decl))
    {
      if (!coro_handle_error_emitted)
        error_at (kw, "coroutines require a handle class template;"
                  " cannot find %<%E::%E%>", std_node, coro_handle_identifier);
      coro_handle_error_emitted = true;
      return NULL_TREE;
    }
  else
    return handle_decl;
}

/* Instantiate the handle template for a given promise type.  */

static tree
instantiate_coro_handle_for_promise_type (location_t kw, tree promise_type)
{
  /* So now build up a type list for the template, one entry, the promise.  */
  tree targ = make_tree_vec (1);
  TREE_VEC_ELT (targ, 0) = promise_type;
  tree handle_type
    = lookup_template_class (coro_handle_identifier, targ,
                             /* in_decl=*/NULL_TREE,
                             /* context=*/std_node,
                             /* entering scope=*/false, tf_warning_or_error);

  if (handle_type == error_mark_node)
    {
      error_at (kw, "cannot instantiate a %<coroutine handle%> for"
                " promise type %qT", promise_type);
      return NULL_TREE;
    }

  return handle_type;
}

/* Look for the promise_type in the instantiated traits.  */

static tree
find_promise_type (tree traits_class)
{
  tree promise_type
    = lookup_member (traits_class, coro_promise_type_identifier,
                     /* protect=*/1, /*want_type=*/true, tf_warning_or_error);

  if (promise_type)
    promise_type
      = complete_type_or_else (TREE_TYPE (promise_type), promise_type);

  /* NULL_TREE on fail.  */
  return promise_type;
}

static bool
coro_promise_type_found_p (tree fndecl, location_t loc)
{
  gcc_assert (fndecl != NULL_TREE);

  if (!coro_initialized)
    {
      /* Trees we only need to create once.
         Set up the identifiers we will use.  */
      coro_init_identifiers ();

      /* Coroutine traits template.  */
      coro_traits_templ = find_coro_traits_template_decl (loc);
      if (coro_traits_templ == NULL_TREE)
        return false;

      /*  coroutine_handle<> template.  */
      coro_handle_templ = find_coro_handle_template_decl (loc);
      if (coro_handle_templ == NULL_TREE)
        return false;

      /*  We can also instantiate the void coroutine_handle<>  */
      void_coro_handle_type =
        instantiate_coro_handle_for_promise_type (loc, NULL_TREE);
      if (void_coro_handle_type == NULL_TREE)
        return false;

      /* A table to hold the state, per coroutine decl.  */
      gcc_checking_assert (coroutine_info_table == NULL);
      coroutine_info_table =
        hash_table<coroutine_info_hasher>::create_ggc (11);

      if (coroutine_info_table == NULL)
        return false;

      coro_initialized = true;
    }

  /* Save the coroutine data on the side to avoid the overhead on every
     function decl tree.  */

  coroutine_info *coro_info = get_or_insert_coroutine_info (fndecl);
  /* Without this, we cannot really proceed.  */
  gcc_checking_assert (coro_info);

  /* If we don't already have a current promise type, try to look it up.  */
  if (coro_info->promise_type == NULL_TREE)
    {
      /* Get the coroutine traits template class instance for the function
         signature we have - coroutine_traits <R, ...>  */

      tree templ_class = instantiate_coro_traits (fndecl, loc);

      /* Find the promise type for that.  */
      coro_info->promise_type = find_promise_type (templ_class);

      /* If we don't find it, punt on the rest.  */
      if (coro_info->promise_type == NULL_TREE)
        {
          if (!coro_info->coro_promise_error_emitted)
            error_at (loc, "unable to find the promise type for"
                      " this coroutine");
          coro_info->coro_promise_error_emitted = true;
          return false;
        }

      /* Try to find the handle type for the promise.  */
      tree handle_type =
        instantiate_coro_handle_for_promise_type (loc, coro_info->promise_type);
      if (handle_type == NULL_TREE)
        return false;

      /* Complete this, we're going to use it.  */
      coro_info->handle_type = complete_type_or_else (handle_type, fndecl);

      /* Diagnostic would be emitted by complete_type_or_else.  */
      if (!coro_info->handle_type)
        return false;

      /* Build a proxy for a handle to "self" as the param to
         await_suspend() calls.  */
      coro_info->self_h_proxy
        = build_lang_decl (VAR_DECL, get_identifier ("self_h.proxy"),
                           coro_info->handle_type);

      /* Build a proxy for the promise so that we can perform lookups.  */
      coro_info->promise_proxy
        = build_lang_decl (VAR_DECL, get_identifier ("promise.proxy"),
                           coro_info->promise_type);

      /* Note where we first saw a coroutine keyword.  */
      coro_info->first_coro_keyword = loc;
    }

  return true;
}

/* These functions assumes that the caller has verified that the state for
   the decl has been initialized, we try to minimize work here.  */

static tree
get_coroutine_promise_type (tree decl)
{
  if (coroutine_info *info = get_coroutine_info (decl))
    return info->promise_type;

  return NULL_TREE;
}

static tree
get_coroutine_handle_type (tree decl)
{
  if (coroutine_info *info = get_coroutine_info (decl))
    return info->handle_type;

  return NULL_TREE;
}

static tree
get_coroutine_self_handle_proxy (tree decl)
{
  if (coroutine_info *info = get_coroutine_info (decl))
    return info->self_h_proxy;

  return NULL_TREE;
}

static tree
get_coroutine_promise_proxy (tree decl)
{
  if (coroutine_info *info = get_coroutine_info (decl))
    return info->promise_proxy;

  return NULL_TREE;
}

static tree
lookup_promise_method (tree fndecl, tree member_id, location_t loc,
                       bool musthave)
{
  tree promise = get_coroutine_promise_type (fndecl);
  tree pm_memb
    = lookup_member (promise, member_id,
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
  if (musthave && pm_memb == NULL_TREE)
    {
      error_at (loc, "no member named %qE in %qT", member_id, promise);
      return error_mark_node;
    }
  return pm_memb;
}

/* Lookup an Awaitable member, which should be await_ready, await_suspend
   or await_resume.  */

static tree
lookup_awaitable_member (tree await_type, tree member_id, location_t loc)
{
  tree aw_memb
    = lookup_member (await_type, member_id,
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
  if (aw_memb == NULL_TREE)
    {
      error_at (loc, "no member named %qE in %qT", member_id, await_type);
      return error_mark_node;
    }
  return aw_memb;
}

/* Here we check the constraints that are common to all keywords (since the
   presence of a coroutine keyword makes the function into a coroutine).  */

static bool
coro_common_keyword_context_valid_p (tree fndecl, location_t kw_loc,
                                     const char *kw_name)
{
  if (fndecl == NULL_TREE)
    {
      error_at (kw_loc, "%qs cannot be used outside a function", kw_name);
      return false;
    }

  /* This is arranged in order of prohibitions in the std.  */
  if (DECL_MAIN_P (fndecl))
    {
      /* [basic.start.main] 3. The function main shall not be a coroutine.  */
      error_at (kw_loc, "%qs cannot be used in the %<main%> function",
                kw_name);
      return false;
    }

  if (DECL_DECLARED_CONSTEXPR_P (fndecl))
    {
      /* [dcl.constexpr] 3.3 it shall not be a coroutine.  */
      error_at (kw_loc, "%qs cannot be used in a %<constexpr%> function",
                kw_name);
      cp_function_chain->invalid_constexpr = true;
      return false;
    }

  if (FNDECL_USED_AUTO (fndecl))
    {
      /* [dcl.spec.auto] 15. A function declared with a return type that uses
         a placeholder type shall not be a coroutine.  */
      error_at (kw_loc,
                "%qs cannot be used in a function with a deduced return type",
                kw_name);
      return false;
    }

  if (varargs_function_p (fndecl))
    {
      /* [dcl.fct.def.coroutine] The parameter-declaration-clause of the
         coroutine shall not terminate with an ellipsis that is not part
         of a parameter-declaration.  */
      error_at (kw_loc,
                "%qs cannot be used in a varargs function", kw_name);
      return false;
    }

  if (DECL_CONSTRUCTOR_P (fndecl))
    {
      /* [class.ctor] 7. a constructor shall not be a coroutine.  */
      error_at (kw_loc, "%qs cannot be used in a constructor", kw_name);
      return false;
    }

  if (DECL_DESTRUCTOR_P (fndecl))
    {
      /* [class.dtor] 21. a destructor shall not be a coroutine.  */
      error_at (kw_loc, "%qs cannot be used in a destructor", kw_name);
      return false;
    }

  return true;
}

/* Here we check the constraints that are not per keyword.  */

static bool
coro_function_valid_p (tree fndecl)
{
  location_t f_loc = DECL_SOURCE_LOCATION (fndecl);

  /* For cases where fundamental information cannot be found, e.g. the
     coroutine traits are missing, we need to punt early.  */
  if (!coro_promise_type_found_p (fndecl, f_loc))
    return false;

  /* Since we think the function is a coroutine, that implies we parsed
     a keyword that triggered this.  Keywords check promise validity for
     their context and thus the promise type should be known at this point.  */
  if (get_coroutine_handle_type (fndecl) == NULL_TREE
      || get_coroutine_promise_type (fndecl) == NULL_TREE)
    return false;

  if (current_function_returns_value || current_function_returns_null)
    {
       /* TODO: record or extract positions of returns (and the first coro
          keyword) so that we can add notes to the diagnostic about where
          the bad keyword is and what made the function into a coro.  */
      error_at (f_loc, "a %<return%> statement is not allowed in coroutine;"
                        " did you mean %<co_return%>?");
      return false;
    }

  return true;
}

enum suspend_point_kind {
  CO_AWAIT_SUSPEND_POINT = 0,
  CO_YIELD_SUSPEND_POINT,
  INITIAL_SUSPEND_POINT,
  FINAL_SUSPEND_POINT
};

/*  This performs [expr.await] bullet 3.3 and validates the interface obtained.
    It is also used to build the initial and final suspend points.

    'a', 'o' and 'e' are used as per the description in the section noted.

    A, the original yield/await expr, is found at source location LOC.

    We will be constructing a CO_AWAIT_EXPR for a suspend point of one of
    the four suspend_point_kind kinds.  This is indicated by SUSPEND_KIND.  */

static tree
build_co_await (location_t loc, tree a, suspend_point_kind suspend_kind)
{
  /* Try and overload of operator co_await, .... */
  tree o;
  if (MAYBE_CLASS_TYPE_P (TREE_TYPE (a)))
    {
      o = build_new_op (loc, CO_AWAIT_EXPR, LOOKUP_NORMAL, a, NULL_TREE,
                        NULL_TREE, NULL, tf_warning_or_error);
      /* If no viable functions are found, o is a.  */
      if (!o || o == error_mark_node)
        o = a;
    }
  else
    o = a; /* This is most likely about to fail anyway.  */

  tree o_type = TREE_TYPE (o);
  if (o_type && !VOID_TYPE_P (o_type))
    o_type = complete_type_or_else (o_type, o);

  if (!o_type)
    return error_mark_node;

  if (TREE_CODE (o_type) != RECORD_TYPE)
    {
      error_at (loc, "awaitable type %qT is not a structure",
                o_type);
      return error_mark_node;
    }

  /* Check for required awaitable members and their types.  */
  tree awrd_meth
    = lookup_awaitable_member (o_type, coro_await_ready_identifier, loc);
  if (!awrd_meth || awrd_meth == error_mark_node)
    return error_mark_node;
  tree awsp_meth
    = lookup_awaitable_member (o_type, coro_await_suspend_identifier, loc);
  if (!awsp_meth || awsp_meth == error_mark_node)
    return error_mark_node;

  /* The type of the co_await is the return type of the awaitable's
     await_resume, so we need to look that up.  */
  tree awrs_meth
    = lookup_awaitable_member (o_type, coro_await_resume_identifier, loc);
  if (!awrs_meth || awrs_meth == error_mark_node)
    return error_mark_node;

  /* To complete the lookups, we need an instance of 'e' which is built from
     'o' according to [expr.await] 3.4.  However, we don't want to materialize
     'e' here (it might need to be placed in the coroutine frame) so we will
     make a temp placeholder instead.  If 'o' is a parameter or a local var,
     then we do not need an additional var (parms and local vars are already
     copied into the frame and will have lifetimes according to their original
     scope).  */
  tree e_proxy = STRIP_NOPS (o);
  if (INDIRECT_REF_P (e_proxy))
    e_proxy = TREE_OPERAND (e_proxy, 0);
  if (TREE_CODE (e_proxy) == PARM_DECL
      || (VAR_P (e_proxy) && (!DECL_ARTIFICIAL (e_proxy)
                              || DECL_HAS_VALUE_EXPR_P (e_proxy))))
    e_proxy = o;
  else
    {
      e_proxy = build_lang_decl (VAR_DECL, NULL_TREE, o_type);
      DECL_ARTIFICIAL (e_proxy) = true;
    }

  /* I suppose we could check that this is contextually convertible to bool.  */
  tree awrd_func = NULL_TREE;
  tree awrd_call
    = build_new_method_call (e_proxy, awrd_meth, NULL, NULL_TREE, LOOKUP_NORMAL,
                             &awrd_func, tf_warning_or_error);

  if (!awrd_func || !awrd_call || awrd_call == error_mark_node)
    return error_mark_node;

  /* The suspend method may return one of three types:
      1. void (no special action needed).
      2. bool (if true, we don't need to suspend).
      3. a coroutine handle, we execute the handle.resume() call.  */
  tree awsp_func = NULL_TREE;
  tree h_proxy = get_coroutine_self_handle_proxy (current_function_decl);
  vec<tree, va_gc> *args = make_tree_vector_single (h_proxy);
  tree awsp_call
    = build_new_method_call (e_proxy, awsp_meth, &args, NULL_TREE,
                             LOOKUP_NORMAL, &awsp_func, tf_warning_or_error);

  release_tree_vector (args);
  if (!awsp_func || !awsp_call || awsp_call == error_mark_node)
    return error_mark_node;

  bool ok = false;
  tree susp_return_type = TREE_TYPE (TREE_TYPE (awsp_func));
  if (same_type_p (susp_return_type, void_type_node))
    ok = true;
  else if (same_type_p (susp_return_type, boolean_type_node))
    ok = true;
  else if (TREE_CODE (susp_return_type) == RECORD_TYPE
           && CLASS_TYPE_P (susp_return_type))
    {
      tree tt = CLASSTYPE_TI_TEMPLATE (susp_return_type);
      if (tt == coro_handle_templ)
        ok = true;
    }

  if (!ok)
    {
      error_at (loc, "%<await_suspend%> must return %<void%>, %<bool%> or"
                     " a coroutine handle");
      return error_mark_node;
    }

  /* Finally, the type of e.await_resume() is the co_await's type.  */
  tree awrs_func = NULL_TREE;
  tree awrs_call
    = build_new_method_call (e_proxy, awrs_meth, NULL, NULL_TREE, LOOKUP_NORMAL,
                             &awrs_func, tf_warning_or_error);

  if (!awrs_func || !awrs_call || awrs_call == error_mark_node)
    return error_mark_node;

  /* We now have three call expressions, in terms of the promise, handle and
     'e' proxies.  Save them in the await expression for later expansion.  */

  tree awaiter_calls = make_tree_vec (3);
  TREE_VEC_ELT (awaiter_calls, 0) = awrd_call; /* await_ready().  */
  TREE_VEC_ELT (awaiter_calls, 1) = awsp_call; /* await_suspend().  */
  tree te = NULL_TREE;
  if (TREE_CODE (awrs_call) == TARGET_EXPR)
    {
      te = awrs_call;
      awrs_call = TREE_OPERAND (awrs_call, 1);
    }
  TREE_VEC_ELT (awaiter_calls, 2) = awrs_call; /* await_resume().  */

  tree await_expr = build5_loc (loc, CO_AWAIT_EXPR,
                                TREE_TYPE (TREE_TYPE (awrs_func)),
                                a, e_proxy, o, awaiter_calls,
                                build_int_cst (integer_type_node,
                                               (int) suspend_kind));
  if (te)
    {
      TREE_OPERAND (te, 1) = await_expr;
      await_expr = te;
    }
  tree t = convert_from_reference (await_expr);
  return t;
}

tree
finish_co_await_expr (location_t kw, tree expr)
{
  if (!expr || error_operand_p (expr))
    return error_mark_node;

  if (!coro_common_keyword_context_valid_p (current_function_decl, kw,
                                            "co_await"))
    return error_mark_node;

  /* The current function has now become a coroutine, if it wasn't already.  */
  DECL_COROUTINE_P (current_function_decl) = 1;

  if (processing_template_decl)
    {
      current_function_returns_value = 1;

      if (check_for_bare_parameter_packs (expr))
        return error_mark_node;

      /* If we don't know the promise type, we can't proceed.  */
      tree functype = TREE_TYPE (current_function_decl);
      if (dependent_type_p (functype) || type_dependent_expression_p (expr))
        return build5_loc (kw, CO_AWAIT_EXPR, unknown_type_node, expr,
                           NULL_TREE, NULL_TREE, NULL_TREE, integer_zero_node);
    }

  /* We must be able to look up the "await_transform" method in the scope of
     the promise type, and obtain its return type.  */
  if (!coro_promise_type_found_p (current_function_decl, kw))
    return error_mark_node;

  /* [expr.await] 3.2
     The incoming cast expression might be transformed by a promise
     'await_transform()'.  */
  tree at_meth
    = lookup_promise_method (current_function_decl,
                             coro_await_transform_identifier, kw,
                             /*musthave=*/false);
  if (at_meth == error_mark_node)
    return error_mark_node;

  tree a = expr;
  if (at_meth)
    {
      /* try to build a = p.await_transform (e). */
      vec<tree, va_gc> *args = make_tree_vector_single (expr);
      a = build_new_method_call (get_coroutine_promise_proxy (
                                   current_function_decl),
                                 at_meth, &args, NULL_TREE, LOOKUP_NORMAL,
                                 NULL, tf_warning_or_error);

      /* As I read the section.
         We saw an await_transform method, so it's mandatory that we replace
         expr with p.await_transform (expr), therefore if the method call fails
         (presumably, we don't have suitable arguments) then this part of the
         process fails.  */
      if (a == error_mark_node)
        return error_mark_node;
    }

  /* Now we want to build co_await a.  */
  tree op = build_co_await (kw, a, CO_AWAIT_SUSPEND_POINT);
  if (op != error_mark_node)
    {
      TREE_SIDE_EFFECTS (op) = 1;
      SET_EXPR_LOCATION (op, kw);
    }

  return op;
}

/* Take the EXPR given and attempt to build:
     co_await p.yield_value (expr);
   per [expr.yield] para 1. */

tree
finish_co_yield_expr (location_t kw, tree expr)
{
  if (!expr || error_operand_p (expr))
    return error_mark_node;

  /* Check the general requirements and simple syntax errors.  */
  if (!coro_common_keyword_context_valid_p (current_function_decl, kw,
                                            "co_yield"))
    return error_mark_node;

  /* The current function has now become a coroutine, if it wasn't already.  */
  DECL_COROUTINE_P (current_function_decl) = 1;

  if (processing_template_decl)
    {
      current_function_returns_value = 1;

      if (check_for_bare_parameter_packs (expr))
        return error_mark_node;

      tree functype = TREE_TYPE (current_function_decl);
      /* If we don't know the promise type, we can't proceed.  */
      if (dependent_type_p (functype) || type_dependent_expression_p (expr))
        return build2_loc (kw, CO_YIELD_EXPR, unknown_type_node, expr,
                           NULL_TREE);
    }

  if (!coro_promise_type_found_p (current_function_decl, kw))
    /* We must be able to look up the "yield_value" method in the scope of
       the promise type, and obtain its return type.  */
    return error_mark_node;

  /* The incoming expr is "e" per [expr.yield] para 1, lookup and build a
     call for p.yield_value(e).  */
  tree y_meth = lookup_promise_method (current_function_decl,
                                       coro_yield_value_identifier, kw,
                                       /*musthave=*/true);
  if (!y_meth || y_meth == error_mark_node)
    return error_mark_node;

  /* [expr.yield] / 1
     Let e be the operand of the yield-expression and p be an lvalue naming
     the promise object of the enclosing coroutine, then the yield-expression
     is equivalent to the expression co_await p.yield_value(e).
     build p.yield_value(e):  */
  vec<tree, va_gc> *args = make_tree_vector_single (expr);
  tree yield_call = build_new_method_call
    (get_coroutine_promise_proxy (current_function_decl), y_meth, &args,
     NULL_TREE, LOOKUP_NORMAL, NULL, tf_warning_or_error);

