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

[Jakub Jelinek <jakub@redhat.com>]

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.

	Jakub