https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80829
--- Comment #1 from Benjamin Bannier <bbannier+gcc_bugs at gmail dot com> --- We believe this code is legal C++11 (AFAICT no explicit restrictions on implicit conversions of `constexpr` vars), and was compiling successfully with e.g., 6.3, so we strongly suspect a GCC regression.
