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

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

And likewise in other type-id contexts such as trailing-return-type:

  auto g() -> const void (*)();

This patch limits the warning to the function declaration context only.

	PR c++/107492

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Only emit a -Wignored-qualifiers warning
	when funcdecl_p.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wignored-qualifiers3.C: New test.

gcc/cp/decl.cc

@@ -13038,7 +13038,11 @@ grokdeclarator (const cp_declarator *declarator,
 	    if (type_quals != TYPE_UNQUALIFIED)
 	      {
-		if (SCALAR_TYPE_P (type) || VOID_TYPE_P (type))
+		/* It's wrong, for instance, to issue a -Wignored-qualifiers
+		   warning for
+		    static_assert(!is_same_v<void(*)(), const void(*)()>);
+		    because there the qualifier matters.  */
+		if (funcdecl_p && (SCALAR_TYPE_P (type) || VOID_TYPE_P (type)))
 		  warning_at (typespec_loc, OPT_Wignored_qualifiers, "type "
 			      "qualifiers ignored on function return type");
 		/* [dcl.fct] "A volatile-qualified return type is

gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C

+// PR c++/107492
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-Wignored-qualifiers" }
+
+// Here the 'const' matters, so don't warn.
+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" }
+
+// Here the 'const' matters as well -> don't warn.
+auto g() -> const void (*)(); // { dg-bogus "ignored" }
+auto g() -> const void (*)() { return nullptr; } // { dg-bogus "ignored" }
+
+// Here as well.
+const void (*h)() = static_cast<const void (*)()>(h); // { dg-bogus "ignored" }
+
+// But let's keep the warning here.
+const void f(); // { dg-warning "ignored" }
+const void f() { } // { dg-warning "ignored" }