illegal member access of incomplete type

[kornelski]

This pattern is used in my <math.h> (OS X 10.11, clang 8.0.0 (clang-800.0.38))

void test() {
    union { float a; unsigned int b; } c;
    c.a = 0;
}

I don't see any problem with it. The type is complete. It is unsafe by Rust's standards, but I can't help what <math.h> does :(