Annex D (normative) Compatibility features [depr]

D.1 Redeclaration of static constexpr data members [depr.static_constexpr]

1
#
For compatibility with prior C++ International Standards, a constexpr static data member may be redundantly redeclared outside the class with no initializer.
This usage is deprecated.
[ Example
: 
struct A {
  static constexpr int n = 5;  // definition (declaration in C++ 2014)
};

constexpr int A::n;  // redundant declaration (definition in C++ 2014)
— end example
 ]