[temp.mem.class]

A member class of a class template may be defined outside the class template definition in which it is declared.

[Note 1:

The member class must be defined before its first use that requires an instantiation ([temp.inst]).

For example, template<class T> struct A { class B; }; A<int>::B* b1; / OK, requires A to be defined but not A::B template<class T> class A<T>::B { }; A<int>::B b2; / OK, requires A::B to be defined

— end note]