[Issue 13537] Unions may break immutability

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13537

--- Comment #1 from hst...@quickfur.ath.cx ---
Basically, immutable cannot be allowed to overlap with anything mutable,
otherwise there will be a way to break the immutability guarantee.

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