[GHC] #11428: ImpredicativeTypes causes GHC panic with 8.0.1-rc1

[GHC]

#11428: ImpredicativeTypes causes GHC panic with 8.0.1-rc1
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  ImpredicativeTypes                 |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I noticed this issue when attempting to compile `conduit` with GHC
 8.0.1-rc1 (specifically, the
 [https://github.com/snoyberg/conduit/blob/master/conduit/Data/Conduit/Internal/Pipe.hs
 Data.Conduit.Internal.Pipe] module). This (greatly simplified) code, which
 compiles without issue on GHC 7.10.3:

 {{{#!hs
 {-# 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). Still, this
 currently happens on code in the wild, and perhaps it would be worth
 turning this into a more sensible error.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11428>
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