[Bug c++/100995] Extend std::is_constant_evaluated in if warning

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100995

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:26dbe85a3781af913639b17bc966f4a0b8209f3b

commit r12-1375-g26dbe85a3781af913639b17bc966f4a0b8209f3b
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Jun 9 15:18:39 2021 -0400

    c++: Extend std::is_constant_evaluated in if warning [PR100995]

    Jakub pointed me at
   
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1938r3.html#compiler-warnings>
    which shows that our existing warning could be extended to handle more
    cases.  This patch implements that.

    A minor annoyance was handling macros, in libstdc++ we have

      reference operator[](size_type __pos) {
          __glibcxx_assert(__pos <= size());
          ...
      }

    wherein __glibcxx_assert expands to

      if (__builtin_is_constant_evaluated() && !bool(__pos <= size())
        ...

    but I'm of a mind to not warn on that.

    Once consteval if makes it in, we should tweak this warning one more
    time.

            PR c++/100995

    gcc/cp/ChangeLog:

            * constexpr.c (maybe_constexpr_fn): New.
            * cp-tree.h (maybe_constexpr_fn): Declare.
            * semantics.c (find_std_constant_evaluated_r): New.
            (maybe_warn_for_constant_evaluated): New.
            (finish_if_stmt_cond): Call it.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/is-constant-evaluated9.C: Add dg-warning.
            * g++.dg/cpp2a/is-constant-evaluated12.C: New test.