Re: [PATCH] c++, v4: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr functions

[Jason Merrill <jason@redhat.com>]

On 11/18/22 11:34, Jakub Jelinek wrote:
> On Fri, Nov 18, 2022 at 11:24:45AM -0500, Jason Merrill wrote:
>>> Right, that's the C++17 implicit constexpr for lambdas, finish_function:
>>>
>>>     /* Lambda closure members are implicitly constexpr if possible.  */
>>>     if (cxx_dialect >= cxx17
>>>         && LAMBDA_TYPE_P (CP_DECL_CONTEXT (fndecl)))
>>>       DECL_DECLARED_CONSTEXPR_P (fndecl)
>>>         = ((processing_template_decl
>>>             || is_valid_constexpr_fn (fndecl, /*complain*/false))
>>>            && potential_constant_expression (DECL_SAVED_TREE (fndecl)));
>>
>> Yeah, I guess potential_constant_expression needs to be stricter in a
>> lambda. Or perhaps any function that isn't already
>> DECL_DECLARED_CONSTEXPR_P?
> 
> potential_constant_expression can't be relied on that it catches up
> everything if it, even a simple if statement with a condition not yet
> known to be 0 or non-0 results in just a requirement that at least
> one of the substatements is potential constant, etc.
> Similarly switch statements etc.
> If there is a way to distinguish between functions with user
> specified constexpr/consteval and DECL_DECLARED_CONSTEXPR_P set
> through the above if condition, sure, cp_finish_decl ->
> check_static_in_constexpr could be perhaps silent about those, but then
> we want to diagnose it during constexpr evaluation at least.  But in that
> case having it a pedwarn rather than "this is a constant expression"
> vs. "this is not a constant expression, if !ctx->quiet emit an error"
> is something I don't see how to handle.  Because something needs
> to be returned, it is a constant expression or it is not.

True.  Let's go with your option 2, then, thanks.

Jason