  /* Now build co_await p.yield_value (e).
     Noting that for co_yield, there is no evaluation of any potential
     promise transform_await(), so we call build_co_await directly.  */

  tree op = build_co_await (kw, yield_call, CO_YIELD_SUSPEND_POINT);
  if (op != error_mark_node)
    {
      if (REFERENCE_REF_P (op))
        op = TREE_OPERAND (op, 0);
      /* If the await expression is wrapped in a TARGET_EXPR, then transfer
         that wrapper to the CO_YIELD_EXPR, since this is just a proxy for
         its contained await.  Otherwise, just build the CO_YIELD_EXPR.  */
      if (TREE_CODE (op) == TARGET_EXPR)
        {
          tree t = TREE_OPERAND (op, 1);
          t = build2_loc (kw, CO_YIELD_EXPR, TREE_TYPE (t), expr, t);
          TREE_OPERAND (op, 1) = t;
        }
      else
        op = build2_loc (kw, CO_YIELD_EXPR, TREE_TYPE (op), expr, op);
      TREE_SIDE_EFFECTS (op) = 1;
      op = convert_from_reference (op);
    }

  return op;
}

/* Check and build a co_return statememt.
   First that it's valid to have a co_return keyword here.
   If it is, then check and build the p.return_{void(),value(expr)}.
   These are built against a proxy for the promise, which will be filled
   in with the actual frame version when the function is transformed.  */

tree
finish_co_return_stmt (location_t kw, tree expr)
{
  if (expr)
    STRIP_ANY_LOCATION_WRAPPER (expr);

  if (error_operand_p (expr))
    return error_mark_node;

  /* If it fails the following test, the function is not permitted to be a
     coroutine, so the co_return statement is erroneous.  */
  if (!coro_common_keyword_context_valid_p (current_function_decl, kw,
                                            "co_return"))
    return error_mark_node;

  /* The current function has now become a coroutine, if it wasn't
     already.  */
  DECL_COROUTINE_P (current_function_decl) = 1;

  /* This function will appear to have no return statement, even if it
     is declared to return non-void (most likely).  This is correct - we
     synthesize the return for the ramp in the compiler.  So suppress any
     extraneous warnings during substitution.  */
  TREE_NO_WARNING (current_function_decl) = true;

  if (processing_template_decl
      && check_for_bare_parameter_packs (expr))
    return error_mark_node;

  /* If we don't know the promise type, we can't proceed, build the
     co_return with the expression unchanged.  */
  tree functype = TREE_TYPE (current_function_decl);
  if (dependent_type_p (functype) || type_dependent_expression_p (expr))
    {
      /* co_return expressions are always void type, regardless of the
         expression type.  */
      expr = build2_loc (kw, CO_RETURN_EXPR, void_type_node,
                         expr, NULL_TREE);
      expr = maybe_cleanup_point_expr_void (expr);
      return add_stmt (expr);
    }

  if (!coro_promise_type_found_p (current_function_decl, kw))
    return error_mark_node;

  /* Suppress -Wreturn-type for co_return, we need to check indirectly
     whether the promise type has a suitable return_void/return_value.  */
  TREE_NO_WARNING (current_function_decl) = true;

  if (!processing_template_decl && warn_sequence_point)
    verify_sequence_points (expr);

  if (expr)
    {
      /* If we had an id-expression obfuscated by force_paren_expr, we need
         to undo it so we can try to treat it as an rvalue below.  */
      expr = maybe_undo_parenthesized_ref (expr);

      if (processing_template_decl)
        expr = build_non_dependent_expr (expr);

      if (error_operand_p (expr))
        return error_mark_node;
    }

  /* If the promise object doesn't have the correct return call then
     there's a mis-match between the co_return <expr> and this.  */
  tree co_ret_call = error_mark_node;
  if (expr == NULL_TREE || VOID_TYPE_P (TREE_TYPE (expr)))
    {
      tree crv_meth
        = lookup_promise_method (current_function_decl,
                                 coro_return_void_identifier, kw,
                                 /*musthave=*/true);
      if (crv_meth == error_mark_node)
        return error_mark_node;

      co_ret_call = build_new_method_call (
        get_coroutine_promise_proxy (current_function_decl), crv_meth, NULL,
        NULL_TREE, LOOKUP_NORMAL, NULL, tf_warning_or_error);
    }
  else
    {
      tree crv_meth
        = lookup_promise_method (current_function_decl,
                                 coro_return_value_identifier, kw,
                                 /*musthave=*/true);
      if (crv_meth == error_mark_node)
        return error_mark_node;

      /* [class.copy.elision] / 3.
         An implicitly movable entity is a variable of automatic storage
         duration that is either a non-volatile object or an rvalue reference
         to a non-volatile object type.  For such objects in the context of
         the co_return, the overload resolution should be carried out first
         treating the object as an rvalue, if that fails, then we fall back
         to regular overload resolution.  */

      if (treat_lvalue_as_rvalue_p (expr, /*parm_ok*/true)
          && CLASS_TYPE_P (TREE_TYPE (expr))
          && !TYPE_VOLATILE (TREE_TYPE (expr)))
        {
          vec<tree, va_gc> *args = make_tree_vector_single (move (expr));
          /* It's OK if this fails... */
          co_ret_call = build_new_method_call
            (get_coroutine_promise_proxy (current_function_decl), crv_meth,
             &args, NULL_TREE, LOOKUP_NORMAL|LOOKUP_PREFER_RVALUE,
             NULL, tf_none);
        }

      if (co_ret_call == error_mark_node)
        {
          vec<tree, va_gc> *args = make_tree_vector_single (expr);
          /* ... but this must succeed if we didn't get the move variant.  */
          co_ret_call = build_new_method_call
            (get_coroutine_promise_proxy (current_function_decl), crv_meth,
             &args, NULL_TREE, LOOKUP_NORMAL, NULL, tf_warning_or_error);
        }
    }

  /* Makes no sense for a co-routine really. */
  if (TREE_THIS_VOLATILE (current_function_decl))
    warning_at (kw, 0,
                "function declared %<noreturn%> has a"
                " %<co_return%> statement");

  expr = build2_loc (kw, CO_RETURN_EXPR, void_type_node, expr, co_ret_call);
  expr = maybe_cleanup_point_expr_void (expr);
  return add_stmt (expr);
}

/* We need to validate the arguments to __builtin_coro_promise, since the
   second two must be constant, and the builtins machinery doesn't seem to
   deal with that properly.  */

tree
coro_validate_builtin_call (tree call, tsubst_flags_t)
{
  tree fn = TREE_OPERAND (CALL_EXPR_FN (call), 0);

  gcc_checking_assert (DECL_BUILT_IN_CLASS (fn) == BUILT_IN_NORMAL);
  switch (DECL_FUNCTION_CODE (fn))
    {
    default:
      return call;

    case BUILT_IN_CORO_PROMISE:
      {
        /* Argument 0 is already checked by the normal built-in machinery
           Argument 1 must be a constant of size type.  It probably makes
           little sense if it's not a power of 2, but that isn't specified
           formally.  */
        tree arg = CALL_EXPR_ARG (call, 1);
        location_t loc = EXPR_LOCATION (arg);

        /* We expect alignof expressions in templates.  */
        if (TREE_CODE (arg) == NON_DEPENDENT_EXPR
            && TREE_CODE (TREE_OPERAND (arg, 0)) == ALIGNOF_EXPR)
          ;
        else if (!TREE_CONSTANT (arg))
          {
            error_at (loc, "the align argument to %<__builtin_coro_promise%>"
                           " must be a constant");
            return error_mark_node;
          }
        /* Argument 2 is the direction - to / from handle address to promise
           address.  */
        arg = CALL_EXPR_ARG (call, 2);
        loc = EXPR_LOCATION (arg);
        if (!TREE_CONSTANT (arg))
          {
            error_at (loc, "the direction argument to"
                           " %<__builtin_coro_promise%> must be a constant");
            return error_mark_node;
          }
        return call;
        break;
      }
    }
}

/* ================= Morph and Expand. =================

   The entry point here is morph_fn_to_coro () which is called from
   finish_function () when we have completed any template expansion.

   This is preceded by helper functions that implement the phases below.

   The process proceeds in four phases.

   A Initial framing.
     The user's function body is wrapped in the initial and final suspend
     points and we begin building the coroutine frame.
     We build empty decls for the actor and destroyer functions at this
     time too.
     When exceptions are enabled, the user's function body will also be
     wrapped in a try-catch block with the catch invoking the promise
     class 'unhandled_exception' method.

   B Analysis.
     The user's function body is analyzed to determine the suspend points,
     if any, and to capture local variables that might persist across such
     suspensions.  In most cases, it is not necessary to capture compiler
     temporaries, since the tree-lowering nests the suspensions correctly.
     However, in the case of a captured reference, there is a lifetime
     extension to the end of the full expression - which can mean across a
     suspend point in which case it must be promoted to a frame variable.

     At the conclusion of analysis, we have a conservative frame layout and
     maps of the local variables to their frame entry points.

   C Build the ramp function.
     Carry out the allocation for the coroutine frame (NOTE; the actual size
     computation is deferred until late in the middle end to allow for future
     optimizations that will be allowed to elide unused frame entries).
     We build the return object.

   D Build and expand the actor and destroyer function bodies.
     The destroyer is a trivial shim that sets a bit to indicate that the
     destroy dispatcher should be used and then calls into the actor.

     The actor function is the implementation of the user's state machine.
     The current suspend point is noted in an index.
     Each suspend point is encoded as a pair of internal functions, one in
     the relevant dispatcher, and one representing the suspend point.

     During this process, the user's local variables and the proxies for the
     self-handle and the promise class instance are re-written to their
     coroutine frame equivalents.

     The complete bodies for the ramp, actor and destroy function are passed
     back to finish_function for folding and gimplification.  */

/* Helpers to build EXPR_STMT and void-cast EXPR_STMT, common ops.  */

static tree
coro_build_expr_stmt (tree expr, location_t loc)
{
  return maybe_cleanup_point_expr_void (build_stmt (loc, EXPR_STMT, expr));
}

static tree
coro_build_cvt_void_expr_stmt (tree expr, location_t loc)
{
  tree t = build1 (CONVERT_EXPR, void_type_node, expr);
  return coro_build_expr_stmt (t, loc);
}

/* Helpers for label creation:
   1. Create a named label in the specified context.  */

static tree
create_anon_label_with_ctx (location_t loc, tree ctx)
{
  tree lab = build_decl (loc, LABEL_DECL, NULL_TREE, void_type_node);

  DECL_CONTEXT (lab) = ctx;
  DECL_ARTIFICIAL (lab) = true;
  DECL_IGNORED_P (lab) = true;
  TREE_USED (lab) = true;
  return lab;
}

/*  2. Create a named label in the specified context.  */

static tree
create_named_label_with_ctx (location_t loc, const char *name, tree ctx)
{
  tree lab_id = get_identifier (name);
  tree lab = define_label (loc, lab_id);
  DECL_CONTEXT (lab) = ctx;
  DECL_ARTIFICIAL (lab) = true;
  TREE_USED (lab) = true;
  return lab;
}

struct proxy_replace
{
  tree from, to;
};

static tree
replace_proxy (tree *here, int *do_subtree, void *d)
{
  proxy_replace *data = (proxy_replace *) d;

  if (*here == data->from)
    {
      *here = data->to;
      *do_subtree = 0;
    }
  else
    *do_subtree = 1;
  return NULL_TREE;
}

/* Support for expansion of co_return statements.  */

struct coro_ret_data
{
  tree promise_proxy;
  tree real_promise;
  tree fs_label;
};

/* If this is a coreturn statement (or one wrapped in a cleanup) then
   return the list of statements to replace it.  */

static tree
coro_maybe_expand_co_return (tree co_ret_expr, coro_ret_data *data)
{
  /* Look inside <(void) (expr)> cleanup */
  if (TREE_CODE (co_ret_expr) == CLEANUP_POINT_EXPR)
    co_ret_expr = TREE_OPERAND (co_ret_expr, 0);

  if (TREE_CODE (co_ret_expr) != CO_RETURN_EXPR)
    return NULL_TREE;

  location_t loc = EXPR_LOCATION (co_ret_expr);
  tree expr = TREE_OPERAND (co_ret_expr, 0);
  tree call = TREE_OPERAND (co_ret_expr, 1);
  tree stmt_list = NULL;
  if (expr && VOID_TYPE_P (TREE_TYPE (expr)))
    {
       /* [stmt.return.coroutine], 2.2 
          If expr is present and void, it is placed immediately before
          the call for return_void;  */
      expr = maybe_cleanup_point_expr_void (expr);
      append_to_statement_list (expr, &stmt_list);
    }

  /* Now replace the promise proxy with its real value.  */
  proxy_replace p_data;
  p_data.from = data->promise_proxy;
  p_data.to = data->real_promise;
  cp_walk_tree (&call, replace_proxy, &p_data, NULL);

  /* The types of p.return_void and p.return_value are not explicitly stated
     at least in n4835, it is expected that they will return void.  */
  call = maybe_cleanup_point_expr_void (call);
  append_to_statement_list (call, &stmt_list);
  tree r = build1_loc (loc, GOTO_EXPR, void_type_node, data->fs_label);
  append_to_statement_list (r, &stmt_list);
  return stmt_list;
}

/* Callback that rewrites co_return as per [stmt.return.coroutine]
   - for co_return;
   { p.return_void (); goto final_suspend; }
   - for co_return [void expr];
   { expr; p.return_void(); goto final_suspend;}
   - for co_return [non void expr];
   { p.return_value(expr); goto final_suspend; }  */

static tree
co_return_expander (tree *stmt, int *do_subtree, void *d)
{
  coro_ret_data *data = (coro_ret_data *) d;

  /* To avoid nesting statement lists, walk them and insert as needed.  */
  if (TREE_CODE (*stmt) == STATEMENT_LIST)
    {
      tree_stmt_iterator i;
      for (i = tsi_start (*stmt); !tsi_end_p (i); tsi_next (&i))
        {
          tree *new_stmt = tsi_stmt_ptr (i);
          tree replace = coro_maybe_expand_co_return (*new_stmt, data);
          /* If we got something, it will be list and we want to splice
             it in.  */
          if (replace != NULL_TREE)
            {
              /* Splice it in ... */
              tsi_link_before (&i, replace, TSI_SAME_STMT);
              /* ... and delete what we expanded.  */
              tsi_delink (&i);
              /* Maybe, even likely, we replaced the last in the list.  */
              if (tsi_end_p (i))
                break;
            }
          else /* Continue the walk.  */
            cp_walk_tree (new_stmt, co_return_expander, d, NULL);
        }
      *do_subtree = 0; /* Done subtrees.  */
    }
  else
    {
      /* We might have a single co_return statement, in which case, we do
         have to replace it with a list.  */
      tree replace = coro_maybe_expand_co_return (*stmt, data);
      if (replace != NULL_TREE)
        {
          *stmt = replace;
          *do_subtree = 0; /* Done here.  */
        }
    }
  return NULL_TREE;
}

/* Walk the original function body, rewriting co_returns.  */

static tree
expand_co_returns (tree *fnbody, tree promise_proxy, tree promise,
                   tree fs_label)
{
  coro_ret_data data = {promise_proxy, promise, fs_label};
  cp_walk_tree (fnbody, co_return_expander, &data, NULL);
  return *fnbody;
}

/* Support for expansion of co_await statements.  */

struct coro_aw_data
{
  tree actor_fn;   /* Decl for context.  */
  tree coro_fp;    /* Frame pointer var.  */
  tree resume_idx; /* This is the index var in the frame.  */
  tree i_a_r_c;    /* initial suspend await_resume() was called if true.  */
  tree self_h;     /* This is a handle to the current coro (frame var).  */
  tree cleanup;    /* This is where to go once we complete local destroy.  */
  tree cororet;    /* This is where to go if we suspend.  */
  tree corocont;   /* This is where to go if we continue.  */
  tree conthand;   /* This is the handle for a continuation.  */
  unsigned index;  /* This is our current resume index.  */
};

/* Lighweight search for the first await expression in tree-walk order.
   returns:
     The first await expression found in STMT.
     NULL_TREE if there are none.
   So can be used to determine if the statement needs to be processed for
   awaits.  */

static tree
co_await_find_in_subtree (tree *stmt, int *do_subtree ATTRIBUTE_UNUSED, void *d)
{
  tree **p = (tree **) d;
  if (TREE_CODE (*stmt) == CO_AWAIT_EXPR)
    {
      *p = stmt;
      return *stmt;
    }
  return NULL_TREE;
}

/* Starting with a statment:

   stmt => some tree containing one or more await expressions.

   We replace the statement with:
   <STATEMENT_LIST> {
      initialise awaitable
      if (!ready)
        {
          suspension context.
        }
      resume:
        revised statement with one await expression rewritten to its
        await_resume() return value.
   }

   We then recurse into the initializer and the revised statement
   repeating this replacement until there are no more await expressions
   in either.  */

static tree *
expand_one_await_expression (tree *stmt, tree *await_expr, void *d)
{
  coro_aw_data *data = (coro_aw_data *) d;

  tree saved_statement = *stmt;
  tree saved_co_await = *await_expr;

  tree actor = data->actor_fn;
  location_t loc = EXPR_LOCATION (*stmt);
  tree var = TREE_OPERAND (saved_co_await, 1);  /* frame slot. */
  tree expr = TREE_OPERAND (saved_co_await, 2); /* initializer.  */
  tree awaiter_calls = TREE_OPERAND (saved_co_await, 3);

  tree source = TREE_OPERAND (saved_co_await, 4);
  bool is_initial =
    (source && TREE_INT_CST_LOW (source) == (int) INITIAL_SUSPEND_POINT);
  bool is_final = (source
                   && TREE_INT_CST_LOW (source) == (int) FINAL_SUSPEND_POINT);
  bool needs_dtor = TYPE_HAS_NONTRIVIAL_DESTRUCTOR (TREE_TYPE (var));
  int resume_point = data->index;
  size_t bufsize = sizeof ("destroy.") + 10;
  char *buf = (char *) alloca (bufsize);
  snprintf (buf, bufsize, "destroy.%d", resume_point);
  tree destroy_label = create_named_label_with_ctx (loc, buf, actor);
  snprintf (buf, bufsize, "resume.%d", resume_point);
  tree resume_label = create_named_label_with_ctx (loc, buf, actor);
  tree empty_list = build_empty_stmt (loc);

  tree dtor = NULL_TREE;
  tree await_type = TREE_TYPE (var);
  if (needs_dtor)
    dtor = build_special_member_call (var, complete_dtor_identifier, NULL,
                                      await_type, LOOKUP_NORMAL,
                                      tf_warning_or_error);

  tree stmt_list = NULL;
  tree t_expr = STRIP_NOPS (expr);
  tree r;
  tree *await_init = NULL;
  if (t_expr == var)
    dtor = NULL_TREE;
  else
    {
      /* Initialize the var from the provided 'o' expression.  */
      r = build2 (INIT_EXPR, await_type, var, expr);
      r = coro_build_cvt_void_expr_stmt (r, loc);
      append_to_statement_list_force (r, &stmt_list);
      /* We have an initializer, which might itself contain await exprs.  */
      await_init = tsi_stmt_ptr (tsi_last (stmt_list));
    }

  /* Use the await_ready() call to test if we need to suspend.  */
  tree ready_cond = TREE_VEC_ELT (awaiter_calls, 0); /* await_ready().  */
  ready_cond = build1_loc (loc, TRUTH_NOT_EXPR, boolean_type_node, ready_cond);
  ready_cond
    = build1_loc (loc, CLEANUP_POINT_EXPR, boolean_type_node, ready_cond);

  tree body_list = NULL;
  tree susp_idx = build_int_cst (short_unsigned_type_node, data->index);
  r = build2_loc (loc, MODIFY_EXPR, short_unsigned_type_node, data->resume_idx,
                  susp_idx);
  r = coro_build_cvt_void_expr_stmt (r, loc);
  append_to_statement_list (r, &body_list);

