[Issue 16471] [CTFE] Incorrect CTFE when assigning to union struct fields

[via Digitalmars-d-bugs]

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

--- Comment #1 from Johan Engelen <jbc.engelen at gmail.com> ---
A simpler testcase:
```
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);
}
```

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