immutable promise broken in unions?

[tsbockman via Digitalmars-d-learn]

On Saturday, 2 January 2016 at 13:15:37 UTC, John Colvin wrote:
> Casting away immutable can sometimes be necessary (e.g. when 
> talking to other languages), so I'm not sure it should be 
> disallowed, but it'd be great if it was somehow easier to catch 
> these bugs.

It should be disallowed in @safe code, though.