  /* Find out what we have to do with the awaiter's suspend method.
     [expr.await]
     (5.1) If the result of await-ready is false, the coroutine is considered
           suspended. Then:
     (5.1.1) If the type of await-suspend is std::coroutine_handle<Z>,
             await-suspend.resume() is evaluated.
     (5.1.2) if the type of await-suspend is bool, await-suspend is evaluated,
             and the coroutine is resumed if the result is false.
     (5.1.3) Otherwise, await-suspend is evaluated.  */

  tree suspend = TREE_VEC_ELT (awaiter_calls, 1); /* await_suspend().  */
  tree susp_type = TREE_TYPE (suspend);

  bool is_cont = false;
  /* NOTE: final suspend can't resume; the "resume" label in that case
     corresponds to implicit destruction.  */
  if (VOID_TYPE_P (susp_type))
    {
      /* We just call await_suspend() and hit the yield.  */
      suspend = coro_build_cvt_void_expr_stmt (suspend, loc);
      append_to_statement_list (suspend, &body_list);
    }
  else if (TREE_CODE (susp_type) == BOOLEAN_TYPE)
    {
      /* Boolean return, continue if the call returns false.  */
      suspend = build1_loc (loc, TRUTH_NOT_EXPR, boolean_type_node, suspend);
      suspend
        = build1_loc (loc, CLEANUP_POINT_EXPR, boolean_type_node, suspend);
      tree go_on = build1_loc (loc, GOTO_EXPR, void_type_node, resume_label);
      r = build3_loc (loc, COND_EXPR, void_type_node, suspend, go_on,
                      empty_list);
      append_to_statement_list (r, &body_list);
    }
  else
    {
      r = build1_loc (loc, CONVERT_EXPR, void_coro_handle_type, suspend);
      r = build2_loc (loc, INIT_EXPR, void_coro_handle_type, data->conthand, r);
      r = build1 (CONVERT_EXPR, void_type_node, r);
      append_to_statement_list (r, &body_list);
      is_cont = true;
    }

  tree d_l = build_address (destroy_label);
  tree r_l = build_address (resume_label);
  tree susp = build_address (data->cororet);
  tree cont = build_address (data->corocont);
  tree final_susp = build_int_cst (integer_type_node, is_final ? 1 : 0);

  susp_idx = build_int_cst (integer_type_node, data->index);

  tree sw = begin_switch_stmt ();
  tree cond = build_decl (loc, VAR_DECL, NULL_TREE, integer_type_node);
  DECL_ARTIFICIAL (cond) = 1;
  DECL_IGNORED_P (cond) = 1;
  layout_decl (cond, 0);

  r = build_call_expr_internal_loc (loc, IFN_CO_YIELD, integer_type_node, 5,
                                    susp_idx, final_susp, r_l, d_l,
                                    data->coro_fp);
  r = build2 (INIT_EXPR, integer_type_node, cond, r);
  finish_switch_cond (r, sw);
  r = build_case_label (build_int_cst (integer_type_node, 0), NULL_TREE,
                        create_anon_label_with_ctx (loc, actor));
  add_stmt (r); /* case 0: */
  /* Implement the suspend, a scope exit without clean ups.  */
  r = build_call_expr_internal_loc (loc, IFN_CO_SUSPN, void_type_node, 1,
                                    is_cont ? cont : susp);
  r = coro_build_cvt_void_expr_stmt (r, loc);
  add_stmt (r); /*   goto ret;  */
  r = build_case_label (build_int_cst (integer_type_node, 1), NULL_TREE,
                        create_anon_label_with_ctx (loc, actor));
  add_stmt (r); /* case 1:  */
  r = build1_loc (loc, GOTO_EXPR, void_type_node, resume_label);
  add_stmt (r); /*  goto resume;  */
  r = build_case_label (NULL_TREE, NULL_TREE,
                        create_anon_label_with_ctx (loc, actor));
  add_stmt (r); /* default:;  */
  r = build1_loc (loc, GOTO_EXPR, void_type_node, destroy_label);
  add_stmt (r); /* goto destroy;  */

  /* part of finish switch.  */
  SWITCH_STMT_BODY (sw) = pop_stmt_list (SWITCH_STMT_BODY (sw));
  pop_switch ();
  tree scope = SWITCH_STMT_SCOPE (sw);
  SWITCH_STMT_SCOPE (sw) = NULL;
  r = do_poplevel (scope);
  append_to_statement_list (r, &body_list);

  destroy_label = build_stmt (loc, LABEL_EXPR, destroy_label);
  append_to_statement_list (destroy_label, &body_list);
  if (needs_dtor)
    append_to_statement_list (dtor, &body_list);
  r = build1_loc (loc, GOTO_EXPR, void_type_node, data->cleanup);
  append_to_statement_list (r, &body_list);

  r = build3_loc (loc, COND_EXPR, void_type_node, ready_cond, body_list,
                  empty_list);

  append_to_statement_list (r, &stmt_list);

  /* Resume point.  */
  resume_label = build_stmt (loc, LABEL_EXPR, resume_label);
  append_to_statement_list (resume_label, &stmt_list);

  if (is_initial)
    {
      /* Note that we are about to execute the await_resume() for the initial
         await expression.  */
      r = build2_loc (loc, MODIFY_EXPR, boolean_type_node, data->i_a_r_c,
                      boolean_true_node);
      r = coro_build_cvt_void_expr_stmt (r, loc);
      append_to_statement_list (r, &stmt_list);
    }

  /* This will produce the value (if one is provided) from the co_await
     expression.  */
  tree resume_call = TREE_VEC_ELT (awaiter_calls, 2); /* await_resume().  */
  if (REFERENCE_REF_P (resume_call))
    /* Sink to await_resume call_expr.  */
    resume_call = TREE_OPERAND (resume_call, 0);

  *await_expr = resume_call; /* Replace the co_await expr with its result.  */
  append_to_statement_list_force (saved_statement, &stmt_list);
  /* Get a pointer to the revised statment.  */
  tree *revised = tsi_stmt_ptr (tsi_last (stmt_list));
  if (needs_dtor)
    append_to_statement_list (dtor, &stmt_list);
  data->index += 2;

  /* Replace the original statement with the expansion.  */
  *stmt = stmt_list;

  /* Now, if the awaitable had an initializer, expand any awaits that might
     be embedded in it.  */
  tree *aw_expr_ptr;
  if (await_init &&
      cp_walk_tree (await_init, co_await_find_in_subtree, &aw_expr_ptr, NULL))
    expand_one_await_expression (await_init, aw_expr_ptr, d);

  /* Expand any more await expressions in the the original statement.  */
  if (cp_walk_tree (revised, co_await_find_in_subtree, &aw_expr_ptr, NULL))
    expand_one_await_expression (revised, aw_expr_ptr, d);

  return NULL;
}

/* Check to see if a statement contains at least one await expression, if
   so, then process that.  */

static tree
process_one_statement (tree *stmt, void *d)
{
  tree *aw_expr_ptr;
  if (cp_walk_tree (stmt, co_await_find_in_subtree, &aw_expr_ptr, NULL))
    expand_one_await_expression (stmt, aw_expr_ptr, d);
  return NULL_TREE;
}

static tree
await_statement_expander (tree *stmt, int *do_subtree, void *d)
{
  tree res = NULL_TREE;

  /* Process a statement at a time.  */
  if (STATEMENT_CLASS_P (*stmt) || TREE_CODE (*stmt) == BIND_EXPR)
    return NULL_TREE; /* Just process the sub-trees.  */
  else if (TREE_CODE (*stmt) == STATEMENT_LIST)
    {
      tree_stmt_iterator i;
      for (i = tsi_start (*stmt); !tsi_end_p (i); tsi_next (&i))
        {
          res = cp_walk_tree (tsi_stmt_ptr (i), await_statement_expander,
                              d, NULL);
          if (res)
            return res;
        }
      *do_subtree = 0; /* Done subtrees.  */
    }
  else if (EXPR_P (*stmt))
    {
      process_one_statement (stmt, d);
      *do_subtree = 0; /* Done subtrees.  */
    }

  /* Continue statement walk, where required.  */
  return res;
}

/* Suspend point hash_map.  */

struct suspend_point_info
{
  /* coro frame field type.  */
  tree awaitable_type;
  /* coro frame field name.  */
  tree await_field_id;
};

static hash_map<tree, suspend_point_info> *suspend_points;

struct await_xform_data
{
  tree actor_fn;   /* Decl for context.  */
  tree actor_frame;
  tree promise_proxy;
  tree real_promise;
  tree self_h_proxy;
  tree real_self_h;
};

/* When we built the await expressions, we didn't know the coro frame
   layout, therefore no idea where to find the promise or where to put
   the awaitables.  Now we know these things, fill them in.  */

static tree
transform_await_expr (tree await_expr, await_xform_data *xform)
{
  suspend_point_info *si = suspend_points->get (await_expr);
  location_t loc = EXPR_LOCATION (await_expr);
  if (!si)
    {
      error_at (loc, "no suspend point info for %qD", await_expr);
      return error_mark_node;
    }

  /* So, on entry, we have:
     in : CO_AWAIT_EXPR (a, e_proxy, o, awr_call_vector, mode)
          We no longer need a [it had diagnostic value, maybe?]
          We need to replace the promise proxy in all elements
          We need to replace the e_proxy in the awr_call.  */

  tree coro_frame_type = TREE_TYPE (xform->actor_frame);

  /* If we have a frame var for the awaitable, get a reference to it.  */
  proxy_replace data;
  if (si->await_field_id)
    {
      tree as_m
         = lookup_member (coro_frame_type, si->await_field_id,
                          /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
      tree as = build_class_member_access_expr (xform->actor_frame, as_m,
                                                NULL_TREE, true,
                                                tf_warning_or_error);

      /* Replace references to the instance proxy with the frame entry now
         computed.  */
      data.from = TREE_OPERAND (await_expr, 1);
      data.to = as;
      cp_walk_tree (&await_expr, replace_proxy, &data, NULL);

      /* .. and replace.  */
      TREE_OPERAND (await_expr, 1) = as;
    }

  /* Now do the self_handle.  */
  data.from = xform->self_h_proxy;
  data.to = xform->real_self_h;
  cp_walk_tree (&await_expr, replace_proxy, &data, NULL);

  /* Now do the promise.  */
  data.from = xform->promise_proxy;
  data.to = xform->real_promise;
  cp_walk_tree (&await_expr, replace_proxy, &data, NULL);

  return await_expr;
}

/* A wrapper for the transform_await_expr function so that it can be a
   callback from cp_walk_tree.  */

static tree
transform_await_wrapper (tree *stmt, int *do_subtree, void *d)
{
  /* Set actor function as new DECL_CONTEXT of label_decl.  */
  struct await_xform_data *xform = (struct await_xform_data *) d;
  if (TREE_CODE (*stmt) == LABEL_DECL
      && DECL_CONTEXT (*stmt) != xform->actor_fn)
    DECL_CONTEXT (*stmt) = xform->actor_fn;

  /* We should have already lowered co_yields to their co_await.  */
  gcc_checking_assert (TREE_CODE (*stmt) != CO_YIELD_EXPR);
  if (TREE_CODE (*stmt) != CO_AWAIT_EXPR)
    return NULL_TREE;

  tree await_expr = *stmt;
  *stmt = transform_await_expr (await_expr, xform);
  if (*stmt == error_mark_node)
    *do_subtree = 0;
  return NULL_TREE;
}

/* This caches information that we determine about function params,
   their uses and copies in the coroutine frame.  */

struct param_info
{
  tree field_id;     /* The name of the copy in the coroutine frame.  */
  vec<tree *> *body_uses; /* Worklist of uses, void if there are none.  */
  tree frame_type;   /* The type used to represent this parm in the frame.  */
  tree orig_type;    /* The original type of the parm (not as passed).  */
  bool by_ref;       /* Was passed by reference.  */
  bool pt_ref;       /* Was a pointer to object.  */
  bool trivial_dtor; /* The frame type has a trivial DTOR.  */
  bool this_ptr;     /* Is 'this' */
  bool lambda_cobj;  /* Lambda capture object */
};

struct local_var_info
{
  tree field_id;
  tree field_idx;
  tree frame_type;
  bool is_lambda_capture;
  bool is_static;
  bool has_value_expr_p;
  location_t def_loc;
};

/* For figuring out what local variable usage we have.  */
struct local_vars_transform
{
  tree context;
  tree actor_frame;
  tree coro_frame_type;
  location_t loc;
  hash_map<tree, local_var_info> *local_var_uses;
};

static tree
transform_local_var_uses (tree *stmt, int *do_subtree, void *d)
{
  local_vars_transform *lvd = (local_vars_transform *) d;

  /* For each var in this bind expr (that has a frame id, which means it was
     accessed), build a frame reference for each and then walk the bind expr
     statements, substituting the frame ref for the original var.  */

  if (TREE_CODE (*stmt) == BIND_EXPR)
    {
      tree lvar;
      for (lvar = BIND_EXPR_VARS (*stmt); lvar != NULL;
           lvar = DECL_CHAIN (lvar))
        {
          bool existed;
          local_var_info &local_var
            = lvd->local_var_uses->get_or_insert (lvar, &existed);
          gcc_checking_assert (existed);

          /* Re-write the variable's context to be in the actor func.  */
          DECL_CONTEXT (lvar) = lvd->context;

        /* For capture proxies, this could include the decl value expr.  */
        if (local_var.is_lambda_capture || local_var.has_value_expr_p)
          {
            tree ve = DECL_VALUE_EXPR (lvar);
            cp_walk_tree (&ve, transform_local_var_uses, d, NULL);
            continue; /* No frame entry for this.  */
          }

          /* TODO: implement selective generation of fields when vars are
             known not-used.  */
          if (local_var.field_id == NULL_TREE)
            continue; /* Wasn't used.  */

          tree fld_ref
            = lookup_member (lvd->coro_frame_type, local_var.field_id,
                             /*protect=*/1, /*want_type=*/0,
                             tf_warning_or_error);
          tree fld_idx = build3_loc (lvd->loc, COMPONENT_REF, TREE_TYPE (lvar),
                                     lvd->actor_frame, fld_ref, NULL_TREE);
          local_var.field_idx = fld_idx;
        }
      /* FIXME: we should be able to do this in the loop above, but (at least
         for range for) there are cases where the DECL_INITIAL contains
         forward references.
         So, now we've built the revised var in the frame, substitute uses of
         it in initializers and the bind expr body.  */
      for (lvar = BIND_EXPR_VARS (*stmt); lvar != NULL;
           lvar = DECL_CHAIN (lvar))
        {
          /* we need to walk some of the decl trees, which might contain
             references to vars replaced at a higher level.  */
          cp_walk_tree (&DECL_INITIAL (lvar), transform_local_var_uses, d,
                        NULL);
          cp_walk_tree (&DECL_SIZE (lvar), transform_local_var_uses, d, NULL);
          cp_walk_tree (&DECL_SIZE_UNIT (lvar), transform_local_var_uses, d,
                        NULL);
        }
      cp_walk_tree (&BIND_EXPR_BODY (*stmt), transform_local_var_uses, d, NULL);

      /* Now we have processed and removed references to the original vars,
         we can drop those from the bind - leaving capture proxies alone.  */
      for (tree *pvar = &BIND_EXPR_VARS (*stmt); *pvar != NULL;)
        {
          bool existed;
          local_var_info &local_var
            = lvd->local_var_uses->get_or_insert (*pvar, &existed);
          gcc_checking_assert (existed);

          /* Leave lambda closure captures alone, we replace the *this
             pointer with the frame version and let the normal process
             deal with the rest.
             Likewise, variables with their value found elsewhere.
             Skip past unused ones too.  */
          if (local_var.is_lambda_capture
             || local_var.has_value_expr_p
             || local_var.field_id == NULL_TREE)
            {
              pvar = &DECL_CHAIN (*pvar);
              continue;
            }

          /* Discard this one, we replaced it.  */
          *pvar = DECL_CHAIN (*pvar);
        }

      *do_subtree = 0; /* We've done the body already.  */
      return NULL_TREE;
    }

  tree var_decl = *stmt;
  /* Look inside cleanups, we don't want to wrap a statement list in a
     cleanup.  */
  bool needs_cleanup = true;
  if (TREE_CODE (var_decl) == CLEANUP_POINT_EXPR)
    var_decl = TREE_OPERAND (var_decl, 0);
  else
    needs_cleanup = false;

  /* Look inside the decl_expr for the actual var.  */
  bool decl_expr_p = TREE_CODE (var_decl) == DECL_EXPR;
  if (decl_expr_p && TREE_CODE (DECL_EXPR_DECL (var_decl)) == VAR_DECL)
    var_decl = DECL_EXPR_DECL (var_decl);
  else if (TREE_CODE (var_decl) != VAR_DECL)
    return NULL_TREE;

  /* VAR_DECLs that are not recorded can belong to the proxies we've placed
     for the promise and coroutine handle(s), to global vars or to compiler
     temporaries.  Skip past these, we will handle them later.  */
  local_var_info *local_var_i = lvd->local_var_uses->get (var_decl);

  if (local_var_i == NULL)
    return NULL_TREE;

  if (local_var_i->is_lambda_capture
      || local_var_i->is_static
      || local_var_i->has_value_expr_p)
    return NULL_TREE;

  /* This is our revised 'local' i.e. a frame slot.  */
  tree revised = local_var_i->field_idx;
  gcc_checking_assert (DECL_CONTEXT (var_decl) == lvd->context);

  if (decl_expr_p && DECL_INITIAL (var_decl))
    {
      location_t loc = DECL_SOURCE_LOCATION (var_decl);
      tree r
        = cp_build_modify_expr (loc, revised, INIT_EXPR,
                                DECL_INITIAL (var_decl), tf_warning_or_error);
      if (needs_cleanup)
        r = coro_build_cvt_void_expr_stmt (r, EXPR_LOCATION (*stmt));
      *stmt = r;
    }
  else
    *stmt = revised;

  if (decl_expr_p)
    *do_subtree = 0; /* We've accounted for the nested use.  */
  return NULL_TREE;
}

/* The actor transform.  */

static void
build_actor_fn (location_t loc, tree coro_frame_type, tree actor, tree fnbody,
                tree orig, hash_map<tree, param_info> *param_uses,
                hash_map<tree, local_var_info> *local_var_uses,
                vec<tree, va_gc> *param_dtor_list, tree initial_await,
                tree final_await, unsigned body_count, tree frame_size)
{
  verify_stmt_tree (fnbody);
  /* Some things we inherit from the original function.  */
  tree coro_frame_ptr = build_pointer_type (coro_frame_type);
  tree handle_type = get_coroutine_handle_type (orig);
  tree self_h_proxy = get_coroutine_self_handle_proxy (orig);
  tree promise_type = get_coroutine_promise_type (orig);
  tree promise_proxy = get_coroutine_promise_proxy (orig);
  tree act_des_fn_type
    = build_function_type_list (void_type_node, coro_frame_ptr, NULL_TREE);
  tree act_des_fn_ptr = build_pointer_type (act_des_fn_type);

  /* One param, the coro frame pointer.  */
  tree actor_fp = DECL_ARGUMENTS (actor);

  /* A void return.  */
  tree resdecl = build_decl (loc, RESULT_DECL, 0, void_type_node);
  DECL_ARTIFICIAL (resdecl) = 1;
  DECL_IGNORED_P (resdecl) = 1;
  DECL_RESULT (actor) = resdecl;
  DECL_COROUTINE_P (actor) = 1;

  /* We have a definition here.  */
  TREE_STATIC (actor) = 1;

  tree actor_outer = push_stmt_list ();
  current_stmt_tree ()->stmts_are_full_exprs_p = 1;
  tree stmt = begin_compound_stmt (BCS_FN_BODY);

  tree actor_bind = build3 (BIND_EXPR, void_type_node, NULL, NULL, NULL);
  tree top_block = make_node (BLOCK);
  BIND_EXPR_BLOCK (actor_bind) = top_block;

  tree continuation = build_lang_decl (VAR_DECL,
                                       get_identifier ("actor.continue"),
                                       void_coro_handle_type);
  DECL_ARTIFICIAL (continuation) = 1;
  DECL_IGNORED_P (continuation) = 1;
  DECL_CONTEXT (continuation) = actor;
  BIND_EXPR_VARS (actor_bind) = continuation;

  /* Update the block associated with the outer scope of the orig fn.  */
  tree first = expr_first (fnbody);
  if (first && TREE_CODE (first) == BIND_EXPR)
    {
      /* We will discard this, since it's connected to the original scope
         nest.  */
      tree block = BIND_EXPR_BLOCK (first);
      if (block) /* For this to be missing is probably a bug.  */
        {
          gcc_assert (BLOCK_SUPERCONTEXT (block) == NULL_TREE);
          gcc_assert (BLOCK_CHAIN (block) == NULL_TREE);
          BLOCK_SUPERCONTEXT (block) = top_block;
          BLOCK_SUBBLOCKS (top_block) = block;
        }
    }

  add_stmt (actor_bind);
  tree actor_body = push_stmt_list ();

