13 Templates [temp]

13.7 Template declarations [temp.decls]

13.7.1 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.
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