If we're forced into a situation where we need to factor it into a separate class and build aliases like this, then switching to undecidable instances and encoding that as

class (Monad m, Fail m) => MonadFail m

instance (Monad m, Fail m) => MonadFail m

etc.

would be much better than using the ConstraintKinds extension and a type alias. The type synonym you gave doesn't let you use "MonadFail" as a first class inhabitant of kind (* -> *) -> Constraint, you can only pass it once it has been applied to an `m`. This rules out many useful type level tricks.

-Edward