[Bug c++/87389] if constexpr not working properly with static_assert

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87389

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Specifically, the not-taken branch of an if constexpr is only discarded when
inside a template. In a non-template both branches are compiled as normal, and
so both static assertions get compiled. Since both cannot possibly be true at
once, the program can never compile.