[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
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             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:                    |
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Comment (by RyanGlScott):

 Actually, `vector` isn't needed here. Here's an example that only requires
 `QuickCheck`:

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

 import Test.QuickCheck
 import Text.Show.Functions ()

 main :: IO ()
 main = verboseCheck foldlTest

 type FoldlTest a = (a -> a -> a) -> a -> [a] -> Bool

 foldlTest :: FoldlTest (Int, Int)
 foldlTest f (i, b) v =
   foldl f (i, b) v == foldl (\x -> f (unmodel x)) (i, b) v

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

 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)
 }}}

 Another observation is that the type `(Int, Int)` is crucial for
 triggering the infinite loop. If you use, say, `FoldlTest Int` instead of
 `FoldlTest (Int, Int)`, then it terminates again.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13536#comment:2>
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