Annex D (normative) Compatibility features [depr]

D.6 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
 ]