ImpredicativeTypes causes GHC panic with 8.0.1-rc1

I noticed this issue when attempting to compile conduit with GHC 8.0.1-rc1 (specifically, the Data.Conduit.Internal.Pipe module). This (greatly simplified) code, which compiles without issue on GHC 7.10.3:

{-# LANGUAGE ImpredicativeTypes #-}
module Data.Conduit.Internal.Pipe where

data Pipe o r =
    HaveOutput (Pipe o r) o

mapOutputMaybe :: (o1 -> Maybe o2) -> Pipe o1 r -> Pipe o2 r
mapOutputMaybe f (HaveOutput p o) =
  maybe id (\o' p' -> HaveOutput p' o') (f o) (mapOutputMaybe f p)

emits a GHC panic with GHC 8.0.1-rc1:

[1 of 1] Compiling Data.Conduit.Internal.Pipe ( Wat.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.0.20160111 for x86_64-unknown-linux):
	matchExpectedFunTys
  <>
  a_a15Z[tau:5] -> b_a15Y[tau:5]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Note that this code does not require -XImpredicativeTypes, and removing the pragma makes the code compile again.

Marking as high since it's a regression, but not highest because -XImpredicativeTypes has long been broken (see also #11319 (closed)). Still, this currently happens on code in the wild, and perhaps it would be worth turning this into a more sensible error.

Trac metadata

Trac field	Value
Version	8.0.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
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