For compatibility with prior revisions of C++, a constexpr
static data member may be redundantly redeclared outside the class with no
initializer ([basic.def], [class.static.data]).
[Example 1: struct A {staticconstexprint n =5; / definition (declaration in C++ 2014)};
constexprint A::n; / redundant declaration (definition in C++ 2014) — end example]