Coercible regression from 7.10 to HEAD

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.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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