Consider the following example:
class A : public list < A >
Does your compiler accept it? According to the Standard, using an incomplete type as a template parameter in a template instantiation results in undefined behavior. In this example, A is an incomplete type when it is used as a template argument of list. Certain compilers, GCC for instance, do accept this code. However, most compilers don't. For this reason, you should avoid this programming style if you're writing portable code.