"opt_univ fell into a hole" panic from rewrite rule

Roboguy reports (via IRC) that the following causes a GHC panic:

{-# LANGUAGE RankNTypes #-}

module Crash where

data Exp a = Exp

fromExp :: Exp a -> a
fromExp _ = error "Impossible"

toExp :: a -> Exp a
toExp _ = Exp



newtype Iter a b = Iter { getIter :: forall r. (a -> r) -> (b -> r) -> r }

iterLoop :: (a -> Iter a b) -> a -> b
iterLoop f x = getIter (f x) (iterLoop f) id


  -- This rewrite rule results in a GHC panic: "opt_univ fell into a hole" on GHC 8.0.1, 8.0.2, and 8.1.
{-# RULES "fromExp-into-iterLoop" [~]
    forall (f :: Int -> Iter (Exp Int) (Exp Char))
           (init :: Int).
    fromExp (iterLoop f init)
      =
    fromExp (iterLoop (f . fromExp) (toExp init))
  #-}

I have confirmed this, and see the following error:

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20170305 for x86_64-unknown-linux):
        opt_univ fell into a hole
  {a1WD}
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1191:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in ghc:Outputable
        pprPanic, called at compiler/types/OptCoercion.hs:398:29 in ghc:OptCoercion

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	roboguy
Operating system
Architecture