[Issue 15575] partial initialization of a union member causes wrong diagnostic during CTFE

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=15575

Basile-z <b2.temp at gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|ice                         |CTFE, rejects-valid
                 CC|                            |b2.temp at gmx.com
            Summary|Wrong value for static      |partial initialization of a
                   |immutable struct with union |union member causes wrong
                   |                            |diagnostic during CTFE

--- Comment #3 from Basile-z <b2.temp at gmx.com> ---
updated test case:

```
struct B15575 {
    union {
        ubyte[2] a;
        struct {
            ubyte b;
            ubyte c;
        }
    }

    this(ubyte b) {
        if (b)
            this.a[0] = b;
        else
            this.b = b;
        this.c = 0;
    }
}

void main()
{
    auto a = B15575(7).a;     // run-time OK
    pragma(msg, B15575(7).a); // CTFE NG
}     
```

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