[Issue 18561] postblit should allow writing const/immutable members just like constructors

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18561

--- Comment #8 from Ate Eskola <Ajieskola at gmail.com> ---
And the issue is in practice solved by copy constructors anyway, thanks to you
:). They didn't exist when I raised this issue but now they do.

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