What does VMT mean? Virtual Method Table? When was this introduced to C? It looks like it was either in C89 or earlier according to both GCC and Clang.

#define ARR_LEN(a) (sizeof (a) / sizeof (a))

int main(void) { int a[5] = {0, 1, 2, 3, 4};

        int (*p)[ARR_LEN(a)] = &a;
        (*p)[6] = 1;
        return ARR_LEN(*p); // 5

}

I had no idea this was possible in C. It is unfortunate that GCC seems to ignore this information. Clang on the other hand uses it.