[Issue 6777] alias this disables casting for classes

[d-bugmail at puremagic.com]

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

calex <calex+bugzilla-mail at aristoweb.net> changed:

           What    |Removed                     |Added
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                 CC|                            |calex+bugzilla-mail at aristow
                   |                            |eb.net

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