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

[Jason Merrill <jason@redhat.com>]

On 11/16/22 09:08, Jakub Jelinek wrote:
> On Wed, Nov 16, 2022 at 08:20:34AM -0500, Jason Merrill wrote:
>>> 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.
> 
> Yes.
> Though, with -std=c++20 we are rejected already in start_decl's
>    if (current_function_decl && VAR_P (decl)
>        && DECL_DECLARED_CONSTEXPR_P (current_function_decl)
>        && cxx_dialect < cxx23)
>      {
>        bool ok = false;
>        if (CP_DECL_THREAD_LOCAL_P (decl) && !DECL_REALLY_EXTERN (decl))
>          error_at (DECL_SOURCE_LOCATION (decl),
>                    "%qD defined %<thread_local%> in %qs function only "
>                    "available with %<-std=c++2b%> or %<-std=gnu++2b%>", decl,
>                    DECL_IMMEDIATE_FUNCTION_P (current_function_decl)
>                    ? "consteval" : "constexpr");
>        else if (TREE_STATIC (decl))
>          error_at (DECL_SOURCE_LOCATION (decl),
>                    "%qD defined %<static%> in %qs function only available "
>                    "with %<-std=c++2b%> or %<-std=gnu++2b%>", decl,
>                    DECL_IMMEDIATE_FUNCTION_P (current_function_decl)
>                    ? "consteval" : "constexpr");
>        else
>          ok = true;
>        if (!ok)
>          cp_function_chain->invalid_constexpr = true;
>      }
> and at that point I fear decl_maybe_constant_var_p will not work
> properly.  Shall this hunk be moved somewhere else (cp_finish_decl?)
> where we can already call it, or do the above in start_decl for
> cxx_dialect < cxx20 and add a cxx_dialect == cxx20 hunk in cp_finish_decl?

Hmm, I'd expect decl_maybe_constant_var_p to work fine at this point.

Jason