[Issue 6777] alias this disables casting for classes

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=6777

github-bugzilla at puremagic.com changed:

           What    |Removed                     |Added
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             Status|REOPENED                    |RESOLVED
         Resolution|---                         |FIXED

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