[GHC] #13658: Assertion failure on HEAD: "optCoercion changed types!"

[GHC]

#13658: Assertion failure on HEAD: "optCoercion changed types!"
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by slyfox):

 Slightly shorter example without foldl:

 {{{#!hs
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE NoImplicitPrelude #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE BangPatterns #-}

 {- # OPTIONS_GHC -Werror #-}
 {-# OPTIONS_GHC -g -O2 #-}

 module Bug (bug) where

 -- import GHC.Base (seq)
 import Unsafe.Coerce (unsafeCoerce)

 undefined :: a
 undefined = undefined

 data TypeRep (a :: k) where
     TrTyCon :: TypeRep (a :: k)
     TrApp   :: forall k1 k2 (a :: k1 -> k2) (b :: k1).
                TypeRep (a b)

 data SomeTypeRep where
     SomeTypeRep :: forall k (a :: k).
                    TypeRep a
                 -> SomeTypeRep

 mkTrApp :: forall k1 k2 (a :: k1 -> k2) (b :: k1).
            TypeRep (a :: k1 -> k2)
         -> TypeRep (a b)
 mkTrApp TrTyCon = undefined
 mkTrApp TrApp   = undefined

 bug :: SomeTypeRep
 -- bug = f x -- this works
 bug = f (f x)
   where x = SomeTypeRep TrTyCon
         f :: SomeTypeRep -> SomeTypeRep
         f (SomeTypeRep acc) = SomeTypeRep (mkTrApp (unsafeCoerce acc))
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13658#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler