[GHC] #15428: GHC 8.6+ panics (piResultTys2), older GHCs don't

[GHC]

#15428: GHC 8.6+ panics (piResultTys2), older GHCs don't
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  TypeFamilies                       |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following program panics when compiled with GHC 8.6.1 or HEAD:

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

 import Data.Kind
 import Data.Proxy
 import Data.Type.Equality

 type family Flurmp :: k
 type family Pure (x :: a) :: f a

 wat :: forall (f :: Type -> Type) (p :: Type).
        Proxy (f p) -> ()
 wat _ =
   let s ::     (Flurmp :: f p)
            :~: Pure (Flurmp :: p -> p) (Flurmp :: p)
       s = undefined
   in ()
 }}}
 {{{
 $ /opt/ghc/8.6.1/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.0.20180714 for x86_64-unknown-linux):
         piResultTys2
   f_aAT a_aAU
   [(->) p_a1uI[sk:1], p_a1uI[sk:1] -> p_a1uI[sk:1], Flurmp, Flurmp]
   [Flurmp]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:1041:9 in ghc:Type
 }}}

 On GHC 8.4 and earlier, this simply gives an error message:

 {{{
 $ /opt/ghc/8.4.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:18:16: error:
     • Expecting one more argument to ‘Pure (Flurmp :: p -> p)’
       Expected kind ‘p -> f p’,
         but ‘Pure (Flurmp :: p -> p)’ has kind ‘p -> p -> p’
     • In the second argument of ‘(:~:)’, namely
         ‘Pure (Flurmp :: p -> p) (Flurmp :: p)’
       In the type signature:
         s :: (Flurmp :: f p) :~: Pure (Flurmp :: p -> p) (Flurmp :: p)
       In the expression:
         let
           s :: (Flurmp :: f p) :~: Pure (Flurmp :: p -> p) (Flurmp :: p)
           s = undefined
         in ()
     • Relevant bindings include
         wat :: Proxy (f p) -> () (bound at Bug.hs:16:1)
    |
 18 |            :~: Pure (Flurmp :: p -> p) (Flurmp :: p)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15428>
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