Rarely implemented part of C++

This post is to show the relevant part of the standard which relates to the following:

This is a little known and a rarely implement part of C++. A constant integral class member type can be initialised in-line yet according to the standard it also requires a definition without an initial value if it is used.

9.4.2.4
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 scope if it is used in the program and the namespace scope definition shall not contain an initializer.

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