  /* The entry point for the actor code from the ramp.  */
  tree actor_begin_label
    = create_named_label_with_ctx (loc, "actor.begin", actor);
  tree actor_frame = build1_loc (loc, INDIRECT_REF, coro_frame_type, actor_fp);

  /* Declare the continuation handle.  */
  add_decl_expr (continuation);

  /* Re-write param references in the body, no code should be generated
     here.  */
  if (DECL_ARGUMENTS (orig))
    {
      tree arg;
      for (arg = DECL_ARGUMENTS (orig); arg != NULL; arg = DECL_CHAIN (arg))
        {
          bool existed;
          param_info &parm = param_uses->get_or_insert (arg, &existed);
          if (!parm.body_uses)
            continue; /* Wasn't used in the orignal function body.  */

          tree fld_ref = lookup_member (coro_frame_type, parm.field_id,
                                        /*protect=*/1, /*want_type=*/0,
                                        tf_warning_or_error);
          tree fld_idx = build3_loc (loc, COMPONENT_REF, parm.frame_type,
                                     actor_frame, fld_ref, NULL_TREE);

          /* We keep these in the frame as a regular pointer, so convert that
           back to the type expected.  */
          if (parm.pt_ref)
            fld_idx = build1_loc (loc, CONVERT_EXPR, TREE_TYPE (arg), fld_idx);

          int i;
          tree *puse;
          FOR_EACH_VEC_ELT (*parm.body_uses, i, puse)
            *puse = fld_idx;
        }
    }

  /* Re-write local vars, similarly.  */
  local_vars_transform xform_vars_data
    = {actor, actor_frame, coro_frame_type, loc, local_var_uses};
  cp_walk_tree (&fnbody, transform_local_var_uses, &xform_vars_data, NULL);

  tree resume_idx_name = get_identifier ("__resume_at");
  tree rat_field = lookup_member (coro_frame_type, resume_idx_name, 1, 0,
                                  tf_warning_or_error);
  tree rat = build3 (COMPONENT_REF, short_unsigned_type_node, actor_frame,
                     rat_field, NULL_TREE);

  tree ret_label
    = create_named_label_with_ctx (loc, "actor.suspend.ret", actor);

  tree continue_label
    = create_named_label_with_ctx (loc, "actor.continue.ret", actor);

  tree lsb_if = begin_if_stmt ();
  tree chkb0 = build2 (BIT_AND_EXPR, short_unsigned_type_node, rat,
                       build_int_cst (short_unsigned_type_node, 1));
  chkb0 = build2 (NE_EXPR, short_unsigned_type_node, chkb0,
                  build_int_cst (short_unsigned_type_node, 0));
  finish_if_stmt_cond (chkb0, lsb_if);

  tree destroy_dispatcher = begin_switch_stmt ();
  finish_switch_cond (rat, destroy_dispatcher);
  tree ddeflab = build_case_label (NULL_TREE, NULL_TREE,
                                   create_anon_label_with_ctx (loc, actor));
  add_stmt (ddeflab);
  tree b = build_call_expr_loc (loc, builtin_decl_explicit (BUILT_IN_TRAP), 0);
  b = coro_build_cvt_void_expr_stmt (b, loc);
  add_stmt (b);

  short unsigned lab_num = 3;
  for (unsigned destr_pt = 0; destr_pt < body_count + 2; destr_pt++)
    {
      tree l_num = build_int_cst (short_unsigned_type_node, lab_num);
      b = build_case_label (l_num, NULL_TREE,
                            create_anon_label_with_ctx (loc, actor));
      add_stmt (b);
      b = build_call_expr_internal_loc (loc, IFN_CO_ACTOR, void_type_node, 1,
                                        l_num);
      b = coro_build_cvt_void_expr_stmt (b, loc);
      add_stmt (b);
      b = build1 (GOTO_EXPR, void_type_node, CASE_LABEL (ddeflab));
      add_stmt (b);
      lab_num += 2;
    }

  /* Insert the prototype dispatcher.  */
  finish_switch_stmt (destroy_dispatcher);

  finish_then_clause (lsb_if);

  tree dispatcher = begin_switch_stmt ();
  finish_switch_cond (rat, dispatcher);
  b = build_case_label (build_int_cst (short_unsigned_type_node, 0), NULL_TREE,
                        create_anon_label_with_ctx (loc, actor));
  add_stmt (b);
  b = build1 (GOTO_EXPR, void_type_node, actor_begin_label);
  add_stmt (b);

  tree rdeflab = build_case_label (NULL_TREE, NULL_TREE,
                                   create_anon_label_with_ctx (loc, actor));
  add_stmt (rdeflab);
  b = build_call_expr_loc (loc, builtin_decl_explicit (BUILT_IN_TRAP), 0);
  b = coro_build_cvt_void_expr_stmt (b, loc);
  add_stmt (b);

  lab_num = 2;
  /* The final resume should be made to hit the default (trap, UB) entry.  */
  for (unsigned resu_pt = 0; resu_pt < body_count + 1; resu_pt++)
    {
      tree l_num = build_int_cst (short_unsigned_type_node, lab_num);
      b = build_case_label (l_num, NULL_TREE,
                            create_anon_label_with_ctx (loc, actor));
      add_stmt (b);
      b = build_call_expr_internal_loc (loc, IFN_CO_ACTOR, void_type_node, 1,
                                        l_num);
      b = coro_build_cvt_void_expr_stmt (b, loc);
      add_stmt (b);
      b = build1 (GOTO_EXPR, void_type_node, CASE_LABEL (rdeflab));
      add_stmt (b);
      lab_num += 2;
    }

  /* Insert the prototype dispatcher.  */
  finish_switch_stmt (dispatcher);

  finish_if_stmt (lsb_if);

  tree r = build_stmt (loc, LABEL_EXPR, actor_begin_label);
  add_stmt (r);

  /* actor's version of the promise.  */
  tree ap_m = lookup_member (coro_frame_type, get_identifier ("__p"), 1, 0,
                             tf_warning_or_error);
  tree ap = build_class_member_access_expr (actor_frame, ap_m, NULL_TREE, false,
                                            tf_warning_or_error);

  /* actor's coroutine 'self handle'.  */
  tree ash_m = lookup_member (coro_frame_type, get_identifier ("__self_h"), 1,
                              0, tf_warning_or_error);
  tree ash = build_class_member_access_expr (actor_frame, ash_m, NULL_TREE,
                                             false, tf_warning_or_error);
  /* So construct the self-handle from the frame address.  */
  tree hfa_m = lookup_member (handle_type, coro_from_address_identifier, 1,
                              0, tf_warning_or_error);

  r = build1 (CONVERT_EXPR, build_pointer_type (void_type_node), actor_fp);
  vec<tree, va_gc> *args = make_tree_vector_single (r);
  tree hfa = build_new_method_call (ash, hfa_m, &args, NULL_TREE, LOOKUP_NORMAL,
                                    NULL, tf_warning_or_error);
  r = build2 (INIT_EXPR, handle_type, ash, hfa);
  r = coro_build_cvt_void_expr_stmt (r, loc);
  add_stmt (r);
  release_tree_vector (args);

  /* Now we know the real promise, and enough about the frame layout to
     decide where to put things.  */

  await_xform_data xform
    = {actor, actor_frame, promise_proxy, ap, self_h_proxy, ash};

  /* Get a reference to the initial suspend var in the frame.  */
  transform_await_expr (initial_await, &xform);
  tree initial_await_stmt = coro_build_expr_stmt (initial_await, loc);

  /* co_return branches to the final_suspend label, so declare that now.  */
  tree fs_label = create_named_label_with_ctx (loc, "final.suspend", actor);

  /* Expand co_returns in the saved function body  */
  fnbody = expand_co_returns (&fnbody, promise_proxy, ap, fs_label);

  /* Specific behaviour to treat exceptions thrown by the await_resume ()
     of the initial suspend expression.  In order to implement this, we
     need to treat the initial_suspend expression as if it were part of the
     user-authored function body.  This only applies if exceptions are
     enabled.  */
  if (flag_exceptions)
    {
      tree outer = fnbody;
      if (TREE_CODE (outer) == BIND_EXPR)
        outer = BIND_EXPR_BODY (outer);
      gcc_checking_assert (TREE_CODE (outer) == TRY_BLOCK);
      tree sl = TRY_STMTS (outer);
      if (TREE_CODE (sl) == STATEMENT_LIST)
        {
          tree_stmt_iterator si = tsi_start (sl);
          tsi_link_before (&si, initial_await_stmt, TSI_NEW_STMT);
        }
      else
        {
          tree new_try = NULL_TREE;
          append_to_statement_list (initial_await_stmt, &new_try);
          append_to_statement_list (sl, &new_try);
          TRY_STMTS (outer) = new_try;
        }
    }
  else
    add_stmt (initial_await_stmt);

  /* Transform the await expressions in the function body.  Only do each
     await tree once!  */
  hash_set<tree> pset;
  cp_walk_tree (&fnbody, transform_await_wrapper, &xform, &pset);

  /* Add in our function body with the co_returns rewritten to final form.  */
  add_stmt (fnbody);

  /* Final suspend starts here.  */
  r = build_stmt (loc, LABEL_EXPR, fs_label);
  add_stmt (r);

  /* Set the actor pointer to null, so that 'done' will work.
     Resume from here is UB anyway - although a 'ready' await will
     branch to the final resume, and fall through to the destroy.  */
  tree resume_m
    = lookup_member (coro_frame_type, get_identifier ("__resume"),
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
  tree res_x = build_class_member_access_expr (actor_frame, resume_m, NULL_TREE,
                                               false, tf_warning_or_error);
  r = build1 (CONVERT_EXPR, act_des_fn_ptr, integer_zero_node);
  r = build2 (INIT_EXPR, act_des_fn_ptr, res_x, r);
  r = coro_build_cvt_void_expr_stmt (r, loc);
  add_stmt (r);

  /* Get a reference to the final suspend var in the frame.  */
  transform_await_expr (final_await, &xform);
  r = coro_build_expr_stmt (final_await, loc);
  add_stmt (r);

  /* now do the tail of the function.  */
  tree del_promise_label
    = create_named_label_with_ctx (loc, "coro.delete.promise", actor);
  r = build_stmt (loc, LABEL_EXPR, del_promise_label);
  add_stmt (r);

  /* Destructors for the things we built explicitly.  */
  r = build_special_member_call (ap, complete_dtor_identifier, NULL,
                                 promise_type, LOOKUP_NORMAL,
                                 tf_warning_or_error);
  add_stmt (r);

  tree del_frame_label
    = create_named_label_with_ctx (loc, "coro.delete.frame", actor);
  r = build_stmt (loc, LABEL_EXPR, del_frame_label);
  add_stmt (r);

  /* Here deallocate the frame (if we allocated it), which we will have at
     present.  */
  tree fnf_m
    = lookup_member (coro_frame_type, get_identifier ("__frame_needs_free"), 1,
                     0, tf_warning_or_error);
  tree fnf2_x = build_class_member_access_expr (actor_frame, fnf_m, NULL_TREE,
                                                false, tf_warning_or_error);

  tree need_free_if = begin_if_stmt ();
  fnf2_x = build1 (CONVERT_EXPR, integer_type_node, fnf2_x);
  tree cmp = build2 (NE_EXPR, integer_type_node, fnf2_x, integer_zero_node);
  finish_if_stmt_cond (cmp, need_free_if);
  if (param_dtor_list != NULL)
    {
      int i;
      tree pid;
      FOR_EACH_VEC_ELT (*param_dtor_list, i, pid)
        {
          tree m
            = lookup_member (coro_frame_type, pid, 1, 0, tf_warning_or_error);
          tree a = build_class_member_access_expr (actor_frame, m, NULL_TREE,
                                                   false, tf_warning_or_error);
          tree t = TREE_TYPE (a);
          tree dtor;
          dtor
            = build_special_member_call (a, complete_dtor_identifier, NULL, t,
                                         LOOKUP_NORMAL, tf_warning_or_error);
          add_stmt (dtor);
        }
    }

  /* [dcl.fct.def.coroutine] / 12
     The deallocation function’s name is looked up in the scope of the promise
     type.  If this lookup fails, the deallocation function’s name is looked up
     in the global scope.  If deallocation function lookup finds both a usual
     deallocation function with only a pointer parameter and a usual
     deallocation function with both a pointer parameter and a size parameter,
     then the selected deallocation function shall be the one with two
     parameters.  Otherwise, the selected deallocation function shall be the
     function with one parameter.  If no usual deallocation function is found
     the program is ill-formed.  The selected deallocation function shall be
     called with the address of the block of storage to be reclaimed as its
     first argument.  If a deallocation function with a parameter of type
     std::size_t is used, the size of the block is passed as the corresponding
     argument.  */

  tree del_coro_fr = NULL_TREE;
  tree frame_arg = build1 (CONVERT_EXPR, ptr_type_node, actor_fp);

  tree delname = ovl_op_identifier (false, DELETE_EXPR);
  tree fns = lookup_promise_method (orig, delname, loc, /*musthave=*/false);
  if (fns && BASELINK_P (fns))
    {
      /* Look for sized version first, since this takes precedence.  */
      vec<tree, va_gc> *args = make_tree_vector ();
      vec_safe_push (args, frame_arg);
      vec_safe_push (args, frame_size);
      tree dummy_promise = build_dummy_object (promise_type);

      /* It's OK to fail for this one... */
      del_coro_fr = build_new_method_call (dummy_promise, fns, &args,
                                           NULL_TREE, LOOKUP_NORMAL, NULL,
                                           tf_none);

      if (!del_coro_fr || del_coro_fr == error_mark_node)
        {
          release_tree_vector (args);
          args = make_tree_vector_single (frame_arg);
          del_coro_fr = build_new_method_call (dummy_promise, fns, &args,
                                               NULL_TREE, LOOKUP_NORMAL, NULL,
                                               tf_none);
        }

      /* But one of them must succeed, or the program is ill-formed.  */
      if (!del_coro_fr || del_coro_fr == error_mark_node)
        {
          error_at (loc, "%qE is provided by %qT but is not usable with"
                  " the function signature %qD", delname, promise_type, orig);
          del_coro_fr = error_mark_node;
        }
    }
  else
    {
      del_coro_fr = build_op_delete_call (DELETE_EXPR, frame_arg, frame_size,
                                          /*global_p=*/true, /*placement=*/NULL,
                                          /*alloc_fn=*/NULL,
                                          tf_warning_or_error);
      if (!del_coro_fr || del_coro_fr == error_mark_node)
        del_coro_fr = error_mark_node;
    }

  del_coro_fr = coro_build_cvt_void_expr_stmt (del_coro_fr, loc);
  add_stmt (del_coro_fr);
  finish_then_clause (need_free_if);
  tree scope = IF_SCOPE (need_free_if);
  IF_SCOPE (need_free_if) = NULL;
  r = do_poplevel (scope);
  add_stmt (r);

  /* done.  */
  r = build_stmt (loc, RETURN_EXPR, NULL);
  TREE_NO_WARNING (r) |= 1; /* We don't want a warning about this.  */
  r = maybe_cleanup_point_expr_void (r);
  add_stmt (r);

  /* This is the suspend return point.  */
  r = build_stmt (loc, LABEL_EXPR, ret_label);
  add_stmt (r);

  r = build_stmt (loc, RETURN_EXPR, NULL);
  TREE_NO_WARNING (r) |= 1; /* We don't want a warning about this.  */
  r = maybe_cleanup_point_expr_void (r);
  add_stmt (r);

  /* This is the 'continuation' return point.  For such a case we have a coro
     handle (from the await_suspend() call) and we want handle.resume() to
     execute as a tailcall allowing arbitrary chaining of coroutines.  */
  r = build_stmt (loc, LABEL_EXPR, continue_label);
  add_stmt (r);

  /* We want to force a tail-call even for O0/1, so this expands the resume
     call into its underlying implementation.  */
  tree addr = lookup_member (void_coro_handle_type, coro_address_identifier,
                               1, 0, tf_warning_or_error);
  addr = build_new_method_call (continuation, addr, NULL, NULL_TREE,
                                  LOOKUP_NORMAL, NULL, tf_warning_or_error);
  tree resume = build_call_expr_loc
    (loc, builtin_decl_explicit (BUILT_IN_CORO_RESUME), 1, addr);

  /* In order to support an arbitrary number of coroutine continuations,
     we must tail call them.  However, some targets do not support indirect
     tail calls to arbitrary callees.  See PR94359.  */
  CALL_EXPR_TAILCALL (resume) = true;
  resume = coro_build_cvt_void_expr_stmt (resume, loc);
  add_stmt (resume);

  r = build_stmt (loc, RETURN_EXPR, NULL);
  gcc_checking_assert (maybe_cleanup_point_expr_void (r) == r);
  add_stmt (r);

  /* We will need to know which resume point number should be encoded.  */
  tree res_idx_m
    = lookup_member (coro_frame_type, resume_idx_name,
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
  tree resume_pt_number
    = build_class_member_access_expr (actor_frame, res_idx_m, NULL_TREE, false,
                                      tf_warning_or_error);

  /* Boolean value to flag that the initial suspend expression's
     await_resume () has been called, and therefore we are in the user's
     function body for the purposes of handing exceptions.  */
  tree i_a_r_c_m
    = lookup_member (coro_frame_type, get_identifier ("__i_a_r_c"),
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
  tree i_a_r_c
    = build_class_member_access_expr (actor_frame, i_a_r_c_m, NULL_TREE,
                                      false, tf_warning_or_error);

  /* We've now rewritten the tree and added the initial and final
     co_awaits.  Now pass over the tree and expand the co_awaits.  */

  coro_aw_data data = {actor, actor_fp, resume_pt_number, i_a_r_c,
                       ash, del_promise_label, ret_label,
                       continue_label, continuation, 2};
  cp_walk_tree (&actor_body, await_statement_expander, &data, NULL);

  BIND_EXPR_BODY (actor_bind) = pop_stmt_list (actor_body);
  TREE_SIDE_EFFECTS (actor_bind) = true;

  finish_compound_stmt (stmt);
  DECL_SAVED_TREE (actor) = pop_stmt_list (actor_outer);
  verify_stmt_tree (DECL_SAVED_TREE (actor));
}

/* The prototype 'destroy' function :
   frame->__resume_at |= 1;
   actor (frame);  */

static void
build_destroy_fn (location_t loc, tree coro_frame_type, tree destroy,
                  tree actor)
{
  /* One param, the coro frame pointer.  */
  tree destr_fp = DECL_ARGUMENTS (destroy);

  /* A void return.  */
  tree resdecl = build_decl (loc, RESULT_DECL, 0, void_type_node);
  DECL_ARTIFICIAL (resdecl) = 1;
  DECL_IGNORED_P (resdecl) = 1;
  DECL_RESULT (destroy) = resdecl;

  /* We have a definition here.  */
  TREE_STATIC (destroy) = 1;
  DECL_COROUTINE_P (destroy) = 1;

  tree destr_outer = push_stmt_list ();
  current_stmt_tree ()->stmts_are_full_exprs_p = 1;
  tree dstr_stmt = begin_compound_stmt (BCS_FN_BODY);

  tree destr_frame = build1 (INDIRECT_REF, coro_frame_type, destr_fp);

  tree resume_idx_name = get_identifier ("__resume_at");
  tree rat_field = lookup_member (coro_frame_type, resume_idx_name, 1, 0,
                                  tf_warning_or_error);
  tree rat = build3 (COMPONENT_REF, short_unsigned_type_node, destr_frame,
                     rat_field, NULL_TREE);

  /* _resume_at |= 1 */
  tree dstr_idx = build2 (BIT_IOR_EXPR, short_unsigned_type_node, rat,
                          build_int_cst (short_unsigned_type_node, 1));
  tree r = build2 (MODIFY_EXPR, short_unsigned_type_node, rat, dstr_idx);
  r = coro_build_cvt_void_expr_stmt (r, loc);
  add_stmt (r);

  /* So .. call the actor ..  */
  r = build_call_expr_loc (loc, actor, 1, destr_fp);
  r = coro_build_cvt_void_expr_stmt (r, loc);
  add_stmt (r);

