Illegal variance position in Mods

[mielientiev]

https://gist.github.com/mielientiev/1873b2aea8f6f2bff81e13b597a7849c

In Scala 2.11 that code compiles. (due to bug in type checker). After https://github.com/scala/scala#5280 fix
But now it fails because of type alias is in the contravariant position.
The workaround is to add @uncheckedVariance.

We should spend some time for reviewing this problem and find better solution