Annex D (normative) Compatibility features [depr]

D.5 Redeclaration of static constexpr data members [depr.static.constexpr]

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