[GHC] #13951: InScope set assertion failure from monad-skeleton

[GHC]

#13951: InScope set assertion failure from monad-skeleton
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
           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:
-------------------------------------+-------------------------------------
 {{{
 {-# LANGUAGE PolyKinds, GADTs, Rank2Types, ScopedTypeVariables,
 Trustworthy #-}
 module Control.Monad.Skeleton.Internal where

 data Cat k a b where
   Empty :: Cat k a a
   Leaf :: k a b -> Cat k a b
   Tree :: Cat k a b -> Cat k b c -> Cat k a c

 viewL :: forall k a b r. Cat k a b
   -> ((a ~ b) => r)
   -> (forall x. k a x -> Cat k x b -> r)
   -> r
 viewL Empty e _ = e
 viewL (Leaf k) _ r = k `r` Empty
 viewL (Tree a b) e r = go a b where
   go :: Cat k a x -> Cat k x b -> r
   go Empty t = viewL t e r
   go (Leaf k) t = r k t
   go (Tree c d) t = go c (Tree d t)
 }}}

 Leads to the assertion failure

 {{{
 [1 of 1] Compiling Control.Monad.Skeleton.Internal ( Internal.hs,
 Internal.o )
 WARNING: file compiler/simplCore/OccurAnal.hs, line 2160 Just 3 []
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170708 for x86_64-unknown-linux):
         ASSERT failed!
   in_scope InScope {x_avF ds_d14c}
   tenv [avF :-> x_avF]
   tenvFVs [avF :-> x_avF, a11Z :-> k_a11Z]
   cenv []
   cenvFVs []
   tys [k1_a120 a_a121 x_avF]
   cos []
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1188:22 in
 ghc:Outputable
         assertPprPanic, called at compiler/types/TyCoRep.hs:2088:56 in
 ghc:TyCoRep
         checkValidSubst, called at compiler/types/TyCoRep.hs:2121:29 in
 ghc:TyCoRep
         substTy, called at compiler/coreSyn/CoreArity.hs:1197:19 in
 ghc:CoreArity
   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/utils/Outputable.hs:1186:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/types/TyCoRep.hs:2088:56 in
 ghc:TyCoRep
         checkValidSubst, called at compiler/types/TyCoRep.hs:2121:29 in
 ghc:TyCoRep
         substTy, called at compiler/coreSyn/CoreArity.hs:1197:19 in
 ghc:CoreArity

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

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13951>
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