[Bug c++/80829] Use of constexpr constructors with base type instantiation fails compilation

[bbannier+gcc_bugs at gmail dot com]

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.