Given the following example:
---
struct foo {
        int a;
        union {
                int a;
                double b;
        };
};

int main(void)
{
        struct foo f;
        f.a = 123;
        return 0;
}
---

How it came to be:
Accidental error where I had forgotten to remove foo.a after having added foo.{anonymous}.a.

What has been observed:
It compiles fine, which caused this programming error of mine to go unnoticed for half a day. It seems gcc gives foo.a precedence over foo.{anonymous}.a when accessing it through f.a=....

What had been expected:
I would have liked that gcc emit at least a warning because f.a is ambiguous.