  /* done. */
  r = build_stmt (loc, RETURN_EXPR, NULL);
  r = maybe_cleanup_point_expr_void (r);
  add_stmt (r);

  finish_compound_stmt (dstr_stmt);
  DECL_SAVED_TREE (destroy) = pop_stmt_list (destr_outer);
}

/* Helper that returns an identifier for an appended extension to the
   current un-mangled function name.  */

static tree
get_fn_local_identifier (tree orig, const char *append)
{
  /* Figure out the bits we need to generate names for the outlined things
     For consistency, this needs to behave the same way as
     ASM_FORMAT_PRIVATE_NAME does. */
  tree nm = DECL_NAME (orig);
  const char *sep, *pfx = "";
#ifndef NO_DOT_IN_LABEL
  sep = ".";
#else
#ifndef NO_DOLLAR_IN_LABEL
  sep = "$";
#else
  sep = "_";
  pfx = "__";
#endif
#endif

  char *an;
  if (DECL_ASSEMBLER_NAME (orig))
    an = ACONCAT ((IDENTIFIER_POINTER (DECL_ASSEMBLER_NAME (orig)), sep, append,
                   (char *) 0));
  else if (DECL_USE_TEMPLATE (orig) && DECL_TEMPLATE_INFO (orig)
           && DECL_TI_ARGS (orig))
    {
      tree tpl_args = DECL_TI_ARGS (orig);
      an = ACONCAT ((pfx, IDENTIFIER_POINTER (nm), (char *) 0));
      for (int i = 0; i < TREE_VEC_LENGTH (tpl_args); ++i)
        {
          tree typ = DECL_NAME (TYPE_NAME (TREE_VEC_ELT (tpl_args, i)));
          an = ACONCAT ((an, sep, IDENTIFIER_POINTER (typ), (char *) 0));
        }
      an = ACONCAT ((an, sep, append, (char *) 0));
    }
  else
    an = ACONCAT ((pfx, IDENTIFIER_POINTER (nm), sep, append, (char *) 0));

  return get_identifier (an);
}

static tree
build_init_or_final_await (location_t loc, bool is_final)
{
  tree suspend_alt = is_final ? coro_final_suspend_identifier
                              : coro_initial_suspend_identifier;
  tree setup_meth = lookup_promise_method (current_function_decl, suspend_alt,
                                           loc, /*musthave=*/true);
  if (!setup_meth || setup_meth == error_mark_node)
    return error_mark_node;

  tree s_fn = NULL_TREE;
  tree setup_call = build_new_method_call (
    get_coroutine_promise_proxy (current_function_decl), setup_meth, NULL,
    NULL_TREE, LOOKUP_NORMAL, &s_fn, tf_warning_or_error);

  if (!s_fn || setup_call == error_mark_node)
    return error_mark_node;

  /* So build the co_await for this */
  /* For initial/final suspends the call is "a" per [expr.await] 3.2.  */
  return build_co_await (loc, setup_call, (is_final ? FINAL_SUSPEND_POINT
                                                    : INITIAL_SUSPEND_POINT));
}

/* Callback to record the essential data for each await point found in the
   function.  */

static bool
register_await_info (tree await_expr, tree aw_type, tree aw_nam)
{
  bool seen;
  suspend_point_info &s
    = suspend_points->get_or_insert (await_expr, &seen);
  if (seen)
    {
      error_at (EXPR_LOCATION (await_expr), "duplicate info for %qE",
                await_expr);
      return false;
    }
  s.awaitable_type = aw_type;
  s.await_field_id = aw_nam;
  return true;
}

/* Small helper for the repetitive task of adding a new field to the coro
   frame type.  */

static tree
coro_make_frame_entry (tree *field_list, const char *name, tree fld_type,
                       location_t loc)
{
  tree id = get_identifier (name);
  tree decl = build_decl (loc, FIELD_DECL, id, fld_type);
  DECL_CHAIN (decl) = *field_list;
  *field_list = decl;
  return id;
}

/* This data set is used when analyzing statements for await expressions.  */
struct susp_frame_data
{
  /* Function-wide.  */
  tree *field_list; /* The current coroutine frame field list.  */
  tree handle_type; /* The self-handle type for this coroutine.  */
  vec<tree, va_gc> *block_stack; /* Track block scopes.  */
  vec<tree, va_gc> *bind_stack;  /* Track current bind expr.  */
  unsigned await_number;         /* Which await in the function.  */
  unsigned cond_number;          /* Which replaced condition in the fn.  */
  /* Temporary values for one statement or expression being analyzed.  */
  hash_set<tree> captured_temps; /* The suspend captured these temps.  */
  vec<tree, va_gc> *to_replace;  /* The VAR decls to replace.  */
  hash_set<tree> *truth_aoif_to_expand; /* The set of TRUTH exprs to expand.  */
  unsigned saw_awaits;           /* Count of awaits in this statement  */
  bool captures_temporary;       /* This expr captures temps by ref.  */
  bool needs_truth_if_exp;       /* We must expand a truth_if expression.  */
};

/* Walk the sub-tree looking for call expressions that both capture
   references and have compiler-temporaries as parms.  */

static tree
captures_temporary (tree *stmt, int *do_subtree, void *d)
{
  /* We should have already lowered co_yields to their co_await.  */
  gcc_checking_assert (TREE_CODE (*stmt) != CO_YIELD_EXPR);

  /* Stop recursing if we see an await expression, the subtrees
     of that will be handled when it is processed.  */
  if (TREE_CODE (*stmt) == CO_AWAIT_EXPR)
    {
      *do_subtree = 0;
      return NULL_TREE;
    }

  /* We're only interested in calls.  */
  if (TREE_CODE (*stmt) != CALL_EXPR)
    return NULL_TREE;

  /* Does this call capture references?
     Strip the ADDRESS_EXPR to get the fn decl and inspect it.  */
  tree fn = TREE_OPERAND (CALL_EXPR_FN (*stmt), 0);
  bool is_meth = TREE_CODE (TREE_TYPE (fn)) == METHOD_TYPE;
  tree arg = TYPE_ARG_TYPES (TREE_TYPE (fn));
  unsigned offset = 3;
  for (unsigned anum = 0; arg != NULL; arg = TREE_CHAIN (arg), anum++)
    {
      tree parm_type = TREE_VALUE (arg);
      if (anum == 0 && is_meth && INDIRECT_TYPE_P (parm_type))
        {
          /* Account for 'this' when the fn is a method.  Unless it
             belongs to a CTOR or DTOR.  */
          if (DECL_CONSTRUCTOR_P (fn) || DECL_DESTRUCTOR_P (fn))
            continue;
        }
      else if (!TYPE_REF_P (parm_type))
        /* If it's not a reference, we don't care.  */
        continue;

      /* Fetch the value presented to the fn.  */
      tree parm = TREE_OPERAND (*stmt, anum + offset);

      while (TREE_CODE (parm) == NOP_EXPR)
        parm = TREE_OPERAND (parm, 0);

      /* We only care if we're taking the addr of a temporary.  */
      if (TREE_CODE (parm) != ADDR_EXPR)
        continue;

      parm = TREE_OPERAND (parm, 0);

      /* In case of component_ref, we need to capture the object of base
         class as if it is temporary object.  There are two possibilities:
         (*base).field and base->field.  */
      while (TREE_CODE (parm) == COMPONENT_REF)
        {
          parm = TREE_OPERAND (parm, 0);
          if (TREE_CODE (parm) == INDIRECT_REF)
            parm = TREE_OPERAND (parm, 0);
          STRIP_NOPS (parm);
        }

      /* This isn't a temporary.  */
      if ((VAR_P (parm)
           && (!DECL_ARTIFICIAL (parm) || DECL_HAS_VALUE_EXPR_P (parm)))
          || TREE_CODE (parm) == PARM_DECL
          || TREE_CODE (parm) == NON_LVALUE_EXPR)
        continue;

      if (TREE_CODE (parm) == TARGET_EXPR)
        {
          /* We're taking the address of a temporary and using it as a ref.  */
          tree tvar = TREE_OPERAND (parm, 0);
          gcc_checking_assert (DECL_ARTIFICIAL (tvar));

          susp_frame_data *data = (susp_frame_data *) d;
          data->captures_temporary = true;
          /* Record this one so we don't duplicate, and on the first
             occurrence note the target expr to be replaced.  */
          if (!data->captured_temps.add (tvar))
            vec_safe_push (data->to_replace, parm);
          /* Now see if the initializer contains any more cases.  */
          hash_set<tree> visited;
          tree res = cp_walk_tree (&TREE_OPERAND (parm, 1),
                                   captures_temporary, d, &visited);
          if (res)
            return res;
          /* Otherwise, we're done with sub-trees for this.  */
        }
      else if (TREE_CODE (parm) == CO_AWAIT_EXPR)
        {
          /* CO_AWAIT expressions behave in a similar manner to target
             expressions when the await_resume call is contained in one.  */
          tree awr = TREE_OPERAND (parm, 3); /* call vector.  */
          awr = TREE_VEC_ELT (awr, 2); /* resume call.  */
          if (TREE_CODE (awr) == TARGET_EXPR)
            {
              tree tvar = TREE_OPERAND (awr, 0);
              gcc_checking_assert (DECL_ARTIFICIAL (tvar));

              susp_frame_data *data = (susp_frame_data *) d;
              data->captures_temporary = true;
              /* Use this as a place-holder.  */
              if (!data->captured_temps.add (tvar))
                vec_safe_push (data->to_replace, parm);
            }
        /* We will walk the sub-trees of this co_await separately.  */
        }
      else
        gcc_unreachable ();
    }
  /* As far as it's necessary, we've walked the subtrees of the call
     expr.  */
  *do_subtree = 0;
  return NULL_TREE;
}

/* If this is an await, then register it and decide on what coro
   frame storage is needed.
   If this is a co_yield (which embeds an await), drop the yield
   and record the await (the yield was kept for diagnostics only).  */

static tree
register_awaits (tree *stmt, int *do_subtree ATTRIBUTE_UNUSED, void *d)
{
  susp_frame_data *data = (susp_frame_data *) d;

  /* We should have already lowered co_yields to their co_await.  */
  gcc_checking_assert (TREE_CODE (*stmt) != CO_YIELD_EXPR);

  if (TREE_CODE (*stmt) != CO_AWAIT_EXPR)
    return NULL_TREE;

  tree aw_expr = *stmt;
  location_t aw_loc = EXPR_LOCATION (aw_expr); /* location of the co_xxxx.  */

  /* If the awaitable is a parm or a local variable, then we already have
     a frame copy, so don't make a new one.  */
  tree aw = TREE_OPERAND (aw_expr, 1);
  tree aw_field_type = TREE_TYPE (aw);
  tree aw_field_nam = NULL_TREE;
  if (INDIRECT_REF_P (aw))
    aw = TREE_OPERAND (aw, 0);
  if (TREE_CODE (aw) == PARM_DECL
      || (VAR_P (aw) && (!DECL_ARTIFICIAL (aw)
                         || DECL_HAS_VALUE_EXPR_P (aw))))
    ; /* Don't make an additional copy.  */
  else
    {
      /* The required field has the same type as the proxy stored in the
         await expr.  */
      char *nam = xasprintf ("__aw_s.%d", data->await_number);
      aw_field_nam = coro_make_frame_entry (data->field_list, nam,
                                            aw_field_type, aw_loc);
      free (nam);
    }

  tree o = TREE_OPERAND (aw_expr, 2); /* Initialiser for the frame var.  */
  /* If this is a target expression, then we need to remake it to strip off
     any extra cleanups added.  */
  if (TREE_CODE (o) == TARGET_EXPR)
    TREE_OPERAND (aw_expr, 2) = get_target_expr (TREE_OPERAND (o, 1));

  tree v = TREE_OPERAND (aw_expr, 3);
  o = TREE_VEC_ELT (v, 1);
  if (TREE_CODE (o) == TARGET_EXPR)
    TREE_VEC_ELT (v, 1) = get_target_expr (TREE_OPERAND (o, 1));

  register_await_info (aw_expr, aw_field_type, aw_field_nam);

  /* Count how many awaits the current expression contains.  */
  data->saw_awaits++;
  /* Each await suspend context is unique, this is a function-wide value.  */
  data->await_number++;

  /* We now need to know if to take special action on lifetime extension
     of temporaries captured by reference.  This can only happen if such
     a case appears in the initializer for the awaitable.  The callback
     records captured temporaries including subtrees of initializers.  */
  hash_set<tree> visited;
  tree res = cp_walk_tree (&TREE_OPERAND (aw_expr, 2), captures_temporary, d,
                           &visited);
  return res;
}

/* The gimplifier correctly extends the lifetime of temporaries captured
   by reference (per. [class.temporary] (6.9) "A temporary object bound
   to a reference parameter in a function call persists until the completion
   of the full-expression containing the call").  However, that is not
   sufficient to work across a suspension - and we need to promote such
   temporaries to be regular vars that will then get a coro frame slot.
   We don't want to incur the effort of checking for this unless we have
   an await expression in the current full expression.  */

/* This takes the statement which contains one or more temporaries that have
   been 'captured' by reference in the initializer(s) of co_await(s).
   The statement is replaced by a bind expression that has actual variables
   to replace the temporaries.  These variables will be added to the coro-
   frame in the same manner as user-authored ones.  */

static void
replace_statement_captures (tree *stmt, void *d)
{
  susp_frame_data *awpts = (susp_frame_data *) d;
  location_t sloc = EXPR_LOCATION (*stmt);
  tree aw_bind
    = build3_loc (sloc, BIND_EXPR, void_type_node, NULL, NULL, NULL);

  /* Any cleanup point expression might no longer be necessary, since we
     are removing one or more temporaries.  */
  tree aw_statement_current = *stmt;
  if (TREE_CODE (aw_statement_current) == CLEANUP_POINT_EXPR)
    aw_statement_current = TREE_OPERAND (aw_statement_current, 0);

  /* Collected the scope vars we need move the temps to regular. */
  tree aw_bind_body = push_stmt_list ();
  tree varlist = NULL_TREE;
  int vnum = -1;
  while (!awpts->to_replace->is_empty ())
    {
      tree to_replace = awpts->to_replace->pop ();
      tree orig_temp;
      if (TREE_CODE (to_replace) == CO_AWAIT_EXPR)
        {
          orig_temp = TREE_OPERAND (to_replace, 3);
          orig_temp = TREE_VEC_ELT (orig_temp, 2);
          orig_temp = TREE_OPERAND (orig_temp, 0);
        }
      else
        orig_temp = TREE_OPERAND (to_replace, 0);

      tree var_type = TREE_TYPE (orig_temp);
      gcc_checking_assert (same_type_p (TREE_TYPE (to_replace), var_type));
      /* Build a variable to hold the captured value, this will be included
         in the frame along with any user-authored locals.  */
      char *nam = xasprintf ("aw_%d.tmp.%d", awpts->await_number, ++vnum);
      tree newvar = build_lang_decl (VAR_DECL, get_identifier (nam), var_type);
      free (nam);
      /* If we have better location than the whole expression use that, else
         fall back to the expression loc.  */
      DECL_CONTEXT (newvar) = DECL_CONTEXT (orig_temp);
      if (DECL_SOURCE_LOCATION (orig_temp))
        sloc = DECL_SOURCE_LOCATION (orig_temp);
     else
        sloc = EXPR_LOCATION (*stmt);
      DECL_SOURCE_LOCATION (newvar) = sloc;
      DECL_CHAIN (newvar) = varlist;
      varlist = newvar; /* Chain it onto the list for the bind expr.  */
      /* Declare and initialize it in the new bind scope.  */
      add_decl_expr (newvar);
      tree new_s = build2_loc (sloc, INIT_EXPR, var_type, newvar, to_replace);
      new_s = coro_build_cvt_void_expr_stmt (new_s, sloc);
      add_stmt (new_s);

     /* Replace all instances of that temp in the original expr.  */
      proxy_replace pr = {to_replace, newvar};
       cp_walk_tree (&aw_statement_current, replace_proxy, &pr, NULL);
    }

  /* What's left should be the original statement with any co_await captured
     temporaries broken out.  Other temporaries might remain so see if we
     need to wrap the revised statement in a cleanup.  */
  aw_statement_current = maybe_cleanup_point_expr_void (aw_statement_current);
  add_stmt (aw_statement_current);

  BIND_EXPR_BODY (aw_bind) = pop_stmt_list (aw_bind_body);
  awpts->captured_temps.empty ();

  BIND_EXPR_VARS (aw_bind) = nreverse (varlist);
  tree b_block = make_node (BLOCK);
  if (!awpts->block_stack->is_empty ())
    {
      tree s_block = awpts->block_stack->last ();
      if (s_block)
        {
        BLOCK_SUPERCONTEXT (b_block) = s_block;
        BLOCK_CHAIN (b_block) = BLOCK_SUBBLOCKS (s_block);
        BLOCK_SUBBLOCKS (s_block) = b_block;
        }
    }
  BIND_EXPR_BLOCK (aw_bind) = b_block;
  TREE_SIDE_EFFECTS (aw_bind) = TREE_SIDE_EFFECTS (BIND_EXPR_BODY (aw_bind));
  *stmt = aw_bind;
}

/* This is called for single statements from the co-await statement walker.
   It checks to see if the statement contains any co-awaits and, if so,
   whether any of these 'capture' a temporary by reference.  */

static tree
maybe_promote_captured_temps (tree *stmt, void *d)
{
  susp_frame_data *awpts = (susp_frame_data *) d;
  hash_set<tree> visited;
  awpts->saw_awaits = 0;

  /* When register_awaits sees an await, it walks the initializer for
     that await looking for temporaries captured by reference and notes
     them in awpts->captured_temps.  */

  if (tree res = cp_walk_tree (stmt, register_awaits, d, &visited))
    return res; /* We saw some reason to abort the tree walk.  */

  /* We only need to take any action here if the statement contained any
     awaits and any of those had temporaries captured by reference in their
     initializers. */

  if (awpts->saw_awaits > 0 && !awpts->captured_temps.is_empty ())
    replace_statement_captures (stmt, d);

  return NULL_TREE;
}

/* Lightweight callback to determine two key factors:
   1) If the statement/expression contains any await expressions.
   2) If the statement/expression potentially requires a re-write to handle
      TRUTH_{AND,OR}IF_EXPRs since, in most cases, they will need expansion
      so that the await expressions are not processed in the case of the
      short-circuit arm.
   CO_YIELD expressions are re-written to their underlying co_await.  */

static tree
analyze_expression_awaits (tree *stmt, int *do_subtree, void *d)
{
  susp_frame_data *awpts = (susp_frame_data *) d;

  switch (TREE_CODE (*stmt))
    {
      default: return NULL_TREE;
      case CO_YIELD_EXPR:
        /* co_yield is syntactic sugar, re-write it to co_await.  */
        *stmt = TREE_OPERAND (*stmt, 1);
        /* FALLTHROUGH */
      case CO_AWAIT_EXPR:
        awpts->saw_awaits++;
        break;
      case TRUTH_ANDIF_EXPR:
      case TRUTH_ORIF_EXPR:
        {
          /* We don't need special action for awaits in the always-executed
             arm of a TRUTH_IF.  */
          if (tree res = cp_walk_tree (&TREE_OPERAND (*stmt, 0),
                                       analyze_expression_awaits, d, NULL))
            return res;
          /* However, if there are await expressions on the conditionally
             executed branch, we must expand the TRUTH_IF to ensure that the
             expanded await expression control-flow is fully contained in the
             conditionally executed code.  */
          unsigned aw_count = awpts->saw_awaits;
          if (tree res = cp_walk_tree (&TREE_OPERAND (*stmt, 1),
                                       analyze_expression_awaits, d, NULL))
            return res;
          if (awpts->saw_awaits > aw_count)
            {
              awpts->truth_aoif_to_expand->add (*stmt);
              awpts->needs_truth_if_exp = true;
            }
          /* We've done the sub-trees here.  */
          *do_subtree = 0;
        }
        break;
    }

  return NULL_TREE; /* Recurse until done.  */
}

/* Given *EXPR
   If EXPR contains a TRUTH_{AND,OR}IF_EXPR, TAOIE with an await expr on
   the conditional branch expand this to:

   bool not_expr = TAOIE == TRUTH_ORIF_EXPR ? NOT : NOP;
   A) bool t = always exec expr
      if (not_expr (t))
   B)   t = conditionally exec expr
   c) EXPR' = EXPR with  TAOIE replaced by t.

