c++: Implement C++26 P2169R4 - Placeholder variables with no name [PR110349]

c++: Implement C++26 P2169R4 - Placeholder variables with no name [PR110349]

The following patch implements the C++26 P2169R4 paper.
As written in the PR, the patch expects that:
1) https://eel.is/c++draft/expr.prim.lambda.capture#2
   "Ignoring appearances in initializers of init-captures, an identifier
    or this shall not appear more than once in a lambda-capture."
   is adjusted such that name-independent lambda captures with initializers
   can violate this rule (but lambda captures which aren't name-independent
   can't appear after name-independent ones)
2) https://eel.is/c++draft/class.mem#general-5
   "A member shall not be declared twice in the member-specification,
    except that"
   having an exception that name-independent non-static data member
   declarations can appear multiple times (but again, if there is
   a member which isn't name-independent, it can't appear after
   name-independent ones)
3) it assumes that any name-independent declarations which weren't
   previously valid result in the _ lookups being ambiguous, not just
   if there are 2 _ declarations in the same scope, in particular the
   https://eel.is/c++draft/basic.scope#block-2 mentioned cases
4) it assumes that _ in static function/block scope structured bindings
   is never name-independent like in namespace scope structured bindings;
   it matches clang behavior and is consistent with e.g. static type _;
   not being name-independent both at namespace scope and at function/block
   scope

As you preferred in the PR, for local scope bindings, the ambiguous cases
use a TREE_LIST with the ambiguous cases which can often be directly fed
into print_candidates.  For member_vec after sorting/deduping, I chose to use
instead OVERLOAD with a new flag but only internally inside of the
member_vec, get_class_binding_direct turns it into a TREE_LIST.  There are
2 reasons for that, in order to keep the member_vec binary search fast, I
think it is better to keep OVL_NAME usable on all elements because having
to special case TREE_LIST would slow everything down, and the callers need
to be able to chain the results anyway and so need an unshared TREE_LIST
they can tweak/destroy anyway.
name-independent declarations (even in older standards) will not have
-Wunused{,-variable,-but-set-variable} or -Wshadow* warnings diagnosed, but
unlike e.g. the clang implementation, this patch does diagnose
-Wunused-parameter for parameters with _ names because they aren't
name-independent and one can just omit their name instead.

2023-11-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/110349
gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Predefine
	__cpp_placeholder_variables=202306L for C++26.
gcc/cp/
	* cp-tree.h: Implement C++26 P2169R4 - Placeholder variables with no
	name.
	(OVL_NAME_INDEPENDENT_DECL_P): Define.
	(add_capture): Add unsigned * argument.
	(name_independent_decl_p): New inline function.
	* name-lookup.cc (class name_lookup): Make ambiguous and
	add_value members public.
	(name_independent_linear_search): New function.
	(get_class_binding_direct): Handle member_vec_binary_search
	returning OVL_NAME_INDEPENDENT_DECL_P OVERLOAD.  Use
	name_independent_linear_search rather than fields_linear_search
	for linear lookup of _ name if !want_type.
	(member_name_cmp): Sort name-independent declarations first.
	(member_vec_dedup): Handle name-independent declarations.
	(pop_local_binding): Handle binding->value being a TREE_LIST for
	ambiguous name-independent declarations.
	(supplement_binding): Handle name-independent declarations.
	(update_binding): Likewise.
	(check_local_shadow): Return tree rather than void, normally
	NULL_TREE but old for name-independent declarations which used
	to conflict with outer scope declaration.  Don't emit -Wshadow*
	warnings for name-independent declarations.
	(pushdecl): Handle name-independent declarations.
	* search.cc (lookup_field_r): Handle nval being a TREE_LIST.
	* lambda.cc (build_capture_proxy): Adjust for ___.<number>
	names of members.
	(add_capture): Add NAME_INDEPENDENT_CNT argument.  Use ___.<number>
	name rather than ___ for second and following capture with
	_ name.
	(add_default_capture): Adjust add_capture caller.
	* decl.cc (poplevel): Don't warn about name-independent declarations.
	(duplicate_decls): If in C++26 a _ named declaration conflicts with
	earlier declarations, emit explaining note why the new declaration
	is not name-independent.
	(reshape_init_class): If field is a TREE_LIST, emit an ambiguity
	error with list of candidates rather than error about non-existing
	non-static data member.
	* parser.cc (cp_parser_lambda_introducer): Adjust add_capture callers.
	Allow name-independent capture redeclarations.
	(cp_parser_decomposition_declaration): Set decl_specs.storage_class
	to sc_static for static structured bindings.
	* pt.cc (tsubst_lambda_expr): Adjust add_capture caller.
gcc/testsuite/
	* g++.dg/cpp26/name-independent-decl1.C: New test.
	* g++.dg/cpp26/name-independent-decl2.C: New test.
	* g++.dg/cpp26/name-independent-decl3.C: New test.
	* g++.dg/cpp26/name-independent-decl4.C: New test.
	* g++.dg/cpp26/name-independent-decl5.C: New test.
	* g++.dg/cpp26/name-independent-decl6.C: New test.
	* g++.dg/cpp26/feat-cxx26.C: Add __cpp_placeholder_variables test.