[GHC] #15170: GHC HEAD panic (dischargeFmv)

[GHC]

#15170: GHC HEAD panic (dischargeFmv)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Here's as small of an example as I can muster:

 {{{#!hs
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Data.Kind
 import Data.Proxy

 data TyFun :: Type -> Type -> Type
 type a ~> b = TyFun a b -> Type
 infixr 0 ~>

 type family Apply (f :: k1 ~> k2) (x :: k1) :: k2
 type f @@ x = Apply f x
 infixl 9 @@

 wat :: forall (a :: Type)
               (b :: a ~> Type)
               (c :: forall (x :: a). Proxy x ~> b @@ x ~> Type)
               (f :: forall (x :: a) (y :: b @@ x). Proxy x ~> Proxy y ~> c
 @@ ('Proxy :: Proxy x) @@ y)
               (x :: a).
       (forall (xx :: a) (yy :: b @@ xx). Proxy (f @@ ('Proxy :: Proxy xx)
 @@ ('Proxy :: Proxy yy)))
    -> ()
 wat _ = ()
 }}}
 {{{
 $ /opt/ghc/head/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.5.20180501 for x86_64-unknown-linux):
         dischargeFmv
   [D] _ {0}:: (Apply
                  (f_a1rx[sk:2] xx_a1rH[tau:3] yy_a1rI[tau:3] |> Sym
 ((TyFun
 <Proxy xx_a1rH[tau:3]>_N
 ((TyFun
 (Proxy
 (Sym {co_a1rP})
 (Coh <yy_a1rI[tau:3]>_N
 {co_a1rP}))_N
 (Sym {co_a1s4} ; (Apply
 (Sym {co_a1rP})
 <*>_N
 (Coh ((Coh <s_a1rQ[fmv:0]>_N
 ((TyFun
 (Sym {co_a1rP})
 <*>_N)_N
 ->_N <*>_N) ; Sym {co_a1s3}) ; (Apply
 <Proxy
 xx_a1rH[tau:3]>_N
 ((TyFun
 (Sym {co_a1rP})
 <*>_N)_N
 ->_N <*>_N)
 (Coh <c_a1rw[sk:2] xx_a1rH[tau:3]>_N
 (Sym ((TyFun
 <Proxy
 xx_a1rH[tau:3]>_N
 ((TyFun
 (Sym {co_a1rP})
 <*>_N)_N
 ->_N <*>_N))_N
 ->_N <*>_N)))
 <'Proxy>_N)_N)
 (Sym ((TyFun
 (Sym {co_a1rP})
 <*>_N)_N
 ->_N <*>_N)))
 (Coh <yy_a1rI[tau:3]>_N
 {co_a1rP}))_N))_N
 ->_N <*>_N))_N
                                                                      ->_N
 <*>_N))
                  'Proxy :: (Proxy (yy_a1rI[tau:3] |> {co_a1rP}) ~>
 s_a1rS[fmv:0]))
               ~# (s_a1sj[fmv:0] :: (Proxy (yy_a1rI[tau:3] |> {co_a1rP})
                                     ~> s_a1rS[fmv:0]))
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1162:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/TcSMonad.hs:3047:25 in
 ghc:TcSMonad
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15170#comment:1>
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