Patch: <https://gcc.gnu.org/pipermail/gcc-patches/2022-November/605773.html>.

https://gcc.gnu.org/g:32d16fe9d7e347bc58e7fad316ed7923e1d0f65c

commit r13-4161-g32d16fe9d7e347bc58e7fad316ed7923e1d0f65c
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sat Nov 19 09:26:44 2022 +0100

    c++: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr functions
    
    The following patch implements this paper.
    Per further discussions it is implemented for C++23 only, so isn't
    treated as a DR, e.g. because the part of the standard the paper is
    changing didn't even exist in C++20.
    And we gave up on trying to implement it as a pedwarn rather than
    error for C++20 and older, because of implicit constexpr lambdas or
    -fimplicit-constexpr reasons.
    For C++20 and older, the only change is that passing through
    definitions of static or thread_local vars usable in constant expressions
    is now accepted in statement expressions if they aren't inside of constexpr
    or consteval functions.
    
    2022-11-19  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_EXPRs of decl_constant_var_p static or thread_local vars.
            (potential_constant_expression_1): Similarly, except use
            decl_maybe_constant_var_p instead of decl_constant_var_p if
            processing_template_decl.
    gcc/testsuite/
            * g++.dg/cpp23/constexpr-nonlit17.C: New test.
            * g++.dg/cpp23/constexpr-nonlit18.C: New test.
            * g++.dg/cpp23/feat-cxx2b.C: Adjust expected __cpp_constexpr
            value.
            * g++.dg/ext/stmtexpr19.C: Don't expect an error.
            * g++.dg/ext/stmtexpr25.C: New test.