Annex D (normative) Compatibility features [depr]

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

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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 ]