[GHC] #10642: Coercible regression from 7.10 to HEAD

15 Jul 2015

      #10642: Coercible regression from 7.10 to HEAD
-------------------------------------+-------------------------------------
              Reporter:  darchon     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 This started out with code that compiled on 7.10, but fails on HEAD
 (20150711):

 {{{
 {-# LANGUAGE TypeFamilies, StandaloneDeriving, UndecidableInstances #-}
 module StandaloneDeriving where

 type family F a

 newtype D a = D (F a)

 -- | This works on 7.10.1 and HEAD (20150711)
 deriving instance Eq (F a) => Eq (D a)

 -- | This works on 7.10.1, but fails on HEAD (20150711)
 deriving instance Bounded (F a) => Bounded (D a)
 }}}

 which fails on HEAD with:

 {{{
 GHCi, version 7.11.20150711: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling StandaloneDeriving ( StandaloneDeriving.hs, interpreted
 )

 StandaloneDeriving.hs:12:1: error:
     Couldn't match representation of type ‘a0’ with that of ‘F a’
       arising from a use of ‘coerce’
     Relevant bindings include
       minBound :: D a (bound at StandaloneDeriving.hs:12:1)
     In the expression: coerce (minBound :: F a) :: D a
     In an equation for ‘minBound’:
         minBound = coerce (minBound :: F a) :: D a
     When typechecking the code for ‘minBound’
       in a derived instance for ‘Bounded (D a)’:
       To see the code I am typechecking, use -ddump-deriv
     In the instance declaration for ‘Bounded (D a)’

 StandaloneDeriving.hs:12:1: error:
     Couldn't match representation of type ‘a1’ with that of ‘F a’
       arising from a use of ‘coerce’
     Relevant bindings include
       maxBound :: D a (bound at StandaloneDeriving.hs:12:1)
     In the expression: coerce (maxBound :: F a) :: D a
     In an equation for ‘maxBound’:
         maxBound = coerce (maxBound :: F a) :: D a
     When typechecking the code for ‘maxBound’
       in a derived instance for ‘Bounded (D a)’:
       To see the code I am typechecking, use -ddump-deriv
     In the instance declaration for ‘Bounded (D a)’
 Failed, modules loaded: none.
 }}}

 Which I managed to reduce to:

 {{{
 {-# LANGUAGE TypeFamilies, FlexibleContexts #-}
 module CoerceFail where

 import Data.Coerce

 type family F a

 newtype D a = D (F a)

 -- | This works on 7.10.1, but fails on HEAD (20150711)
 coerceD :: Coercible (F a) (D a) => F a -> D a
 coerceD = coerce
 }}}

 Which also works on 7.10.1 but fails on HEAD with:

 {{{
 GHCi, version 7.11.20150711: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling CoerceFail       ( CoerceFail.hs, interpreted )

 CoerceFail.hs:12:11: error:
     Couldn't match representation of type ‘a0’ with that of ‘F a’
       arising from a use of ‘coerce’
     Relevant bindings include
       coerceD :: F a -> D a (bound at CoerceFail.hs:12:1)
     In the expression: coerce
     In an equation for ‘coerceD’: coerceD = coerce
 }}}

 I don't know if this was never supposed to work, and the behaviour on HEAD
 is correct, or, if this is truly a regression from 7.10 to HEAD.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10642
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