[Bug c++/88815] [9 Regression] is_constexpr (based on narrowing conversion and expression SFINAE) broken

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88815

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
That is most likely a bug in the package.  GCC 8 wasn't very good at detecting
narrow conversions, but in GCC 9 things have improved, so probably we just
detect invalid code now.  The problem in this PR is that we still don't detect
narrow conversions in certain contexts (here, decltype).