[Issue 18417] Make const and immutable postblit constructors illegal

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18417

Cauterite <cauterite at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |cauterite at gmail.com

--- Comment #1 from Cauterite <cauterite at gmail.com> ---
i'm wondering, after this is fixed, whether traits.hasElaborateCopyConstructor
should return false for immutable types?

currently it seems if a struct has a (mutable) postblit, there will be a
mutable __xpostblit that does the work, and an immutable __xpostblit that does
nothing.
hasElaborateCopyConstructor finds the second one and returns true.

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