struct A
{  
    this(uint a, uint b)
    {
	begin = a;
	end = b;
    }
    union
    {
        struct
        {
            uint begin, end;
        }
	uint[2] arr;
    }    
}

void main(){
	enum y = A(10, 20);
	A y2 = A(10, 20); 
	assert(y2.begin == y.begin && y2.end == y.end);  //passes
	assert(y.arr == [0,0]);  //passes, wtf ?!
	assert(y.arr == y2.arr); //fails
}

I dunno what was the last stance on unions in CTFE, but this bug also affects constfolded globals (which is tricky and nasty). So at minimum it's better to just reject it.