I spoke too soon; I'm seeing the same problem with MonadError.

    Prelude> :set -XFlexibleContexts

    Prelude> import Control.Monad.Except

    Prelude Control.Monad.Except> let f = (fail "be Left!" :: (MonadError String m) => m ())

    Loading package transformers-0.4.2.0 ... linking ... done.

    Loading package mtl-2.2.1 ... linking ... done.

    Prelude Control.Monad.Except> f :: Either String ()

    *** Exception: be Left!

    Prelude Control.Monad.Except>