[GHC] #11364: Possible type-checker regression in GHC 8.0

[GHC]

#11364: Possible type-checker regression in GHC 8.0
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           Reporter:  hvr            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.3
  (Type checker)                     |
           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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 The following code fragment works with GHCs prior to GHC 8.0 but not with
 GHC 8.0:

 {{{#!hs
 {-# LANGUAGE RoleAnnotations #-}
 {-# LANGUAGE IncoherentInstances #-}
 {-# LANGUAGE RankNTypes #-}

 module Issue where

 import Data.Coerce (coerce)

 data Proxy a = Proxy

 newtype Catch a = Catch a

 class Throws e

 type role Throws representational

 instance Throws (Catch e)

 newtype Wrap e a = Wrap { unWrap :: Throws e => a }

 coerceWrap :: Wrap e a -> Wrap (Catch e) a
 coerceWrap = coerce

 unthrow :: proxy e -> (Throws e => a) -> a
 unthrow _ = unWrap . coerceWrap . Wrap

 {- this works in GHC 7.10.3, but fails in GHC 8.0 with

 GHCi, version 8.0.0.20160105: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Issue            ( Issue.hs, interpreted )

 Issue.hs:25:13: error:
     • Could not deduce (Throws e)
       from the context: Throws e0
         bound by a type expected by the context:
                    Throws e0 => a
         at Issue.hs:26:13-38
       Possible fix:
         add (Throws e) to the context of
           the type signature for:
             unthrow :: proxy e -> (Throws e => a) -> a
     • In the expression: unWrap . coerceWrap . Wrap
       In an equation for ‘unthrow’: unthrow _ = unWrap . coerceWrap . Wrap
 Failed, modules loaded: none.
 -}
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11364>
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