Is &errno legal C?

Question

Per 7.5,

[errno] expands to a modifiable lvalue175) that has type int, the value of which is set to a positive error number by several library functio

Answer 1

This seems like a valid implementation where &errno would be a constraint violation:

struct __errno_struct {
    signed int __val:12;
} *__errno_location(void);

#define errno (__errno_location()->__val)

So I think the answer is probably no...