[GHC] #11347: No skolem info: b_azg[sk]

[GHC]

#11347: No skolem info: b_azg[sk]
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           Reporter:  nomeata        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           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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 I was testing this code, which is from our ICFP paper on Coercible:

 {{{#!hs
 {-# 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.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11347>
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