Re: [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
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by MitchellSalad:

Old description:
...
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
New description:

 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)   #-}

 }}}

 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#comment:1
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