New GeneralisedNewtypeDeriving needs help with higher rank types

Consider

{-# LANGUAGE RankNTypes, GeneralizedNewtypeDeriving #-}
module Foo where

class C a where
  op :: (forall b. b -> a) -> a

newtype T x = MkT x deriving( C )

With the new "coerce" implementation of GND, this fails:

Foo.hs:7:31:
    Cannot instantiate unification variable ‛b0’
    with a type involving foralls: (forall b. b -> T x) -> T x
      Perhaps you want ImpredicativeTypes
    In the expression:
        GHC.Prim.coerce (op :: (forall (b :: *). b -> x) -> x) ::
          (forall (b :: *). b -> T x) -> T x
    In an equation for ‛op’:
        op
          = GHC.Prim.coerce (op :: (forall (b :: *). b -> x) -> x) ::
              (forall (b :: *). b -> T x) -> T x

We want to coerce betweeen

   (forall b. b -> x) -> x   ~R   (forall b. b -> T x) -> T x

There are two difficulties with the new coerce approach:

• Regarded as source code, instance declaration

  instance C x => C (T x) where
    op = coerce (op :: (forall b. b -> x) -> x)

requires impredicative instantiation.

• We probably don't have a decomposition rule for Coercible (forall a. t1) (forall a. t2)

There is no difficulty in principle here, but it's not quite obvious what the best approach to a fix is. But it would be good to fix before release; we don't want to break conduit

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Blocking
CC	goldfire, nomeata
Operating system
Architecture