[Issue 17206] [Tracking] Check that opEquals and toHash are both defined or neither are defined

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17206
Issue 17206 depends on issue 18676, which changed state.

Issue 18676 Summary: std.datetime.date.DateTime has opCmp but no toHash
https://issues.dlang.org/show_bug.cgi?id=18676

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

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