[Issue 21821] Optimizer assumes immutables do not change, but they can in @system code

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21821

Mathias LANG <pro.mathias.lang at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |pro.mathias.lang at gmail.com

--- Comment #2 from Mathias LANG <pro.mathias.lang at gmail.com> ---
Where is this specified ? `immutable` should never change, that's the point!
I've never seen, nor heard, that `immutable` could be violated in `@system`.
This code in `core.lifetime` exhibits UB and should be changed to apply
`immutable` after the `memcpy`, e.g. as `assummeUnique` does.

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