   Then repeat this for A, B and C.  */

struct truth_if_transform {
  tree *orig_stmt;
  tree scratch_var;
  hash_set<tree> *truth_aoif_to_expand;
};

static tree
expand_one_truth_if (tree *expr, int *do_subtree, void *d)
{
  truth_if_transform *xform = (truth_if_transform *) d;

  bool needs_not = false;
  switch (TREE_CODE (*expr))
    {
      default: break;
      case TRUTH_ORIF_EXPR:
        needs_not = true;
        /* FALLTHROUGH */
      case TRUTH_ANDIF_EXPR:
        {
          if (!xform->truth_aoif_to_expand->contains (*expr))
            break;

          location_t sloc = EXPR_LOCATION (*expr);
          tree type = TREE_TYPE (xform->scratch_var);
          gcc_checking_assert (TREE_CODE (type) == BOOLEAN_TYPE);
          tree new_list = push_stmt_list ();
          /* Init our scratch with the unconditionally-evaluated expr.  */
          tree new_s = build2_loc (sloc, INIT_EXPR, boolean_type_node,
                                   xform->scratch_var,
                                   TREE_OPERAND (*expr, 0));
          finish_expr_stmt (new_s);
          tree *pre = tsi_stmt_ptr (tsi_last (new_list));
          tree if_cond = xform->scratch_var;
          if (needs_not)
            if_cond = build1 (TRUTH_NOT_EXPR, boolean_type_node, if_cond);
          tree if_stmt = begin_if_stmt ();
          finish_if_stmt_cond (if_cond, if_stmt);
          /* If we take the if branch, then overwrite scratch with the cond
             executed branch.  */
          new_s = build2 (INIT_EXPR, boolean_type_node,
                          xform->scratch_var, TREE_OPERAND (*expr, 1));
          finish_expr_stmt (new_s);
          finish_then_clause (if_stmt);
          finish_if_stmt (if_stmt);
          *expr = xform->scratch_var; /* now contains the result.  */
          /* So now we've got a statement list expanding one TAOIe.  */
          add_stmt (*xform->orig_stmt);
          tree *post = tsi_stmt_ptr (tsi_last (new_list));
          *xform->orig_stmt = pop_stmt_list (new_list);
          /* Now recurse into the pre, if and post parts.  */
          truth_if_transform sub_data = {pre, xform->scratch_var,
                                         xform->truth_aoif_to_expand};
          if (tree res = cp_walk_tree (pre, expand_one_truth_if, &sub_data,
                                       NULL))
            return res;
          sub_data.orig_stmt = &THEN_CLAUSE (if_stmt);
          if (tree res = cp_walk_tree (&THEN_CLAUSE (if_stmt),
                                       expand_one_truth_if, &sub_data, NULL))
            return res;
          sub_data.orig_stmt = post;
          if (tree res = cp_walk_tree (post, expand_one_truth_if, &sub_data,
                                       NULL))
            return res;
          /* We've done the sub-trees here.  */
          *do_subtree = 0;
        }
        break;
    }
  return NULL_TREE;
}

/* Helper that adds a new variable of VAR_TYPE to a bind scope BIND, the
   name is made up from NAM_ROOT, NAM_VERS.  */

static tree
add_var_to_bind (tree& bind, tree var_type,
                 const char *nam_root, unsigned nam_vers)
{

  tree b_vars = BIND_EXPR_VARS (bind);
  /* Build a variable to hold the condition, this will be included in the
     frame as a local var.  */
  char *nam = xasprintf ("%s.%d", nam_root, nam_vers);
  tree newvar = build_lang_decl (VAR_DECL, get_identifier (nam), var_type);
  free (nam);
  DECL_CHAIN (newvar) = b_vars;
  BIND_EXPR_VARS (bind) = newvar;
  return newvar;
}

/* Helper to build and add if (!cond) break;  */

static void
coro_build_add_if_not_cond_break (tree cond)
{
  tree if_stmt = begin_if_stmt ();
  tree invert = build1 (TRUTH_NOT_EXPR, boolean_type_node, cond);
  finish_if_stmt_cond (invert, if_stmt);
  finish_break_stmt ();
  finish_then_clause (if_stmt);
  finish_if_stmt (if_stmt);
}

/* Tree walk callback to analyze, register and pre-process statements that
   contain await expressions.  */

static tree
await_statement_walker (tree *stmt, int *do_subtree, void *d)
{
  tree res = NULL_TREE;
  susp_frame_data *awpts = (susp_frame_data *) d;

  /* Process a statement at a time.  */
  if (TREE_CODE (*stmt) == BIND_EXPR)
    {
      /* For conditional expressions, we might wish to add an artificial var
         to their containing bind expr.  */
      vec_safe_push (awpts->bind_stack, *stmt);
      /* We might need to insert a new bind expression, and want to link it
         into the correct scope, so keep a note of the current block scope.  */
      tree blk = BIND_EXPR_BLOCK (*stmt);
      vec_safe_push (awpts->block_stack, blk);
      res = cp_walk_tree (&BIND_EXPR_BODY (*stmt), await_statement_walker,
                          d, NULL);
      awpts->block_stack->pop ();
      awpts->bind_stack->pop ();
      *do_subtree = 0; /* Done subtrees.  */
      return res;
    }
  else if (TREE_CODE (*stmt) == STATEMENT_LIST)
    {
      tree_stmt_iterator i;
      for (i = tsi_start (*stmt); !tsi_end_p (i); tsi_next (&i))
        {
          res = cp_walk_tree (tsi_stmt_ptr (i), await_statement_walker,
                              d, NULL);
          if (res)
            return res;
        }
      *do_subtree = 0; /* Done subtrees.  */
      return NULL_TREE;
    }

  /* We have something to be handled as a single statement.  */
  hash_set<tree> visited;
  awpts->saw_awaits = 0;
  hash_set<tree> truth_aoif_to_expand;
  awpts->truth_aoif_to_expand = &truth_aoif_to_expand;
  awpts->needs_truth_if_exp = false;

  if (STATEMENT_CLASS_P (*stmt))
    switch (TREE_CODE (*stmt))
      {
        /* Unless it's a special case, just walk the subtrees as usual.  */
        default: return NULL_TREE;

        /* When we have a conditional expression, which contains one or more
           await expressions, we have to break the condition out into a
           regular statement so that the control flow introduced by the await
           transforms can be implemented.  */
        case IF_STMT:
          {
            /* Transform 'if (cond with awaits) then stmt1 else stmt2' into
               bool cond = cond with awaits.
               if (cond) then stmt1 else stmt2.  */
            tree if_stmt = *stmt;
            /* We treat the condition as if it was a stand-alone statement,
               to see if there are any await expressions which will be analysed
               and registered.  */
            if ((res = cp_walk_tree (&IF_COND (if_stmt),
                analyze_expression_awaits, d, &visited)))
              return res;
            if (!awpts->saw_awaits)
              return NULL_TREE; /* Nothing special to do here.  */

            gcc_checking_assert (!awpts->bind_stack->is_empty());
            tree& bind_expr = awpts->bind_stack->last ();
            tree newvar = add_var_to_bind (bind_expr, boolean_type_node,
                                           "ifcd", awpts->cond_number++);
            tree insert_list = push_stmt_list ();
            tree cond_inner = IF_COND (if_stmt);
            if (TREE_CODE (cond_inner) == CLEANUP_POINT_EXPR)
              cond_inner = TREE_OPERAND (cond_inner, 0);
            add_decl_expr (newvar);
            location_t sloc = EXPR_LOCATION (IF_COND (if_stmt));
            /* We want to initialize the new variable with the expression
               that contains the await(s) and potentially also needs to
               have truth_if expressions expanded.  */
            tree new_s = build2_loc (sloc, MODIFY_EXPR, boolean_type_node,
                                     newvar, cond_inner);
            finish_expr_stmt (new_s);
            if (awpts->needs_truth_if_exp)
              {
                tree *sp = tsi_stmt_ptr (tsi_last (insert_list));
                truth_if_transform xf = {sp, newvar, &truth_aoif_to_expand};
                if ((res = cp_walk_tree (sp, expand_one_truth_if, &xf, NULL)))
                  return res;
              }
            IF_COND (if_stmt) = newvar;
            add_stmt (if_stmt);
            *stmt = pop_stmt_list (insert_list);
            /* So now walk the new statement list.  */
            res = cp_walk_tree (stmt, await_statement_walker, d, NULL);
            *do_subtree = 0; /* Done subtrees.  */
            return res;
          }
          break;
        case WHILE_STMT:
          {
            /* We turn 'while (cond with awaits) stmt' into
               while (true) {
                  if (!(cond with awaits))
                    break;
                  stmt..
                } */
            tree while_stmt = *stmt;
            if ((res = cp_walk_tree (&WHILE_COND (while_stmt),
                analyze_expression_awaits, d, &visited)))
              return res;
            if (!awpts->saw_awaits)
              return NULL_TREE; /* Nothing special to do here.  */

            tree insert_list = push_stmt_list ();
            coro_build_add_if_not_cond_break (WHILE_COND (while_stmt));
            /* The original while body.  */
            add_stmt (WHILE_BODY (while_stmt));
            /* The new while body.  */
            WHILE_BODY (while_stmt) = pop_stmt_list (insert_list);
            WHILE_COND (while_stmt) = boolean_true_node;
            /* So now walk the new statement list.  */
            res = cp_walk_tree (&WHILE_BODY (while_stmt),
                                await_statement_walker, d, NULL);
            *do_subtree = 0; /* Done subtrees.  */
            return res;
          }
          break;
        case DO_STMT:
          {
            /* We turn do stmt while (cond with awaits) into:
               do {
                  stmt..
                  if (!(cond with awaits))
                    break;
               } while (true); */
            tree do_stmt = *stmt;
            if ((res = cp_walk_tree (&DO_COND (do_stmt),
                analyze_expression_awaits, d, &visited)))
              return res;
            if (!awpts->saw_awaits)
              return NULL_TREE; /* Nothing special to do here.  */

            tree insert_list = push_stmt_list ();
            /* The original do stmt body.  */
            add_stmt (DO_BODY (do_stmt));
            coro_build_add_if_not_cond_break (DO_COND (do_stmt));
            /* The new while body.  */
            DO_BODY (do_stmt) = pop_stmt_list (insert_list);
            DO_COND (do_stmt) = boolean_true_node;
            /* So now walk the new statement list.  */
            res = cp_walk_tree (&DO_BODY (do_stmt), await_statement_walker,
                                d, NULL);
            *do_subtree = 0; /* Done subtrees.  */
            return res;

          }
          break;
        case SWITCH_STMT:
          {
            /* We turn 'switch (cond with awaits) stmt' into
               switch_type cond = cond with awaits
               switch (cond) stmt.  */
            tree sw_stmt = *stmt;
            if ((res = cp_walk_tree (&SWITCH_STMT_COND (sw_stmt),
                analyze_expression_awaits, d, &visited)))
              return res;
            if (!awpts->saw_awaits)
              return NULL_TREE; /* Nothing special to do here.  */

            gcc_checking_assert (!awpts->bind_stack->is_empty());
            /* Build a variable to hold the condition, this will be
                   included in the frame as a local var.  */
            tree& bind_expr = awpts->bind_stack->last ();
            tree sw_type = SWITCH_STMT_TYPE (sw_stmt);
            tree newvar = add_var_to_bind (bind_expr, sw_type, "swch",
                                           awpts->cond_number++);
            tree insert_list = push_stmt_list ();
            add_decl_expr (newvar);

            tree cond_inner = SWITCH_STMT_COND (sw_stmt);
            if (TREE_CODE (cond_inner) == CLEANUP_POINT_EXPR)
              cond_inner = TREE_OPERAND (cond_inner, 0);
            location_t sloc = EXPR_LOCATION (SWITCH_STMT_COND (sw_stmt));
            tree new_s = build2_loc (sloc, INIT_EXPR, sw_type, newvar,
                                     cond_inner);
            finish_expr_stmt (new_s);
            SWITCH_STMT_COND (sw_stmt) = newvar;
            /* Now add the switch statement with the condition re-
                   written to use the local var.  */
            add_stmt (sw_stmt);
            *stmt = pop_stmt_list (insert_list);
            /* Process the expanded list.  */
            res = cp_walk_tree (stmt, await_statement_walker,
                                d, NULL);
            *do_subtree = 0; /* Done subtrees.  */
            return res;
          }
          break;
      }
  else if (EXPR_P (*stmt))
    {
      if ((res = cp_walk_tree (stmt, analyze_expression_awaits, d, &visited)))
        return res;
      *do_subtree = 0; /* Done subtrees.  */
      if (!awpts->saw_awaits)
        return NULL_TREE; /* Nothing special to do here.  */

      /* Unless we need to expand any truth-and/or-if expressions, then the
         remaining action is to check for temporaries to await expressions
         captured by refence.  */
      if (!awpts->needs_truth_if_exp)
        return maybe_promote_captured_temps (stmt, d);

      gcc_checking_assert (!awpts->bind_stack->is_empty());
      tree& bind_expr = awpts->bind_stack->last ();
      /* Build a variable to hold the condition, this will be
      included in the frame as a local var.  */
      tree newvar = add_var_to_bind (bind_expr, boolean_type_node,
                                     "taoi", awpts->cond_number++);
      tree insert_list = push_stmt_list ();
      add_decl_expr (newvar);
      add_stmt (*stmt);
      tree *sp = tsi_stmt_ptr (tsi_last (insert_list));
      *stmt = pop_stmt_list (insert_list);

      truth_if_transform xf = {sp, newvar, &truth_aoif_to_expand};
      if ((res = cp_walk_tree (sp, expand_one_truth_if, &xf, NULL)))
        return res;
      /* Process the expanded trees.  */
      return cp_walk_tree (stmt, await_statement_walker, d, NULL);
    }
 
  /* Continue recursion, if needed.  */
  return res;
}

/* For figuring out what param usage we have.  */

struct param_frame_data
{
  tree *field_list;
  hash_map<tree, param_info> *param_uses;
  hash_set<tree *> *visited;
  location_t loc;
  bool param_seen;
};

static tree
register_param_uses (tree *stmt, int *do_subtree ATTRIBUTE_UNUSED, void *d)
{
  param_frame_data *data = (param_frame_data *) d;

  /* For lambda closure content, we have to look specifically.  */
  if (TREE_CODE (*stmt) == VAR_DECL && DECL_HAS_VALUE_EXPR_P (*stmt))
    {
      tree t = DECL_VALUE_EXPR (*stmt);
      return cp_walk_tree (&t, register_param_uses, d, NULL);
    }

  if (TREE_CODE (*stmt) != PARM_DECL)
    return NULL_TREE;

  /* If we already saw the containing expression, then we're done.  */
  if (data->visited->add (stmt))
    return NULL_TREE;

  bool existed;
  param_info &parm = data->param_uses->get_or_insert (*stmt, &existed);
  gcc_checking_assert (existed);

  if (!parm.body_uses)
    {
      vec_alloc (parm.body_uses, 4);
      parm.body_uses->quick_push (stmt);
      data->param_seen = true;
    }
  else
    parm.body_uses->safe_push (stmt);

  return NULL_TREE;
}

/* For figuring out what local variable usage we have.  */

struct local_vars_frame_data
{
  tree *field_list;
  hash_map<tree, local_var_info> *local_var_uses;
  unsigned int nest_depth, bind_indx;
  location_t loc;
  bool saw_capture;
  bool local_var_seen;
};

static tree
register_local_var_uses (tree *stmt, int *do_subtree, void *d)
{
  local_vars_frame_data *lvd = (local_vars_frame_data *) d;

  /* As we enter a bind expression - record the vars there and then recurse.
     As we exit drop the nest depth.
     The bind index is a growing count of how many bind indices we've seen.
     We build a space in the frame for each local var.  */

  if (TREE_CODE (*stmt) == BIND_EXPR)
    {
      lvd->bind_indx++;
      lvd->nest_depth++;
      tree lvar;
      for (lvar = BIND_EXPR_VARS (*stmt); lvar != NULL;
           lvar = DECL_CHAIN (lvar))
        {
          bool existed;
          local_var_info &local_var
            = lvd->local_var_uses->get_or_insert (lvar, &existed);
          gcc_checking_assert (!existed);
          local_var.def_loc = DECL_SOURCE_LOCATION (lvar);
          tree lvtype = TREE_TYPE (lvar);
          local_var.frame_type = lvtype;
          local_var.field_idx = local_var.field_id = NULL_TREE;

          /* Make sure that we only present vars to the tests below.  */
          if (TREE_CODE (lvar) == TYPE_DECL)
            continue;

          /* We don't move static vars into the frame. */
          local_var.is_static = TREE_STATIC (lvar);
          if (local_var.is_static)
            continue;

          lvd->local_var_seen = true;
          /* If this var is a lambda capture proxy, we want to leave it alone,
             and later rewrite the DECL_VALUE_EXPR to indirect through the
             frame copy of the pointer to the lambda closure object.  */
          local_var.is_lambda_capture = is_capture_proxy (lvar);
          if (local_var.is_lambda_capture)
            continue;

          /* If a variable has a value expression, then that's what needs
             to be processed.  */
          local_var.has_value_expr_p = DECL_HAS_VALUE_EXPR_P (lvar);
          if (local_var.has_value_expr_p)
            continue;

          /* Make names depth+index unique, so that we can support nested
             scopes with identically named locals.  */
          tree lvname = DECL_NAME (lvar);
          char *buf;
          if (lvname != NULL_TREE)
            buf = xasprintf ("__lv.%u.%u.%s", lvd->bind_indx, lvd->nest_depth,
                             IDENTIFIER_POINTER (lvname));
          else
            buf = xasprintf ("__lv.%u.%u.D%u", lvd->bind_indx, lvd->nest_depth,
                             DECL_UID (lvar));
          /* TODO: Figure out if we should build a local type that has any
             excess alignment or size from the original decl.  */
          local_var.field_id
            = coro_make_frame_entry (lvd->field_list, buf, lvtype, lvd->loc);
          free (buf);
          /* We don't walk any of the local var sub-trees, they won't contain
             any bind exprs.  */
        }
      cp_walk_tree (&BIND_EXPR_BODY (*stmt), register_local_var_uses, d, NULL);
      *do_subtree = 0; /* We've done this.  */
      lvd->nest_depth--;
    }
  return NULL_TREE;
}

/* Build, return FUNCTION_DECL node with its coroutine frame pointer argument
   for either actor or destroy functions.  */

static tree
act_des_fn (tree orig, tree fn_type, tree coro_frame_ptr, const char* name)
{
  tree fn_name = get_fn_local_identifier (orig, name);
  tree fn = build_lang_decl (FUNCTION_DECL, fn_name, fn_type);
  DECL_CONTEXT (fn) = DECL_CONTEXT (orig);
  DECL_INITIAL (fn) = error_mark_node;
  tree id = get_identifier ("frame_ptr");
  tree fp = build_lang_decl (PARM_DECL, id, coro_frame_ptr);
  DECL_CONTEXT (fp) = fn;
  DECL_ARG_TYPE (fp) = type_passed_as (coro_frame_ptr);
  DECL_ARGUMENTS (fn) = fp;
  return fn;
}

/* Return a bind expression if we see one, else NULL_TREE.  */
static tree
bind_expr_find_in_subtree (tree *stmt, int *, void *)
{
  if (TREE_CODE (*stmt) == BIND_EXPR)
    return *stmt;
  return NULL_TREE;
}

/* Return the first bind expression that the sub-tree given by STMT
   contains.  */

static tree
coro_body_contains_bind_expr_p (tree *stmt)
{
  hash_set<tree> visited;
  return cp_walk_tree (stmt, bind_expr_find_in_subtree, NULL, &visited);
}

/* Here we:
   a) Check that the function and promise type are valid for a
      coroutine.
   b) Carry out the initial morph to create the skeleton of the
      coroutine ramp function and the rewritten body.

  Assumptions.

  1. We only hit this code once all dependencies are resolved.
  2. The function body will be either a bind expr or a statement list
  3. That cfun and current_function_decl are valid for the case we're
     expanding.
  4. 'input_location' will be of the final brace for the function.

