Can two different objects with automatic storage duration compare equal under address comparison?

Question

In particular, is it allowed for the addresses of two automatic variables in different functions to compare equal as follows:

sink.c

Answer 1

The C11 standard part 6.5.9/6 says:

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.

In this code none of the listed conditions hold; &f1 and &f2 are pointers to different objects, and one is not a subobject of the other.

So the pointers must not compare equal. The compiler reporting equal is non-conforming.

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Note: If anyone has doubts about the legality of Foo f1 = make(&f1);, see this question. It is fine and the automatic object's lifetime begins at the preceding {.