Terrible failure of type inference in visible type application

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!!

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
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