Polymorphic kind annotations on type classes don't always work as expected

Consider the following code for defining Atkey-style parameterised monads:

{-# LANGUAGE
    PolyKinds
 #-}

class PMonad m where
  ret  :: a -> m i i a
  bind :: m i j a -> (a -> m j k b) -> m i k b
class PMonad' (m :: i -> i -> * -> *) where
  ret'  :: a -> m i i a
  bind' :: m i j a -> (a -> m j k b) -> m i k b

The following types are inferred for ret and ret':

*Main> :t ret
ret :: PMonad * m => a -> m i i a
*Main> :t ret'
ret' :: PMonad' m => a -> m BOX BOX a

But ret' should have the former type.

Trac metadata

Trac field	Value
Version	7.6.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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