[Issue 12885] const union wrongly converts implicitly to mutable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=12885
Issue 12885 depends on issue 12883, which changed state.

Issue 12883 Summary: implicit conversion of struct to mutable via alias this
https://issues.dlang.org/show_bug.cgi?id=12883

           What    |Removed                     |Added
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             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |---

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