[Issue 21357] [REG2.093] postblit aliases old and new struct pointers

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21357

--- Comment #3 from Iain Buclaw <ibuclaw at gdcproject.org> ---
(In reply to Boris Carvajal from comment #2)
> Caused by:
> 
> commit 68275da1a9785ad0bf15782da6502948171271bd
> Author: Martin Kinkelin <noone at nowhere.com>
> Date:   Wed May 27 00:58:09 2020 +0200
> 
>     Fix Issue 11292 - Cannot re-initialize a const field in postblit

Confirmed.  The change alters the AssignExp `this.low = this.low.copy()` into a
ConstructExp.  This is wrong, postblits don't initialize new memory.

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