Constraints should commute over ~

Building the smallcheck package fails with this error:

Test/SmallCheck/Property.hs:187:10:
    Could not deduce (Monad n)
      arising from the superclasses of an instance declaration
    from the context: (Monad m, m ~ n)
      bound by the instance declaration
      at Test/SmallCheck/Property.hs:187:10-52
    Possible fix:
      add (Monad n) to the context of the instance declaration
    In the instance declaration for ‘Testable n (Property m)’

Wondering if this is a bug in 7.11's constraint solver or 7.10's, since 7.10 and previous GHCs accept it and 7.11 doesn't.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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