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

[Patrick Palka <ppalka@redhat.com>]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
for trunk and perhaps 13?

-- >8 --

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-declaration, we pass check_dependency_p=true to
cp_parser_nested_name_specifier_opt whereas during parsing of 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
return a TYPE_DECL of TYPENAME_TYPE (sometimes with TYPENAME_IS_CLASS_P
unexpectedly set for the last two type-requirements) whereas passing
=true causes it to return a TEMPLATE_ID_EXPR.

The simplest fix therefore seems to be to pass check_dependency_p=true
from cp_parser_type_requirement, 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.
---
 gcc/cp/parser.cc                              |  4 +--
 .../g++.dg/cpp2a/concepts-requires35.C        | 28 +++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-requires35.C

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 589ac879c6d..2d27376d988 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -30977,7 +30977,7 @@ 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);
 
@@ -30987,7 +30987,7 @@ 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,
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires35.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires35.C
new file mode 100644
index 00000000000..1dad931c01a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires35.C
@@ -0,0 +1,28 @@
+// 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>);
-- 
2.42.0.rc0.25.ga82fb66fed