FYI

>>846
Bug 68012: g++ incorrectly accepts forward declaration of constexpr variable template
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68012

>>848
One definition rule
https://timsong-cpp.github.io/cppwp/n4659/basic.def.odr#1
> No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template.