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

[Jason Merrill <jason@redhat.com>]

On 11/14/22 14:33, Marek Polacek wrote:
> 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.

Well, technically they matter in all contexts, including function 
declaration:

const void f();
template <class T, class U> struct same;
template <class T> struct same<T,T>{};
same<decltype(f),const void()> s;

but much more likely to be a confused user in that case, whereas in a 
template context it's likely to be some deep magic.  :)

> Updated patch below, plus I've extended the testcase.  Thanks,
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >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