NULL is not single type in any conventional sense (and is actually tricky to define in a way that makes it usable in the way most programmers expect).

Thus:

  T1* a = NULL;
  T2* b = NULL
  a == b; /* may be undefined at present, depending on the nature of T1 & T2 */

"NULL" in fact is a macro, not a part of the language. null (zero pointer) is, and it is explicitly defined in standard, that comparison of two null pointers lead to equality. You example simply won't compile, it is not undefined; the pointers simply are of different type, period.

here what standard says:

"A pointer to void may be converted to or from a pointer to any object type.

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal."

therefore, convert any of them or both to void amd compare. you'll get equality.