GHC 7.10 bug

Bug.hs

{-# LANGUAGE PartialTypeSignatures #-}
module Main where

data I a = I a
instance Functor I where
    fmap f (I a) = I (f a)

newtype B t a = B a
instance Functor (B t) where
    fmap f (B a) = B (f a)

newtype H f = H (f ())

app :: H (B t)
app = h (H . I) (B ())

h :: _
--h :: Functor m => (a -> b) -> m a -> H m
h f b = (H . fmap (const ())) (fmap f b)

-- the compiler crashes if the signature is partial (involves underscores)
--
-- it does not crash if the signature is specified completely or isn't
-- specified at all (inferred)

Error message

Bug.hs:15:10:
    Couldn't match type `b' with `H I'
      `b' is untouchable
        inside the constraints ()
        bound by the type signature for app :: H (B t)
        at Bug.hs:14:8-14ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.1 for x86_64-unknown-linux):
	No skolem info: b_a3gc[sk]