Valid to assign to field of struct in union?

[Johan Engelen via Digitalmars-d]

Hi all,
  I have a question about the validity of this code:
```
void main()
{
    struct A {
        int i;
    }
    struct S
    {
        union U
        {
            A first;
            A second;
        }
        U u;
        this(A val)
        {
            u.second = val;
            assign(val);
        }

        void assign(A val)
        {
            u.first.i = val.i+1;
        }
    }
    enum a = S(A(1));

    assert(a.u.first.i == 2);
}
```

My question is: is it allowed to assign to a field of a struct 
inside a union, without there having been an assignment to the 
(full) struct before?

The compiler allows it, but it leads to a bug with CTFE of this 
code: the assert fails.
(changing `enum` to `auto` moves the evaluation to runtime, and 
all works fine)

Reported here: https://issues.dlang.org/show_bug.cgi?id=16471.