Consider

-- optIntArg :: (Maybe Int -> t2) -> (t2,t2)
optIntArg f = (f Nothing, f (Just True))

This is rejected (by HEAD)

T11379a.hs:5:30: error:
    * Couldn't match type `a' with `Bool'
      `a' is a rigid type variable bound by
        a type expected by the context:
          forall a. Maybe a
        at T11379a.hs:5:30
      Expected type: forall a. Maybe a
        Actual type: Maybe Bool
    * In the first argument of `f', namely `(Just True)'
      In the expression: f (Just True)
      In the expression: (f Nothing, f (Just True))

but if you put the tuple components the other way round, it works fine

optIntArg f = (f (Just True), f Nothing)

Adding the commented-out signature also makes it work fine.

I'm almost certain that this is caused by visible type application; perhaps Nothing gets delayed instantiation, and then f's type becomes forall a. Maybe a. Utterly bogus.

I suspect it'll be fixed by Richards ReturnTv work, but let's make sure it is. We can't release this!!