Why is partial specialization of a nested class template allowed, while complete isn't?

Question

    template struct A {                                                                                                    
        template

        

Answer 1

C++ Standard explicitly prohibits full specialization of member template classes in first case. According to 14.7.3/18:

In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well.