c++: typename in out-of-class member function definitions [PR97297]

I was notified that our P0634R3 (Down with typename) implementation has
a flaw: when we have an out-of-class member function definition, we
still required 'typename' for its parameters.  For example here:

  template <typename T> struct S {
    int simple(T::type);
  };
  template <typename T>
  int S<T>::simple(/* typename */T::type) { return 0; }

the 'typename' isn't necessary per [temp.res]/5.2.4.  We have a qualified
name here ("S<T>::simple") so we know it's already been declared so we
can look it up to see if it's a function template or a variable
template.

In this case, the P0634R3 code in cp_parser_direct_declarator wasn't
looking into uninstantiated templates and didn't find the member
function 'simple' -- cp_parser_lookup_name returned a SCOPE_REF which
means that the qualifying scope was dependent.  With this fix, we find
the BASELINK for 'simple', don't clear CP_PARSER_FLAGS_TYPENAME_OPTIONAL
from the flags, and the typename is implicitly assumed.

gcc/cp/ChangeLog:

	PR c++/97297
	* parser.c (cp_parser_direct_declarator): When checking if a
	name is a function template declaration for the P0634R3 case,
	look in uninstantiated templates too.

gcc/testsuite/ChangeLog:

	PR c++/97297
	* g++.dg/cpp2a/typename18.C: New test.

2 files changed, 29 insertions, 2 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index cb44227..2002c05 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -21788,8 +21788,14 @@ cp_parser_direct_declarator (cp_parser* parser,
 		       templates, assume S::p to name a type.  Otherwise,
 		       don't.  */
 		    tree decl
-		      = cp_parser_lookup_name_simple (parser, unqualified_name,
-						      token->location);
+		      = cp_parser_lookup_name (parser, unqualified_name,
+					       none_type,
+					       /*is_template=*/false,
+					       /*is_namespace=*/false,
+					       /*check_dependency=*/false,
+					       /*ambiguous_decls=*/NULL,
+					       token->location);
+
 		    if (!is_overloaded_fn (decl)
 			/* Allow
 			   template<typename T>
diff --git a/gcc/testsuite/g++.dg/cpp2a/typename18.C b/gcc/testsuite/g++.dg/cpp2a/typename18.C
new file mode 100644
index 0000000..9946866
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/typename18.C
@@ -0,0 +1,21 @@
+// PR c++/97297
+// { dg-do compile { target c++20 } }
+
+template <typename T>
+struct S {
+    int simple(T::type);
+
+    template <typename U>
+    int member(U::type);
+};
+
+template <typename T>
+int S<T>::simple(T::type) {
+    return 1;
+}
+
+template <typename T>
+template <typename U>
+int S<T>::member(U::type) {
+    return 2;
+}