[GHC] #14732: -fdefer-type-holes breaks a correct program

28 Jan 2018


      #14732: -fdefer-type-holes breaks a correct program
-------------------------------------+-------------------------------------
           Reporter:  MitchellSalad  |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Here's a bug I discovered in `vector` that Ryan Scott identified as a
 regression from 7.10.3:

 https://github.com/haskell/vector/issues/200

 Here is Ryan's minimal example:

 {{{#!hs

 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 module Bug where

 import Prelude hiding (zip, zipWith)

 zipWith :: (a -> b -> c)
         -> Bundle v a
         -> Bundle v b
         -> Bundle v c
 zipWith = undefined

 class GVector (v :: * -> *) a
 instance GVector Vector a

 data Bundle (v :: * -> *) a
 data Vector a
 class Unbox a

 stream :: GVector v a => v a -> Bundle v a
 {-# INLINE [1] stream #-}
 stream = undefined

 zip :: (Unbox a, Unbox b) => Vector a -> Vector b -> Vector (a, b)
 {-# INLINE [1] zip #-}
 zip = undefined
 {-# RULES "stream/zip [Vector.Unboxed]" forall as bs .
   stream (zip as bs) = zipWith (,) (stream as)
                                    (stream bs)   #-}{#!hs

 }}}

 Output (8.2.2):

 {{{
 Bug.hs:29:11: error:
     • Could not deduce (Unbox a) arising from a use of ‘zip’
       from the context: GVector Vector (a, b)
         bound by the RULE "stream/zip [Vector.Unboxed]"
         at Bug.hs:(28,11)-(30,46)
       Possible fix:
         add (Unbox a) to the context of
           the RULE "stream/zip [Vector.Unboxed]"
     • In the first argument of ‘stream’, namely ‘(zip as bs)’
       In the expression: stream (zip as bs)
       When checking the transformation rule "stream/zip [Vector.Unboxed]"
    |
 29 |   stream (zip as bs) = zipWith (,) (stream as)
    |           ^^^^^^^^^
 }}}

 Similar output in 8.0.1, 8.0.2, 8.2.1

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14732
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