[Issue 23780] Manual __ctor call can mutate immutable object in @safe code

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23780

--- Comment #2 from Paul Backus <snarwin+bugzilla at gmail.com> ---
Unfortunately this will probably be tricky to fix, because a lot of valid code
gets lowered to `obj.__ctor` internally by the DMD frontend.

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