Where should the definition of an explicit specialization of a class template be placed in C++?

Question

According to [temp.spec]/5:

For a given template and a given set of template-arguments,

• ...

• an explicit specialization s

Answer 1

This is a definition (the specialization will be instantiated), not a declaration, it probably ought to go in a specific source file (*.cpp):

template <> struct S;

Note: it doesn't "hurt" to have this in every translation unit... other than the time it takes to instantiate by the compiler, and the bloat for the object files (*.o or *.obj).

This is a declaration (the specialization will not be instantiated), and is okay to put in a header file (*.h):

extern template <> struct S;

The declaration requires C++11 or later.