Equivalence of p[0] and *p for incomplete array types

Question

Consider the following code (it came about as a result of this discussion):

#include 

void foo(int (*p)[]) {          // Argument has incompl         


        

Answer 1

Section 6.5.2.1 of n1570, Array subscripting:

Constraints

One of the expressions shall have type ‘‘pointer to complete object type’’, the other expression shall have integer type, and the result has type ‘‘type’’.

So the standard forbids the expression p[0] if p is a pointer to an incomplete type. There is no such restriction for the indirection operator *.

In older versions/drafts of the standard, however, (n1256 and C99), the word "complete" is absent in that paragraph. Not being involved in any way in the standard procedure, I can only guess whether it's a breaking change or the correction of an omission. The behaviour of the compiler suggests the latter. That is reinforced by the fact that p[i] is per the standard identical to *(p + i) and the latter expression doesn't make sense for a pointer to an incomplete type, so for p[0] to work if p is a pointer to an incomplete type, an explicit special case would be needed.