13 Templates [temp]

13.7 Template declarations [temp.decls]

13.7.2 Class templates [temp.class] Member classes of class templates [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]