[Bug c++/67155] Linking of complex constexpr static class member failing across files

[JamesMikeDuPont at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67155

--- Comment #2 from James Michael DuPont <JamesMikeDuPont at googlemail dot 
com> ---
adding this seems to solve the linker error:

constexpr CU S::u;