c++: dependently scoped template-id in type-req [PR110927]

Here we're incorrectly rejecting the first type-requirement at parse
time with

  concepts-requires35.C:14:56: error: ‘typename A<T>::B’ is not a template [-fpermissive]

We also incorrectly reject the second type-requirement at satisfaction time
with

  concepts-requires35.C:17:34: error: ‘typename A<int>::B’ names ‘template<class U> struct A<int>::B’, which is not a type

and similarly for the third type-requirement.  This seems to happen only
within a type-requirement; if we instead use e.g. an alias template then
it works as expected.

The difference ultimately seems to be that during parsing of a using-decl,
we pass check_dependency_p=true to cp_parser_nested_name_specifier_opt
whereas for a type-requirement we pass check_dependency_p=false.
Passing =false causes cp_parser_template_id for the dependently-scoped
template-id B<bool> to create a TYPE_DECL of TYPENAME_TYPE (with
TYPENAME_IS_CLASS_P unexpectedly set in the last two cases) whereas
passing =true causes it to return a TEMPLATE_ID_EXPR.  We then call
make_typename_type on this TYPE_DECL which does the wrong thing.

Since there seems to be no justification for using check_dependency_p=false
here, the simplest fix seems to be to pass check_dependency_p=true instead,
matching the behavior of cp_parser_elaborated_type_specifier.

	PR c++/110927

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_type_requirement): Pass
	check_dependency_p=true instead of =false.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires35.C: New test.

(cherry picked from commit 63bd36be)

gcc/cp/parser.cc

@@ -30939,7 +30939,7 @@ cp_parser_type_requirement (cp_parser *parser)
   cp_parser_global_scope_opt (parser, /*current_scope_valid_p=*/false);
   cp_parser_nested_name_specifier_opt (parser,
                                        /*typename_keyword_p=*/true,
-                                       /*check_dependency_p=*/false,
+				       /*check_dependency_p=*/true,
                                        /*type_p=*/true,
                                        /*is_declaration=*/false);
@@ -30949,7 +30949,7 @@ cp_parser_type_requirement (cp_parser *parser)
       cp_lexer_consume_token (parser->lexer);
       type = cp_parser_template_id (parser,
                                     /*template_keyword_p=*/true,
-                                    /*check_dependency=*/false,
+				    /*check_dependency_p=*/true,
                                     /*tag_type=*/none_type,
                                     /*is_declaration=*/false);
       type = make_typename_type (parser->scope, type, typename_type,

gcc/testsuite/g++.dg/cpp2a/concepts-requires35.C

+// PR c++/110927
+// { dg-do compile { target c++20 } }
+
+template<class T>
+struct A {
+  template<class U> struct B { using type = B; };
+  template<class U> using type = U;
+};
+
+template<> struct A<void> { };
+
+template<class T>
+concept C1 = requires { typename A<T>::template B<bool>::type; };
+
+template<class T>
+concept C2 = requires { typename A<T>::template B<bool>; };
+
+template<class T>
+concept C3 = requires { typename A<T>::template type<bool>; };
+
+static_assert(C1<int>);
+static_assert(C2<int>);
+static_assert(C3<int>);
+
+static_assert(!C1<void>);
+static_assert(!C2<void>);
+static_assert(!C3<void>);