Each of the following unions give the same error:

arst.d(1): Error: union arst.X overlapping initialization for struct X.y
arst.d(5): Error: union arst.Y overlapping initialization for struct Y.y
arst.d(9): Error: union arst.Z overlapping initialization for struct Z.y

union X {
	int x = void;
	int y = void;
}
union Y {
	int x = void;
	int y = 0;
}
union Z {
	int x = 0;
	int y = void;
}

Removing the "= void", these unions compile (except, for some reason, Y, but that's a separate issue).