Definition of a class member in the primary template and an implicit instantiation during specialization

Question

I have the following example that I've decomposed from §14.7.3/6 [temp.expl.spec] that defines a class member enumeration in the primary template and subsequently specializes it. The following doesn't compile in clang:

template<class T>
struct A {
    enum E : T;
};

template<class T>
enum A<T>::E : T { eT };

template<>
enum A<char>::E : char { echar }; // ill-formed, A<char>::E was instantiated
                                  // when A<char> was instantiated

// error: explicit specialization of 'E' after instantiation

The reason is supposed to be that the definition of the unscoped member enumeration was instantiated before the specialization. 14.7.1 [temp.inst]/1:

The implicit instantiation of a class template specialization causes the implicit instantiation of [...] the definitions of unscoped member enumerations and member anonymous unions.

I'm trying to understand why that is a problem exactly. Is it because if the enumeration already has a definition, then that would cause a redefinition error during the specialization?

Answer 1

you need to specialize for the whole class definition:

template<class T>
struct A {
  enum E : T { eT };
};

template<>
struct A<char> {
  enum E : char { echar };
};