I’m not sure if the summary is the right way to say it, but the following C program fails a static assertion. Without the declaration at global scope it succeeds.

struct Foo {
    int x;
};
_Static_assert(sizeof(struct Foo) == sizeof(int), "");

void one(void){
    struct Foo {
        int y, z;
    };
    struct Foo f = {0};
    _Static_assert(sizeof(struct Foo) == 2*sizeof(int), "");
    struct Foo* pf = &f;
    pf->y = pf->z;
}

void two(void){
    struct Foo {
        int y, z;
    } f = {0};
    _Static_assert(sizeof(f) == 2*sizeof(int), "");
    _Static_assert(sizeof(struct Foo) == 2*sizeof(int), ""); // fails
    // if you comment out the above assertion, then you get the error below
    struct Foo* pf = &f;
    pf->y = pf->z; // Error `y` is not a member of `Foo`.
}