Re: [PATCH] c++, v2: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr functions

[Jason Merrill <jason@redhat.com>]

On 11/16/22 01:19, Jakub Jelinek wrote:
> On Wed, Nov 16, 2022 at 12:27:02AM +0000, Jonathan Wakely wrote:
>> On Tue, 15 Nov 2022 at 23:50, Jakub Jelinek <jakub@redhat.com> wrote:
>>>
>>> On Tue, Nov 15, 2022 at 06:36:38PM -0500, Jason Merrill wrote:
>>>>> Here is an updated patch that passed bootstrap/regtest, the only
>>>>> change is another testcase tweak.
>>>>>
>>>>> 2022-11-13  Jakub Jelinek  <jakub@redhat.com>
>>>>>
>>>>> gcc/c-family/
>>>>>      * c-cppbuiltin.cc (c_cpp_builtins): Bump __cpp_constexpr
>>>>>      value from 202207L to 202211L.
>>>>> gcc/cp/
>>>>>      * constexpr.cc (cxx_eval_constant_expression): Implement C++23
>>>>>      P2647R1 - Permitting static constexpr variables in constexpr functions.
>>>>>      Allow decl_maybe_constant_var_p static or thread_local vars for
>>>>>      C++23.
>>>>
>>>> This was accepted as a DR, so it shouldn't be limited to C++23 mode.
>>>> Certainly it should be allowed in C++20 mode; I don't have a strong opinion
>>>> about C++14/17.  Jonathan, do you?
>>>
>>> How will a feature with feature test macro with multiple values work as DR?
>>> Or will everything but the macro be treated as a DR (so __cpp_constexpr >=
>>> 202211L only for C++23)?
>>
>> Yes, I think so. We just won't be able to advertise this feature as
>> supported in C++20.
> 
> Ok.  But there is another issue, the
> https://eel.is/c++draft/expr.const#5.2
> spot that P2647R1 is changing didn't exist in C++20, it was only added with
> P2242R3.  So, if one would treat P2647R1 as a DR for C++20, one has to come up with
> a different standard modification.
> Probably change the last bullet of:
> [dcl.constexpr]/3
> its function-body shall not enclose
> 
>      a goto statement,
>      an identifier label,
>      a definition of a variable of non-literal type or of static or thread storage duration.
> to
>      a definition of a variable of non-literal type or of a non-constexpr
>        variable of static or thread storage duration.
> or so.

Indeed, though the hypothetical C++20 change could still use the "usable 
in constant expressions" phrase.

Jason