[PATCH] c++: Disable -Wignored-qualifiers for template args [PR107492]

[Marek Polacek <polacek@redhat.com>]

It seems wrong to issue a -Wignored-qualifiers warning for code like:

  static_assert(!is_same_v<void(*)(), const void(*)()>);

because there the qualifier matters.  Likewise in template
specialization:

  template<typename T> struct S { };
  template<> struct S<void(*)()> { };
  template<> struct S<const void(*)()> { }; // OK, not a redefinition

I'm of the mind that we should disable the warning for template
arguments, as in the patch below.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

	PR c++/107492

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_template_type_arg): Suppress -Wignored-qualifiers
	warning.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wignored-qualifiers3.C: New test.
---
 gcc/cp/parser.cc                                 |  4 ++++
 gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C | 12 ++++++++++++
 2 files changed, 16 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index e0e3cf3eaf6..54ad4b98ed3 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -24334,6 +24334,10 @@ cp_parser_template_type_arg (cp_parser *parser)
   const char *saved_message = parser->type_definition_forbidden_message;
   parser->type_definition_forbidden_message
     = G_("types may not be defined in template arguments");
+  /* It's wrong to issue a -Wignored-qualifiers warning for
+      static_assert(!is_same_v<void(*)(), const void(*)()>);
+     because there the qualifier matters.  */
+  warning_sentinel w (warn_ignored_qualifiers);
   r = cp_parser_type_id_1 (parser, CP_PARSER_FLAGS_NONE, true, false, NULL);
   parser->type_definition_forbidden_message = saved_message;
   if (cxx_dialect >= cxx14 && !flag_concepts && type_uses_auto (r))
diff --git a/gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C b/gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C
new file mode 100644
index 00000000000..5696feaaefe
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C
@@ -0,0 +1,12 @@
+// PR c++/107492
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-Wignored-qualifiers" }
+
+template<typename T> struct S { };
+template<> struct S<void(*)()> { };
+template<> struct S<const void(*)()> { }; // { dg-bogus "ignored" }
+
+template<typename T, typename U> constexpr bool is_same_v = false;
+template<typename T> constexpr bool is_same_v<T, T> = true;
+
+static_assert( ! is_same_v< void(*)(), const void(*)() >, ""); // { dg-bogus "ignored" }

base-commit: e7310e24b1c0ca67b1bb507c1330b2bf39e59e32
-- 
2.38.1