class (or struct) self-reference by template

Question

Is the following legal?

template< typename T >
struct tree_node
   {
   T t;
   std::vector children;
   };

A commen

Answer 1

Actually, as a result of N4371 we have (from N4527, [vector.overview], will be in C++17):

An incomplete type T may be used when instantiating vector if the allocator satisfies the allocator completeness requirements 17.6.3.5.1. T shall be complete before any member of the resulting specialization of vector is referenced.

Prior to this, vector could not be constructed with an incomplete type (which tree_node is at that point), and that would be undefined behavior.