[PATCH v2] c++: Disable -Wignored-qualifiers for template args [PR107492]

[Marek Polacek <polacek@redhat.com>]

On Thu, Nov 03, 2022 at 03:22:12PM -0400, Jason Merrill wrote:
> On 11/1/22 13:01, Marek Polacek wrote:
> > 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.
> 
> Hmm, I'm not sure why we would want to treat template arguments differently
> from other type-ids.  Maybe only warn if funcdecl_p?

I think that makes sense.  There are other contexts in which cv-quals
matter, for instance trailing-return-type.  Updated patch below, plus
I've extended the testcase.  Thanks,

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

-- >8 --
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                                |  6 ++++-
 .../g++.dg/warn/Wignored-qualifiers3.C        | 24 +++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wignored-qualifiers3.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 890cfcabd35..67b9f24d7d6 100644
--- 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
index 00000000000..dedb38fc995
--- /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" }

base-commit: c41bbfcaf9d6ef5b57a7e89bba70b861c08a686b
-- 
2.38.1