Type checker fails to recognize equality

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE KindSignatures #-}
module Test where

import qualified Control.Monad.State as S

class HasConns (m :: * -> *) where
    type Conn m

foo :: (HasConns m, Monad m) => S.StateT (Conn m) m ()
foo =
    do _ <- S.get
       return ()

This fails to compile with GHC 7.10.2:

    Could not deduce (S.MonadState (Conn m) (S.StateT (Conn m) m))
      arising from a use of ‘S.get’
    from the context (HasConns m, Monad m)
      bound by the type signature for
                 foo :: (HasConns m, Monad m) => S.StateT (Conn m) m ()

It compiles with GHC 7.8.4, though. Adding "Conn m ~ Conn m" to the type context of foo lets it compile again. This seems not right to me.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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