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.c (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.

diff --git a/gcc/cp/name-lookup.h b/gcc/cp/name-lookup.h
index f63c4f5b8bb885464b66286d9a2de012c506bd8a..c6d0aa96b559f228174dd0881d7be7561aef3c7b 100644 (file)
--- 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.c b/gcc/cp/parser.c
index f41b61c26df5177a40d2f8eb488c0d86102c9a36..90d119eaa28025deabc21ef519f9f608a6317c4c 100644 (file)
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -28849,7 +28849,8 @@ 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 ()
@@ -45383,7 +45384,7 @@ 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 c74f6f00a5c56aa0ba5458a471d03d79e06f9273..bdd5f9699e09f8f341de6066288c8fa3a412d93c 100644 (file)
--- 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 (file)
index 0000000..e632f01
--- /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>);