No skolem info: b_azg[sk]

I was testing this code, which is from our ICFP paper on Coercible:

{-# LANGUAGE TypeFamilies, GeneralizedNewtypeDeriving, MultiParamTypeClasses, FlexibleInstances, AllowAmbiguousTypes #-}

newtype Id1 a = MkId1 a
newtype Id2 a = MkId2 (Id1 a) deriving (UnsafeCast b)

type family Discern a b
type instance Discern (Id1 a) b = a
type instance Discern (Id2 a) b = b

class UnsafeCast to from where
  unsafe :: from -> Discern from to

instance UnsafeCast b (Id1 a) where
  unsafe (MkId1 x) = x

unsafeCoerce :: a -> b
unsafeCoerce x = unsafe (MkId2 (MkId1 x))

without AllowAmbiguousTypes I get

UnsafeCast.hs:11:3: error:
    Couldn't match type ‘Discern from to0’ with ‘Discern from to’
    NB: ‘Discern’ is a type function, and may not be injective
    The type variable ‘to0’ is ambiguous
    Expected type: from -> Discern from to
      Actual type: from -> Discern from to0
    In the ambiguity check for the type signature for ‘unsafe’:
      unsafe :: forall to from.
                UnsafeCast to from =>
                from -> Discern from to
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    When checking the class method:
      unsafe :: forall to from.
                UnsafeCast to from =>
                from -> Discern from to
    In the class declaration for ‘UnsafeCast’

(is that a bug? I feel like it could be, but I’m intimidated by the error message).

So I put in the suggested pragma, and now I get

UnsafeCast.hs:4:41: error:ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.11.20151111 for x86_64-unknown-linux):
	No skolem info: b_azg[sk]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

This is GHC-almost-HEAD (changeset:2f6e87/ghc). I’ll start a rebuild with head and see what has changed.

Trac metadata

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