[palunon]

>For example 0 is a valid address in many cases (even in some versions of UNIX).

0 may be a valid address at runtime, but a NULL pointer is always invalid.

On such platforms, the compiler should handle 0 pointer values correctly - and the NULL pointer may not have a 0 value, and must not compare equal to any valid pointer.

But 0 or NULL constant, when converted to a pointer type, MUST result in a NULL pointer value - which may be nonzero. Dereferencing such a pointer is an UB.