[Bug c++/91428] Please warn on if constexpr (std::is_constant_evaluated())

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91428

--- Comment #5 from Marc Glisse <glisse at gcc dot gnu.org> ---
Would it make sense to add a fixit hint that removes "constexpr"? I think that
might make the warning a bit clearer for some users.
On the other hand, if is_constant_evaluated gets removed by P1938, there is no
point putting more work into it.