Annex D (normative) Compatibility features [depr]

D.6 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
]