From 1de6612d994ada8edaab18bbc6afd8e9a57413aa Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Sun, 27 Mar 2022 22:31:51 -0400
Subject: [PATCH] c++: elaborated-type-spec in requires-expr [PR101677]

We were failing to declare class S in the global namespace because we were
treating the requires-expression parameter scope as a normal block scope, so
the implicit declaration went there.

It seems to me that the requires parameter scope is more like a function
parameter scope (not least in the use of the word "parameter"), so let's
change the scope kind.  But then we need to adjust the prohibition on
placeholders declaring implicit template parameters.

	PR c++/101677

gcc/cp/ChangeLog:

	* name-lookup.h (struct cp_binding_level): Add requires_expression
	bit-field.
	* parser.cc (cp_parser_requires_expression): Set it.
	(synthesize_implicit_template_parm): Check it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-pr67178.C: Adjust error.
	* g++.dg/cpp2a/concepts-requires28.C: New test.
---
 gcc/cp/name-lookup.h                             |  5 ++++-
 gcc/cp/parser.cc                                 |  5 +++--
 gcc/testsuite/g++.dg/cpp2a/concepts-pr67178.C    |  2 +-
 gcc/testsuite/g++.dg/cpp2a/concepts-requires28.C | 13 +++++++++++++
 4 files changed, 21 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-requires28.C

diff --git a/gcc/cp/name-lookup.h b/gcc/cp/name-lookup.h
index 68d08725a001..fa039028847a 100644
--- a/gcc/cp/name-lookup.h
+++ b/gcc/cp/name-lookup.h
@@ -309,7 +309,10 @@ struct GTY(()) cp_binding_level {
   /* true for SK_FUNCTION_PARMS of immediate functions.  */
   unsigned immediate_fn_ctx_p : 1;
 
-  /* 22 bits left to fill a 32-bit word.  */
+  /* True for SK_FUNCTION_PARMS of a requires-expression.  */
+  unsigned requires_expression: 1;
+
+  /* 21 bits left to fill a 32-bit word.  */
 };
 
 /* The binding level currently in effect.  */
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 7e1c777364e7..bfd16e1ef620 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -29964,7 +29964,8 @@ cp_parser_requires_expression (cp_parser *parser)
       scope_sentinel ()
       {
 	++cp_unevaluated_operand;
-	begin_scope (sk_block, NULL_TREE);
+	begin_scope (sk_function_parms, NULL_TREE);
+	current_binding_level->requires_expression = true;
       }
 
       ~scope_sentinel ()
@@ -48082,7 +48083,7 @@ static tree
 synthesize_implicit_template_parm  (cp_parser *parser, tree constr)
 {
   /* A requires-clause is not a function and cannot have placeholders.  */
-  if (current_binding_level->kind == sk_block)
+  if (current_binding_level->requires_expression)
     {
       error ("placeholder type not allowed in this context");
       return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr67178.C b/gcc/testsuite/g++.dg/cpp2a/concepts-pr67178.C
index c74f6f00a5c5..bdd5f9699e09 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-pr67178.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr67178.C
@@ -12,7 +12,7 @@ concept C0 = requires (auto x) { // { dg-error "placeholder type" }
 template<typename T>
 concept C1 = requires (C1 auto x) { // { dg-error "not been declared|placeholder|two or more|in requirements" }
   x; // { dg-error "not declared" }
-  { x } -> c; // { dg-message "is invalid" }
+  { x } -> c; // { dg-message "is invalid|not declared" }
 };
 
 template<typename T>
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires28.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires28.C
new file mode 100644
index 000000000000..e632f0167f4c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires28.C
@@ -0,0 +1,13 @@
+// PR c++/101677
+// { dg-do compile { target c++20 } }
+
+template<class T>
+concept C_bug_with_forward_decl = requires(T& t){
+    t.template f<class S>();
+};
+
+struct good {
+    template<class T> void f() {}
+};
+
+static_assert(C_bug_with_forward_decl<good>);
-- 
2.47.2