What optimizations are enabled by strict aliasing?

Question

Consider this example:

// standard-layout
struct S {
    float x;
    int y;
};

// not inlineable
void f(float* p); /* {
    reinterpret_cast<S*>(p)->y = 43;
} */

int test_f() {
    S s{0.0f, 42};
    // value of p is "pointer to s.x", s.x and s are pointer-interconvertible,
    // so f() could access s.y through p
    float *p = &s.x;
    f(p);
    return s.y; // not 42?
}

void g(int* p);

float test_g() {
    S s{0.0f, 42};
    // value of p is "pointer to s.y", s.y and s are not pointer-interconvertible,
    // so g() can't access s.x through p
    int *p = &s.y;
    g(p);
    return s.x; // always 0.0f
}

Am I correct to assume that f() is allowed access to s.y through &s.x? While test_g() always returns 0.0f, because g() may not access s.x through &s.y.

What if y were std::byte, would the more permissive aliasing preclude this optimization in test_g()?