[Bug c++/114580] Bogus warning on if constexpr

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114580

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:ae3b6dea0445f9650cf1a684527efac06497f1b4

commit r13-8629-gae3b6dea0445f9650cf1a684527efac06497f1b4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Apr 9 09:31:42 2024 +0200

    c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]

    When looking at maybe_warn_for_constant_evaluated for the trivial
    infinite loops patch, I've noticed that it can emit weird diagnostics
    for if constexpr in templates, first warn that std::is_constant_evaluted()
    always evaluates to false (because the function template is not constexpr)
    and then during instantiation warn that std::is_constant_evaluted()
    always evaluates to true (because it is used in if constexpr condition).
    Now, only the latter is actually true, even when the if constexpr
    is in a non-constexpr function, it will still always evaluate to true.

    So, the following patch fixes it to call maybe_warn_for_constant_evaluated
    always with IF_STMT_CONSTEXPR_P (if_stmt) as the second argument rather
than
    true if it is if constexpr with non-dependent condition etc.

    2024-04-09  Jakub Jelinek  <jakub@redhat.com>

            PR c++/114580
            * semantics.cc (finish_if_stmt_cond): Call
            maybe_warn_for_constant_evaluated with IF_STMT_CONSTEXPR_P
(if_stmt)
            as the second argument, rather than true/false depending on if
            it is if constexpr with non-dependent constant expression with
            bool type.

            * g++.dg/cpp2a/is-constant-evaluated15.C: New test.

    (cherry picked from commit cfed80b9e4f562c99679739548df9369117dd791)