S>9.4.2 п.4
S>If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace
S>scope if it is used in the program and the namespace scope definition shall not contain an initializer.