Possible type-checker regression in GHC 8.0

The following code fragment works with GHCs prior to GHC 8.0 but not with GHC 8.0:

{-# 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.
-}

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	bgamari, goldfire, simonpj
Operating system
Architecture