[GHC] #15629: "No skolem info" panic (GHC 8.6 only)

[GHC]

#15629: "No skolem info" panic (GHC 8.6 only)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.6.1-beta1
  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:                    |
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Comment (by RyanGlScott):

 For what it's worth, here is a slightly more minimal version of the
 original program:

 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug (f) where

 import Data.Kind
 import Data.Proxy

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

 type family F x :: Type -> Type
 data F1Sym :: forall x a. x ~> F x a
 data F2Sym :: forall x a. F x a ~> x
 data Comp :: forall b c a. (b ~> c) -> (a ~> b) -> (a ~> c)

 sg :: forall a b c (f :: b ~> c) (g :: a ~> b) (x :: a).
       Proxy f -> Proxy g -> Proxy (Comp f g)
 sg _ _ = Proxy

 f :: forall (x :: Type). Proxy x -> ()
 f _ = ()
   where
     g :: forall ab. Proxy ((Comp (F1Sym :: x ~> F x z) F2Sym) :: F x ab ~>
 F x ab)
     g = sg Proxy Proxy
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15629#comment:7>
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