 We do something like this:
 declare a dummy coro frame.
 struct _R_frame {
  using handle_type = coro::coroutine_handle<coro1::promise_type>;
  void (*__resume)(_R_frame *);
  void (*__destroy)(_R_frame *);
  coro1::promise_type __p;
  bool frame_needs_free; free the coro frame mem if set.
  bool i_a_r_c; [dcl.fct.def.coroutine] / 5.3
  short __resume_at;
  handle_type self_handle;
  (maybe) parameter copies.
  coro1::suspend_never_prt __is;
  coro1::suspend_always_prt __fs;
  (maybe) local variables saved
  (maybe) trailing space.
 };  */

bool
morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
{
  gcc_checking_assert (orig && TREE_CODE (orig) == FUNCTION_DECL);

  *resumer = error_mark_node;
  *destroyer = error_mark_node;
  if (!coro_function_valid_p (orig))
    {
      /* For early errors, we do not want a diagnostic about the missing
         ramp return value, since the user cannot fix this - a 'return' is
         not allowed in a coroutine.  */
      TREE_NO_WARNING (orig) = true;
      /* Discard the body, we can't process it further.  */
      pop_stmt_list (DECL_SAVED_TREE (orig));
      DECL_SAVED_TREE (orig) = push_stmt_list ();
      return false;
    }

  /* We can't validly get here with an empty statement list, since there's no
     way for the FE to decide it's a coroutine in the absence of any code.  */
  tree fnbody = pop_stmt_list (DECL_SAVED_TREE (orig));
  gcc_checking_assert (fnbody != NULL_TREE);

  /* We don't have the locus of the opening brace - it's filled in later (and
     there doesn't really seem to be any easy way to get at it).
     The closing brace is assumed to be input_location.  */
  location_t fn_start = DECL_SOURCE_LOCATION (orig);
  gcc_rich_location fn_start_loc (fn_start);

  /* Initial processing of the function-body.
     If we have no expressions or just an error then punt.  */
  tree body_start = expr_first (fnbody);
  if (body_start == NULL_TREE || body_start == error_mark_node)
    {
      DECL_SAVED_TREE (orig) = push_stmt_list ();
      append_to_statement_list (fnbody, &DECL_SAVED_TREE (orig));
      /* Suppress warnings about the missing return value.  */
      TREE_NO_WARNING (orig) = true;
      return false;
    }

  /* So, we've tied off the original body.  Now start the replacement.
     If we encounter a fatal error we might return a now-empty body.
     TODO: determine if it would help to restore the original.
           determine if looking for more errors in coro_function_valid_p()
           and stashing types is a better solution.  */

  tree newbody = push_stmt_list ();
  DECL_SAVED_TREE (orig) = newbody;

  /* If our original body is noexcept, then that's what we apply to our
     generated functions.  Remember that we're NOEXCEPT and fish out the
     contained list (we tied off to the top level already).  */
  bool is_noexcept = TREE_CODE (body_start) == MUST_NOT_THROW_EXPR;
  if (is_noexcept)
    {
      /* Simplified abstract from begin_eh_spec_block, since we already
         know the outcome.  */
      fnbody = TREE_OPERAND (body_start, 0); /* Stash the original...  */
      add_stmt (body_start);                 /* ... and start the new.  */
      TREE_OPERAND (body_start, 0) = push_stmt_list ();
    }

  /* We can be presented with a function that currently has no outer bind
     expression.  We will insert bind scopes in expanding await expressions,
     and therefore need a top level to the tree, so synthesize an outer bind
     expression and scope.  */
  tree check_bind = expr_first (fnbody);
  if (check_bind && TREE_CODE (check_bind) != BIND_EXPR)
    {
      tree update_body = build3 (BIND_EXPR, void_type_node, NULL, NULL, NULL);
      tree blk = make_node (BLOCK);
      gcc_checking_assert (!coro_body_contains_bind_expr_p (&fnbody));
      BIND_EXPR_BLOCK (update_body) = blk;
      if (TREE_CODE (fnbody) == STATEMENT_LIST)
        BIND_EXPR_BODY (update_body) = fnbody;
      else
        {
          tree tlist = NULL_TREE;
          append_to_statement_list_force (fnbody, &tlist);
          TREE_SIDE_EFFECTS (tlist) = TREE_SIDE_EFFECTS (fnbody);
          BIND_EXPR_BODY (update_body) = tlist;
        }
      tree new_body_list = NULL_TREE;
      TREE_SIDE_EFFECTS (update_body) = true;
      append_to_statement_list (update_body, &new_body_list);
      TREE_SIDE_EFFECTS (new_body_list) = true;
      fnbody = new_body_list;
    }

  /* Create the coro frame type, as far as it can be known at this stage.
     1. Types we already know.  */

  tree fn_return_type = TREE_TYPE (TREE_TYPE (orig));
  tree handle_type = get_coroutine_handle_type (orig);
  tree promise_type = get_coroutine_promise_type (orig);

  /* 2. Types we need to define or look up.  */

  /* We need to know, and inspect, each suspend point in the function
     in several places.  It's convenient to place this map out of line
     since it's used from tree walk callbacks.  */
  suspend_points = new hash_map<tree, suspend_point_info>;

  /* Initial and final suspend types are special in that the co_awaits for
     them are synthetic.  We need to find the type for each awaiter from
     the coroutine promise.  */
  tree initial_await = build_init_or_final_await (fn_start, false);
  if (initial_await == error_mark_node)
    {
      /* Suppress warnings about the missing return value.  */
      TREE_NO_WARNING (orig) = true;
      return false;
    }
  /* The type of the frame var for this is the type of its temp proxy.  */
  tree initial_suspend_type = TREE_TYPE (TREE_OPERAND (initial_await, 1));

  tree final_await = build_init_or_final_await (fn_start, true);
  if (final_await == error_mark_node)
    {
      /* Suppress warnings about the missing return value.  */
      TREE_NO_WARNING (orig) = true;
      return false;
    }

  /* The type of the frame var for this is the type of its temp proxy.  */
  tree final_suspend_type = TREE_TYPE (TREE_OPERAND (final_await, 1));

  tree fr_name = get_fn_local_identifier (orig, "frame");
  tree coro_frame_type = xref_tag (record_type, fr_name, ts_current, false);
  DECL_CONTEXT (TYPE_NAME (coro_frame_type)) = current_scope ();
  tree coro_frame_ptr = build_pointer_type (coro_frame_type);
  tree act_des_fn_type
    = build_function_type_list (void_type_node, coro_frame_ptr, NULL_TREE);
  tree act_des_fn_ptr = build_pointer_type (act_des_fn_type);

  /* Declare the actor and destroyer function.  */
  tree actor = act_des_fn (orig, act_des_fn_type, coro_frame_ptr, "actor");
  tree destroy = act_des_fn (orig, act_des_fn_type, coro_frame_ptr, "destroy");

  /* Build our dummy coro frame layout.  */
  coro_frame_type = begin_class_definition (coro_frame_type);

  tree field_list = NULL_TREE;
  tree resume_name
    = coro_make_frame_entry (&field_list, "__resume", act_des_fn_ptr, fn_start);
  tree destroy_name = coro_make_frame_entry (&field_list, "__destroy",
                                             act_des_fn_ptr, fn_start);
  tree promise_name
    = coro_make_frame_entry (&field_list, "__p", promise_type, fn_start);
  tree fnf_name = coro_make_frame_entry (&field_list, "__frame_needs_free",
                                         boolean_type_node, fn_start);
  tree iarc_name = coro_make_frame_entry (&field_list, "__i_a_r_c",
                                         boolean_type_node, fn_start);
  tree resume_idx_name
    = coro_make_frame_entry (&field_list, "__resume_at",
                             short_unsigned_type_node, fn_start);

  /* We need a handle to this coroutine, which is passed to every
     await_suspend().  There's no point in creating it over and over.  */
  (void) coro_make_frame_entry (&field_list, "__self_h", handle_type, fn_start);

  /* Now add in fields for function params (if there are any).
     We do not attempt elision of copies at this stage, we do analyse the
     uses and build worklists to replace those when the state machine is
     lowered.  */

  hash_map<tree, param_info> *param_uses = NULL;
  if (DECL_ARGUMENTS (orig))
    {
      /* Build a hash map with an entry for each param.
          The key is the param tree.
          Then we have an entry for the frame field name.
          Then a cache for the field ref when we come to use it.
          Then a tree list of the uses.
          The second two entries start out empty - and only get populated
          when we see uses.  */
      param_uses = new hash_map<tree, param_info>;
      bool lambda_p = LAMBDA_FUNCTION_P (orig);

      unsigned no_name_parm = 0;
      for (tree arg = DECL_ARGUMENTS (orig); arg != NULL;
           arg = DECL_CHAIN (arg))
        {
          bool existed;
          param_info &parm = param_uses->get_or_insert (arg, &existed);
          gcc_checking_assert (!existed);
          parm.body_uses = NULL;
          tree actual_type = TREE_TYPE (arg);
          actual_type = complete_type_or_else (actual_type, orig);
          if (actual_type == NULL_TREE)
            actual_type = error_mark_node;
          parm.orig_type = actual_type;
          parm.by_ref = parm.pt_ref = false;
          if (TREE_CODE (actual_type) == REFERENCE_TYPE)
            {
              /* If the user passes by reference, then we will save the
                 pointer to the original.  As noted in
                 [dcl.fct.def.coroutine] / 13, if the lifetime of the
                 referenced item ends and then the coroutine is resumed,
                 we have UB; well, the user asked for it.  */
              actual_type = build_pointer_type (TREE_TYPE (actual_type));
              parm.pt_ref = true;
            }
          else if (TYPE_REF_P (DECL_ARG_TYPE (arg)))
            parm.by_ref = true;

          parm.frame_type = actual_type;

          parm.this_ptr = is_this_parameter (arg);
          /* See PR94807.  When a lambda is in a template instantiation, the
             closure object is named 'this' instead of '__closure'.  */
          if (lambda_p)
            {
              parm.lambda_cobj = DECL_NAME (arg) == closure_identifier;
              gcc_checking_assert (!parm.this_ptr);
            }
          else
            parm.lambda_cobj = false;

          parm.trivial_dtor = TYPE_HAS_TRIVIAL_DESTRUCTOR (parm.frame_type);
          char *buf;
          if (DECL_NAME (arg))
            {
              tree pname = DECL_NAME (arg);
              buf = xasprintf ("__parm.%s", IDENTIFIER_POINTER (pname));
            }
          else
            buf = xasprintf ("__unnamed_parm.%d", no_name_parm++);
          parm.field_id = coro_make_frame_entry
            (&field_list, buf, actual_type, DECL_SOURCE_LOCATION (arg));
          free (buf);
        }

      /* We want to record every instance of param's use, so don't include
         a 'visited' hash_set on the tree walk, but only record a containing
         expression once.  */
      hash_set<tree *> visited;
      param_frame_data param_data
        = {&field_list, param_uses, &visited, fn_start, false};
      cp_walk_tree (&fnbody, register_param_uses, &param_data, NULL);
    }

  /* Initial suspend is mandated.  */
  tree init_susp_name = coro_make_frame_entry (&field_list, "__aw_s.is",
                                               initial_suspend_type, fn_start);

  register_await_info (initial_await, initial_suspend_type, init_susp_name);

  /* Now insert the data for any body await points, at this time we also need
     to promote any temporaries that are captured by reference (to regular
     vars) they will get added to the coro frame along with other locals.  */
  susp_frame_data body_aw_points
    = {&field_list, handle_type, NULL, NULL, 0, 0,
       hash_set<tree> (), NULL, NULL, 0, false, false};
  body_aw_points.block_stack = make_tree_vector ();
  body_aw_points.bind_stack = make_tree_vector ();
  body_aw_points.to_replace = make_tree_vector ();
  cp_walk_tree (&fnbody, await_statement_walker, &body_aw_points, NULL);

  /* Final suspend is mandated.  */
  tree fin_susp_name = coro_make_frame_entry (&field_list, "__aw_s.fs",
                                              final_suspend_type, fn_start);

  register_await_info (final_await, final_suspend_type, fin_susp_name);

  /* 4. Now make space for local vars, this is conservative again, and we
     would expect to delete unused entries later.  */
  hash_map<tree, local_var_info> local_var_uses;
  local_vars_frame_data local_vars_data
    = {&field_list, &local_var_uses, 0, 0, fn_start, false, false};
  cp_walk_tree (&fnbody, register_local_var_uses, &local_vars_data, NULL);

  /* Tie off the struct for now, so that we can build offsets to the
     known entries.  */
  TYPE_FIELDS (coro_frame_type) = field_list;
  TYPE_BINFO (coro_frame_type) = make_tree_binfo (0);
  BINFO_OFFSET (TYPE_BINFO (coro_frame_type)) = size_zero_node;
  BINFO_TYPE (TYPE_BINFO (coro_frame_type)) = coro_frame_type;

  coro_frame_type = finish_struct (coro_frame_type, NULL_TREE);

  /* Ramp: */
  /* Now build the ramp function pieces.  */
  tree ramp_bind = build3 (BIND_EXPR, void_type_node, NULL, NULL, NULL);
  add_stmt (ramp_bind);
  tree ramp_body = push_stmt_list ();

  tree coro_fp = build_lang_decl (VAR_DECL, get_identifier ("coro.frameptr"),
                                  coro_frame_ptr);
  tree varlist = coro_fp;

  /* Collected the scope vars we need ... only one for now. */
  BIND_EXPR_VARS (ramp_bind) = nreverse (varlist);

  /* We're now going to create a new top level scope block for the ramp
     function.  */
  tree top_block = make_node (BLOCK);

  BIND_EXPR_BLOCK (ramp_bind) = top_block;
  BLOCK_VARS (top_block) = BIND_EXPR_VARS (ramp_bind);
  BLOCK_SUBBLOCKS (top_block) = NULL_TREE;

  /* The decl_expr for the coro frame pointer, initialize to zero so that we
     can pass it to the IFN_CO_FRAME (since there's no way to pass a type,
     directly apparently).  This avoids a "used uninitialized" warning.  */
  tree zeroinit = build1 (CONVERT_EXPR, coro_frame_ptr, integer_zero_node);
  DECL_INITIAL (coro_fp) = zeroinit;
  add_decl_expr (coro_fp);

  /* The CO_FRAME internal function is a mechanism to allow the middle end
     to adjust the allocation in response to optimisations.  We provide the
     current conservative estimate of the frame size (as per the current)
     computed layout.  */
  tree frame_size = TYPE_SIZE_UNIT (coro_frame_type);
  tree resizeable
    = build_call_expr_internal_loc (fn_start, IFN_CO_FRAME, size_type_node, 2,
                                    frame_size, coro_fp);

  /* [dcl.fct.def.coroutine] / 10 (part1)
    The unqualified-id get_return_object_on_allocation_failure is looked up
    in the scope of the promise type by class member access lookup.  */

  tree grooaf_meth
    = lookup_promise_method (orig, coro_gro_on_allocation_fail_identifier,
                             fn_start, /*musthave=*/false);

  tree grooaf = NULL_TREE;
  tree dummy_promise = build_dummy_object (get_coroutine_promise_type (orig));

  /* We don't require this, so lookup_promise_method can return NULL...  */
  if (grooaf_meth && BASELINK_P (grooaf_meth))
    {
      /* ... but, if the lookup succeeds, then the function must be
         usable.
         build_new_method_call () wants a valid pointer to (an empty)  args
         list in this case.  */
      vec<tree, va_gc> *args = make_tree_vector ();
      grooaf = build_new_method_call (dummy_promise, grooaf_meth, &args,
                                      NULL_TREE, LOOKUP_NORMAL, NULL,
                                      tf_warning_or_error);
      release_tree_vector (args);
    }

  /* Allocate the frame, this has several possibilities:
     [dcl.fct.def.coroutine] / 9 (part 1)
     The allocation function’s name is looked up in the scope of the promise
     type.  It's not a failure for it to be absent see part 4, below.  */
  tree nwname = ovl_op_identifier (false, NEW_EXPR);
  tree fns = lookup_promise_method (orig, nwname, fn_start,
                                    /*musthave=*/false);
  tree new_fn = NULL_TREE;
  if (fns && BASELINK_P (fns))
    {
      /* [dcl.fct.def.coroutine] / 9 (part 2)
        If the lookup finds an allocation function in the scope of the promise
        type, overload resolution is performed on a function call created by
        assembling an argument list.  The first argument is the amount of space
        requested, and has type std::size_t.  The succeeding arguments are
        those of the original function.  */
      vec<tree, va_gc> *args = make_tree_vector ();
      vec_safe_push (args, resizeable); /* Space needed.  */

      for (tree arg = DECL_ARGUMENTS (orig); arg != NULL;
           arg = DECL_CHAIN (arg))
        {
          param_info *parm_i = param_uses->get (arg);
          gcc_checking_assert (parm_i);
          if (parm_i->lambda_cobj)
            vec_safe_push (args, arg);
          else if (parm_i->this_ptr)
            {
              /* We pass a reference to *this to the allocator lookup.  */
              tree tt = TREE_TYPE (TREE_TYPE (arg));
              tree this_ref = build1 (INDIRECT_REF, tt, arg);
              tt = cp_build_reference_type (tt, false);
              this_ref = convert_to_reference (tt, this_ref, CONV_STATIC,
                                               LOOKUP_NORMAL , NULL_TREE,
                                               tf_warning_or_error);
              vec_safe_push (args, this_ref);
            }
          else
            vec_safe_push (args, arg);
        }

      /* We might need to check that the provided function is nothrow.  */
      tree func;
      /* Failure is OK for the first attempt.  */
      new_fn = build_new_method_call (dummy_promise, fns, &args, NULL,
                                      LOOKUP_NORMAL, &func, tf_none);
      release_tree_vector (args);

      if (!new_fn || new_fn == error_mark_node)
        {
          /* [dcl.fct.def.coroutine] / 9 (part 3)
            If no viable function is found, overload resolution is performed
            again on a function call created by passing just the amount of
            space required as an argument of type std::size_t.  */
          args = make_tree_vector ();
          vec_safe_push (args, resizeable); /* Space needed.  */
          new_fn = build_new_method_call (dummy_promise, fns, &args,
                                          NULL_TREE, LOOKUP_NORMAL, &func,
                                          tf_none);
          release_tree_vector (args);
        }

     /* However, if the initial lookup succeeded, then one of these two
        options must be available.  */
    if (!new_fn || new_fn == error_mark_node)
      {
        error_at (fn_start, "%qE is provided by %qT but is not usable with"
                  " the function signature %qD", nwname, promise_type, orig);
        new_fn = error_mark_node;
      }
    else if (grooaf && !TYPE_NOTHROW_P (TREE_TYPE (func)))
      error_at (fn_start, "%qE is provided by %qT but %qE is not marked"
                " %<throw()%> or %<noexcept%>", grooaf, promise_type, nwname);
    }
  else
    {
      /* [dcl.fct.def.coroutine] / 9 (part 4)
         If this lookup fails, the allocation function’s name is looked up in
         the global scope.  */

      vec<tree, va_gc> *args;
      /* build_operator_new_call () will insert size needed as element 0 of
         this, and we might need to append the std::nothrow constant.  */
      vec_alloc (args, 2);

      if (grooaf)
        {
          /* [dcl.fct.def.coroutine] / 10 (part 2)
           If any declarations (of the get return on allocation fail) are
           found, then the result of a call to an allocation function used
           to obtain storage for the coroutine state is assumed to return
           nullptr if it fails to obtain storage and, if a global allocation
           function is selected, the ::operator new(size_t, nothrow_t) form
           is used.  The allocation function used in this case shall have a
           non-throwing noexcept-specification.  So we need std::nothrow.  */
          tree std_nt = lookup_qualified_name (std_node,
                                               get_identifier ("nothrow"),
                                               0, /*complain=*/true, false);
          vec_safe_push (args, std_nt);
        }

      /* If we get to this point, we must succeed in looking up the global
         operator new for the params provided.  Extract a simplified version
         of the machinery from build_operator_new_call.  This can update the
         frame size.  */
      tree cookie = NULL;
      new_fn = build_operator_new_call (nwname, &args, &frame_size, &cookie,
                                        /*align_arg=*/NULL,
                                        /*size_check=*/NULL, /*fn=*/NULL,
                                        tf_warning_or_error);
      resizeable = build_call_expr_internal_loc
        (fn_start, IFN_CO_FRAME, size_type_node, 2, frame_size, coro_fp);
      CALL_EXPR_ARG (new_fn, 0) = resizeable;

      release_tree_vector (args);
    }

  tree allocated = build1 (CONVERT_EXPR, coro_frame_ptr, new_fn);
  tree r = build2 (INIT_EXPR, TREE_TYPE (coro_fp), coro_fp, allocated);
  r = coro_build_cvt_void_expr_stmt (r, fn_start);
  add_stmt (r);

