[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 #5 from Steven Schveighoffer <schveiguy at yahoo.com> ---
(In reply to anonymous4 from comment #3)
>     a.__ctor(1);

This is another bug. One should only be able to call const __ctor on a struct
once, before using it.

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