Program which terminates instantly in GHC 8.0.2 runs for minutes with 8.2.1

This currently causes the vector test suite to loop forever (see 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:

{-# 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!

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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