[GHC] #14933: DeriveAnyClass can cause "No skolem info" GHC panic

[GHC]

#14933: DeriveAnyClass can cause "No skolem info" GHC panic
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
  (Type checker)                     |
           Keywords:  deriving       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This program panics:

 {{{#!hs
 {-# LANGUAGE DefaultSignatures          #-}
 {-# LANGUAGE DeriveAnyClass             #-}
 {-# LANGUAGE DeriveGeneric              #-}
 {-# LANGUAGE DerivingStrategies         #-}
 {-# LANGUAGE FlexibleContexts           #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE RankNTypes                 #-}
 {-# LANGUAGE ScopedTypeVariables        #-}
 {-# LANGUAGE TypeFamilies               #-}
 module Bug where

 import Control.Concurrent (ThreadId)
 import Control.Monad.Reader

 class Wrapped s where
   type Unwrapped s :: *
   _Wrapped' :: Iso' s (Unwrapped s)

 type Iso' s a = forall f. Functor f => (a -> f a) -> s -> f s

 class Fork m where
     fork :: x -> m () -> m ThreadId

     default fork :: ( Wrapped (m ())
                     , Unwrapped (m ()) ~ t ()
                     , Fork t
                     , Wrapped (m ThreadId)
                     , Unwrapped (m ThreadId) ~ t ThreadId
                     ) => x -> m () -> m ThreadId
     fork = undefined -- view _Unwrapped' . fork . view _Wrapped'

 instance Fork m => Fork (ReaderT e m) where
     fork x action = ReaderT $ \env -> fork x (runReaderT action env)

 data Env

 newtype MyThing m a = MyThing { unMyThing :: ReaderT Env m a }
     deriving newtype  (Functor, Applicative, Monad)
     deriving anyclass (Fork)

 instance Wrapped (MyThing m a) where
     type Unwrapped (MyThing m a) = ReaderT Env m a
     _Wrapped' = undefined -- iso unMyThing MyThing
 }}}

 {{{
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:39:24: error:ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.2 for x86_64-unknown-linux):
         No skolem info:
   m_a1Hs[sk:2]
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/TcErrors.hs:2653:5 in
 ghc:TcErrors
 }}}

 (Program adapted from
 [https://github.com/ekmett/lens/issues/793#issuecomment-369597846 here].)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14933>
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