This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
According to 220.127.116.11 [class.static.data] paragraph 3,
An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer. If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see _N4778_.D.4 [depr.static_constexpr]).
The out-of-class declaration of a static data member was formerly a definition and thus limited to occurring only once. This limitation was lost when the in-class declaration of inline static data members became the definition; the current specification has no apparent prohibition against multiple out-of-class declarations of a constexpr static data member. Should the restriction be reinstated?
Rationale (November, 2018):
Current implementations support this usage and it does not appear to cause any problems.