Annex D (normative) Compatibility features [depr]

D.4 Redeclaration of static constexpr data members [depr.static.constexpr]

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
]