[GHC] #13536: Program which terminated in GHC 8.0.2 loops with 8.2.1

[GHC]

#13536: Program which terminated in GHC 8.0.2 loops with 8.2.1
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This currently causes the `vector` test suite to loop forever (see
 [https://github.com/haskell/vector/pull/161#issuecomment-292031845 here]).
 I've reproduced this with GHC 8.2.1 and HEAD. Unfortunately, it's not easy
 to isolate down to a file with no dependencies, so for now this requires
 `vector` and `QuickCheck` to reproduce. First, install them:

 {{{
 $ cabal install vector QuickCheck --allow-newer -w /opt/ghc/8.2.1/bin/ghc
 }}}

 Then take this file:

 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE TypeFamilies #-}
 module Main (main) where

 import qualified Data.Vector.Generic as V
 import qualified Data.Vector.Unboxed as DVU
 import Test.QuickCheck
 import Text.Show.Functions ()

 main :: IO ()
 main = do
   verboseCheck ((\f (i, b) v -> V.foldl f (i, b) v == foldl (\x -> f
 (unmodel x)) (i, b) (
 DVU.toList v)) :: ((Int, Bool) -> (Int, Bool) -> (Int, Bool))
                                     -> (Int, Bool) -> DVU.Vector (Int,
 Bool) -> Bool)
 instance (Arbitrary a, DVU.Unbox a) => Arbitrary (DVU.Vector a) where
     arbitrary = fmap DVU.fromList arbitrary

 class TestData a where
   type Model a
   unmodel :: Model a -> a

 instance TestData Bool where
   type Model Bool = Bool
   unmodel = id

 instance TestData Int where
   type Model Int = Int
   unmodel = id

 instance (Eq a, Eq b, TestData a, TestData b) => TestData (a,b) where
   type Model (a,b) = (Model a, Model b)
   unmodel (a,b) = (unmodel a, unmodel b)
 }}}

 Then compile it with `/opt/ghc/8.2.1/bin/ghc -O2 Main.hs` (the `-O2` part
 is important). Observe that running it never terminates.

 However, the same program //does// terminate when compiled with 8.0.2!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13536>
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