  /* If the user provided a method to return an object on alloc fail, then
     check the returned pointer and call the func if it's null.
     Otherwise, no check, and we fail for noexcept/fno-exceptions cases.  */

  if (grooaf)
    {
      /* [dcl.fct.def.coroutine] / 10 (part 3)
         If the allocation function returns nullptr,the coroutine returns
         control to the caller of the coroutine and the return value is
         obtained by a call to T::get_return_object_on_allocation_failure(),
         where T is the promise type.  */

      gcc_checking_assert (same_type_p (fn_return_type, TREE_TYPE (grooaf)));
      tree if_stmt = begin_if_stmt ();
      tree cond = build1 (CONVERT_EXPR, coro_frame_ptr, integer_zero_node);
      cond = build2 (EQ_EXPR, boolean_type_node, coro_fp, cond);
      finish_if_stmt_cond (cond, if_stmt);
      if (VOID_TYPE_P (fn_return_type))
        {
          /* Execute the get-return-object-on-alloc-fail call...  */
          finish_expr_stmt (grooaf);
          /* ... but discard the result, since we return void.  */
          finish_return_stmt (NULL_TREE);
        }
      else
        {
          /* Get the fallback return object.  */
          r = build_cplus_new (fn_return_type, grooaf, tf_warning_or_error);
          finish_return_stmt (r);
        }
      finish_then_clause (if_stmt);
      finish_if_stmt (if_stmt);
    }

  /* deref the frame pointer, to use in member access code.  */
  tree deref_fp = build_x_arrow (fn_start, coro_fp, tf_warning_or_error);

  /* For now, once allocation has succeeded we always assume that this needs
     destruction, there's no impl. for frame allocation elision.  */
  tree fnf_m
    = lookup_member (coro_frame_type, fnf_name, 1, 0, tf_warning_or_error);
  tree fnf_x = build_class_member_access_expr (deref_fp, fnf_m, NULL_TREE,
                                               false, tf_warning_or_error);
  r = build2 (INIT_EXPR, boolean_type_node, fnf_x, boolean_true_node);
  r = coro_build_cvt_void_expr_stmt (r, fn_start);
  add_stmt (r);

  /* Put the resumer and destroyer functions in.  */

  tree actor_addr = build1 (ADDR_EXPR, act_des_fn_ptr, actor);
  tree resume_m
    = lookup_member (coro_frame_type, resume_name,
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
  tree resume_x = build_class_member_access_expr (deref_fp, resume_m, NULL_TREE,
                                                  false, tf_warning_or_error);
  r = build2_loc (fn_start, INIT_EXPR, act_des_fn_ptr, resume_x, actor_addr);
  r = coro_build_cvt_void_expr_stmt (r, fn_start);
  add_stmt (r);

  tree destroy_addr = build1 (ADDR_EXPR, act_des_fn_ptr, destroy);
  tree destroy_m
    = lookup_member (coro_frame_type, destroy_name,
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
  tree destroy_x
    = build_class_member_access_expr (deref_fp, destroy_m, NULL_TREE, false,
                                      tf_warning_or_error);
  r = build2_loc (fn_start, INIT_EXPR, act_des_fn_ptr, destroy_x, destroy_addr);
  r = coro_build_cvt_void_expr_stmt (r, fn_start);
  add_stmt (r);

  /* [dcl.fct.def.coroutine] /13
     When a coroutine is invoked, a copy is created for each coroutine
     parameter.  Each such copy is an object with automatic storage duration
     that is direct-initialized from an lvalue referring to the corresponding
     parameter if the parameter is an lvalue reference, and from an xvalue
     referring to it otherwise.  A reference to a parameter in the function-
     body of the coroutine and in the call to the coroutine promise
     constructor is replaced by a reference to its copy.  */

  vec<tree, va_gc> *promise_args = NULL; /* So that we can adjust refs.  */

  /* The initialization and destruction of each parameter copy occurs in the
     context of the called coroutine.  Initializations of parameter copies are
     sequenced before the call to the coroutine promise constructor and
     indeterminately sequenced with respect to each other.  The lifetime of
     parameter copies ends immediately after the lifetime of the coroutine
     promise object ends.  */

  vec<tree, va_gc> *param_dtor_list = NULL;

  if (DECL_ARGUMENTS (orig))
    {
      promise_args = make_tree_vector ();
      for (tree arg = DECL_ARGUMENTS (orig); arg != NULL;
           arg = DECL_CHAIN (arg))
        {
          bool existed;
          param_info &parm = param_uses->get_or_insert (arg, &existed);

          tree fld_ref = lookup_member (coro_frame_type, parm.field_id,
                                        /*protect=*/1, /*want_type=*/0,
                                        tf_warning_or_error);
          tree fld_idx
            = build_class_member_access_expr (deref_fp, fld_ref, NULL_TREE,
                                              false, tf_warning_or_error);

          /* Add this to the promise CTOR arguments list, accounting for
             refs and special handling for method this ptr.  */
          if (parm.lambda_cobj)
            vec_safe_push (promise_args, arg);
          else if (parm.this_ptr)
            {
              /* We pass a reference to *this to the param preview.  */
              tree tt = TREE_TYPE (arg);
              gcc_checking_assert (POINTER_TYPE_P (tt));
              tree ct = TREE_TYPE (tt);
              tree this_ref = build1 (INDIRECT_REF, ct, arg);
              tree rt = cp_build_reference_type (ct, false);
              this_ref = convert_to_reference (rt, this_ref, CONV_STATIC,
                                               LOOKUP_NORMAL , NULL_TREE,
                                               tf_warning_or_error);
              vec_safe_push (promise_args, this_ref);
            }
          else if (parm.by_ref)
            vec_safe_push (promise_args, fld_idx);
          else
            vec_safe_push (promise_args, arg);

          if (TYPE_NEEDS_CONSTRUCTING (parm.frame_type))
            {
              vec<tree, va_gc> *p_in;
              if (CLASS_TYPE_P (parm.frame_type)
                  && classtype_has_non_deleted_move_ctor (parm.frame_type))
                p_in = make_tree_vector_single (move (arg));
              else if (lvalue_p (arg))
                p_in = make_tree_vector_single (rvalue (arg));
              else
                p_in = make_tree_vector_single (arg);
              /* Construct in place or move as relevant.  */
              r = build_special_member_call (fld_idx, complete_ctor_identifier,
                                             &p_in, parm.frame_type,
                                             LOOKUP_NORMAL,
                                             tf_warning_or_error);
              release_tree_vector (p_in);
            }
          else
            {
              if (!same_type_p (parm.frame_type, DECL_ARG_TYPE (arg)))
                r = build1_loc (DECL_SOURCE_LOCATION (arg), CONVERT_EXPR,
                                parm.frame_type, arg);
              else
                r = arg;
              r = build_modify_expr (fn_start, fld_idx, parm.frame_type,
                                     INIT_EXPR, DECL_SOURCE_LOCATION (arg), r,
                                     TREE_TYPE (r));
            }
          r = coro_build_cvt_void_expr_stmt (r, fn_start);
          add_stmt (r);
          if (!parm.trivial_dtor)
            {
              if (param_dtor_list == NULL)
                param_dtor_list = make_tree_vector ();
              vec_safe_push (param_dtor_list, parm.field_id);
            }
        }
    }

  /* Set up the promise.  */
  tree promise_m
    = lookup_member (coro_frame_type, promise_name,
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);

  tree p = build_class_member_access_expr (deref_fp, promise_m, NULL_TREE,
                                           false, tf_warning_or_error);

  if (TYPE_NEEDS_CONSTRUCTING (promise_type))
    {
      /* Do a placement new constructor for the promise type (we never call
         the new operator, just the constructor on the object in place in the
         frame).

         First try to find a constructor with the same parameter list as the
         original function (if it has params), failing that find a constructor
         with no parameter list.  */

      if (DECL_ARGUMENTS (orig))
        {
          r = build_special_member_call (p, complete_ctor_identifier,
                                         &promise_args, promise_type,
                                         LOOKUP_NORMAL, tf_none);
          release_tree_vector (promise_args);
        }
      else
        r = NULL_TREE;

      if (r == NULL_TREE || r == error_mark_node)
        r = build_special_member_call (p, complete_ctor_identifier, NULL,
                                       promise_type, LOOKUP_NORMAL,
                                       tf_warning_or_error);

      r = coro_build_cvt_void_expr_stmt (r, fn_start);
      add_stmt (r);
    }

  /* Set up a new bind context for the GRO.  */
  tree gro_context_bind = build3 (BIND_EXPR, void_type_node, NULL, NULL, NULL);
  /* Make and connect the scope blocks.  */
  tree gro_block = make_node (BLOCK);
  BLOCK_SUPERCONTEXT (gro_block) = top_block;
  BLOCK_SUBBLOCKS (top_block) = gro_block;
  BIND_EXPR_BLOCK (gro_context_bind) = gro_block;
  add_stmt (gro_context_bind);

  tree gro_meth = lookup_promise_method (orig,
                                         coro_get_return_object_identifier,
                                         fn_start, /*musthave=*/true );
  tree get_ro
    = build_new_method_call (p, gro_meth, NULL, NULL_TREE, LOOKUP_NORMAL, NULL,
                             tf_warning_or_error);
  /* Without a return object we haven't got much clue what's going on.  */
  if (get_ro == error_mark_node)
    {
      BIND_EXPR_BODY (ramp_bind) = pop_stmt_list (ramp_body);
      DECL_SAVED_TREE (orig) = newbody;
      /* Suppress warnings about the missing return value.  */
      TREE_NO_WARNING (orig) = true;
      return false;
    }

  tree gro_context_body = push_stmt_list ();
  bool gro_is_void_p = VOID_TYPE_P (TREE_TYPE (get_ro));

  tree gro = NULL_TREE;
  tree gro_bind_vars = NULL_TREE;
  /* We have to sequence the call to get_return_object before initial
     suspend.  */
  if (gro_is_void_p)
    finish_expr_stmt (get_ro);
  else
    {
      gro = build_lang_decl (VAR_DECL, get_identifier ("coro.gro"),
                              TREE_TYPE (get_ro));
      DECL_CONTEXT (gro) = current_scope ();
      DECL_ARTIFICIAL (gro) = true;
      DECL_IGNORED_P (gro) = true;
      add_decl_expr (gro);
      gro_bind_vars = gro;

      r = build2_loc (fn_start, INIT_EXPR, TREE_TYPE (gro), gro, get_ro);
      r = coro_build_cvt_void_expr_stmt (r, fn_start);
      add_stmt (r);
    }

  /* Initialize the resume_idx_name to 0, meaning "not started".  */
  tree resume_idx_m
    = lookup_member (coro_frame_type, resume_idx_name,
                     /*protect=*/1, /*want_type=*/0, tf_warning_or_error);
  tree resume_idx
    = build_class_member_access_expr (deref_fp, resume_idx_m, NULL_TREE, false,
                                      tf_warning_or_error);
  r = build_int_cst (short_unsigned_type_node, 0);
  r = build2_loc (fn_start, INIT_EXPR, short_unsigned_type_node, resume_idx, r);
  r = coro_build_cvt_void_expr_stmt (r, fn_start);
  add_stmt (r);

  /* Initialize 'initial-await-resume-called' as per
     [dcl.fct.def.coroutine] / 5.3 */
  tree i_a_r_c_m
    = lookup_member (coro_frame_type, iarc_name, 1, 0, tf_warning_or_error);
  tree i_a_r_c = build_class_member_access_expr (deref_fp, i_a_r_c_m,
                                                 NULL_TREE, false,
                                                 tf_warning_or_error);
  r = build2 (INIT_EXPR, boolean_type_node, i_a_r_c, boolean_false_node);
  r = coro_build_cvt_void_expr_stmt (r, fn_start);
  add_stmt (r);

  /* So .. call the actor ..  */
  r = build_call_expr_loc (fn_start, actor, 1, coro_fp);
  r = maybe_cleanup_point_expr_void (r);
  add_stmt (r);

  /* Switch to using 'input_location' as the loc, since we're now more
     logically doing things related to the end of the function.  */

  /* The ramp is done, we just need the return value.  */
  if (!same_type_p (TREE_TYPE (get_ro), fn_return_type))
    {
      /* construct the return value with a single GRO param, if it's not
         void.  */
      vec<tree, va_gc> *args = NULL;
      vec<tree, va_gc> **arglist = NULL;
      if (!gro_is_void_p)
        {
          args = make_tree_vector_single (gro);
          arglist = &args;
        }
      r = build_special_member_call (NULL_TREE,
                                     complete_ctor_identifier, arglist,
                                     fn_return_type, LOOKUP_NORMAL,
                                     tf_warning_or_error);
      r = build_cplus_new (fn_return_type, r, tf_warning_or_error);
    }
  else if (!gro_is_void_p)
    r = rvalue (gro); /* The GRO is the return value.  */
  else
    r = NULL_TREE;

  finish_return_stmt (r);

  /* Finish up the ramp function.  */
  BIND_EXPR_VARS (gro_context_bind) = gro_bind_vars;
  BIND_EXPR_BODY (gro_context_bind) = pop_stmt_list (gro_context_body);
  TREE_SIDE_EFFECTS (gro_context_bind) = true;
  BIND_EXPR_BODY (ramp_bind) = pop_stmt_list (ramp_body);
  TREE_SIDE_EFFECTS (ramp_bind) = true;

  /* We know the "real" promise and have a frame layout with a slot for each
     suspend point, so we can build an actor function (which contains the
     functionality for both 'resume' and 'destroy').

     wrap the function body in a try {} catch (...) {} block, if exceptions
     are enabled.  */

  /* First make a new block for the body - that will be embedded in the
     re-written function.  */
  tree first = expr_first (fnbody);
  bool orig_fn_has_outer_bind = false;
  tree replace_blk = NULL_TREE;
  if (first && TREE_CODE (first) == BIND_EXPR)
    {
      orig_fn_has_outer_bind = true;
      tree block = BIND_EXPR_BLOCK (first);
      replace_blk = make_node (BLOCK);
      if (block) /* missing block is probably an error.  */
        {
          gcc_assert (BLOCK_SUPERCONTEXT (block) == NULL_TREE);
          gcc_assert (BLOCK_CHAIN (block) == NULL_TREE);
          BLOCK_VARS (replace_blk) = BLOCK_VARS (block);
          BLOCK_SUBBLOCKS (replace_blk) = BLOCK_SUBBLOCKS (block);
          for (tree b = BLOCK_SUBBLOCKS (replace_blk); b; b = BLOCK_CHAIN (b))
            BLOCK_SUPERCONTEXT (b) = replace_blk;
        }
      BIND_EXPR_BLOCK (first) = replace_blk;
    }

  /* actor's version of the promise.  */
  tree actor_frame = build1_loc (fn_start, INDIRECT_REF, coro_frame_type,
                                 DECL_ARGUMENTS (actor));
  tree ap_m = lookup_member (coro_frame_type, get_identifier ("__p"), 1, 0,
                             tf_warning_or_error);
  tree ap = build_class_member_access_expr (actor_frame, ap_m, NULL_TREE,
                                            false, tf_warning_or_error);

  /* Now we've built the promise etc, process fnbody for co_returns.
     We want the call to return_void () below and it has no params so
     we can create it once here.
     Calls to return_value () will have to be checked and created as
     required.  */

  tree return_void = NULL_TREE;
  tree rvm
    = lookup_promise_method (orig, coro_return_void_identifier, fn_start,
                             /*musthave=*/false);
  if (rvm && rvm != error_mark_node)
    return_void
      = build_new_method_call (ap, rvm, NULL, NULL_TREE, LOOKUP_NORMAL, NULL,
                               tf_warning_or_error);

  /* [stmt.return.coroutine] (2.2 : 3) if p.return_void() is a valid
     expression, flowing off the end of a coroutine is equivalent to
     co_return; otherwise UB.
     We just inject the call to p.return_void() here, and fall through to
     the final_suspend: label (eliding the goto).  If the function body has
     a co_return, then this statement will be unreachable and DCEd.  */
  if (return_void != NULL_TREE)
    {
      tree append = push_stmt_list ();
      add_stmt (fnbody);
      add_stmt (return_void);
      fnbody = pop_stmt_list(append);
    }

  if (flag_exceptions)
    {
      tree ueh_meth
        = lookup_promise_method (orig, coro_unhandled_exception_identifier,
                                 fn_start, /*musthave=*/true);
      /* Build promise.unhandled_exception();  */
      tree ueh
        = build_new_method_call (ap, ueh_meth, NULL, NULL_TREE, LOOKUP_NORMAL,
                                 NULL, tf_warning_or_error);

      /* The try block is just the original function, there's no real
         need to call any function to do this.  */
      fnbody = build_stmt (fn_start, TRY_BLOCK, fnbody, NULL_TREE);
      TRY_HANDLERS (fnbody) = push_stmt_list ();
      /* Mimic what the parser does for the catch.  */
      tree handler = begin_handler ();
      finish_handler_parms (NULL_TREE, handler); /* catch (...) */

      /* Get the initial await resume called value.  */
      tree i_a_r_c = build_class_member_access_expr (actor_frame, i_a_r_c_m,
                                                     NULL_TREE, false,
                                                     tf_warning_or_error);
      tree not_iarc_if = begin_if_stmt ();
      tree not_iarc = build1_loc (fn_start, TRUTH_NOT_EXPR,
                                  boolean_type_node, i_a_r_c);
      finish_if_stmt_cond (not_iarc, not_iarc_if);
      /* If the initial await resume called value is false, rethrow...  */
      tree rethrow = build_throw (fn_start, NULL_TREE);
      TREE_NO_WARNING (rethrow) = true;
      finish_expr_stmt (rethrow);
      finish_then_clause (not_iarc_if);
      tree iarc_scope = IF_SCOPE (not_iarc_if);
      IF_SCOPE (not_iarc_if) = NULL;
      not_iarc_if = do_poplevel (iarc_scope);
      add_stmt (not_iarc_if);
      /* ... else call the promise unhandled exception method.  */
      ueh = maybe_cleanup_point_expr_void (ueh);
      add_stmt (ueh);
      finish_handler (handler);
      TRY_HANDLERS (fnbody) = pop_stmt_list (TRY_HANDLERS (fnbody));
      /* If the function starts with a BIND_EXPR, then we need to create
         one here to contain the try-catch and to link up the scopes.  */
      if (orig_fn_has_outer_bind)
        {
          fnbody = build3 (BIND_EXPR, void_type_node, NULL, fnbody, NULL);
          /* Make and connect the scope blocks.  */
          tree tcb_block = make_node (BLOCK);
          /* .. and connect it here.  */
          BLOCK_SUPERCONTEXT (replace_blk) = tcb_block;
          BLOCK_SUBBLOCKS (tcb_block) = replace_blk;
          BIND_EXPR_BLOCK (fnbody) = tcb_block;
          TREE_SIDE_EFFECTS (fnbody) = true;
        }
    }
  else if (pedantic)
    {
      /* We still try to look for the promise method and warn if it's not
         present.  */
      tree ueh_meth
        = lookup_promise_method (orig, coro_unhandled_exception_identifier,
                                 fn_start, /*musthave=*/false);
      if (!ueh_meth || ueh_meth == error_mark_node)
        warning_at (fn_start, 0, "no member named %qE in %qT",
                    coro_unhandled_exception_identifier,
                    get_coroutine_promise_type (orig));
    }
  /* Else we don't check and don't care if the method is missing.  */

  /* Start to build the final functions.

     We push_deferring_access_checks to avoid these routines being seen as
     nested by the middle end; we are doing the outlining here.  */

  push_deferring_access_checks (dk_no_check);

  /* Build the actor...  */
  build_actor_fn (fn_start, coro_frame_type, actor, fnbody, orig, param_uses,
                  &local_var_uses, param_dtor_list, initial_await, final_await,
                  body_aw_points.await_number, frame_size);

  /* Destroyer ... */
  build_destroy_fn (fn_start, coro_frame_type, destroy, actor);

  pop_deferring_access_checks ();

  DECL_SAVED_TREE (orig) = newbody;
  /* Link our new functions into the list.  */
  TREE_CHAIN (destroy) = TREE_CHAIN (orig);
  TREE_CHAIN (actor) = destroy;
  TREE_CHAIN (orig) = actor;

  *resumer = actor;
  *destroyer = destroy;

  delete suspend_points;
  suspend_points = NULL;
  return true;
}

#include "gt-cp-coroutines.h"