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.

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index c83fdac4984469015ca5ae239728aae52b4380fc..0234919ea572477297649d2303ec6b486f7af905 100644 (file)
--- a/gcc/cp/decl.cc
+++ b/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

diff --git a/gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C b/gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C
new file mode 100644 (file)
index 0000000..dedb38f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C
@@ -0,0 +1,24 @@
+// 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" }