[GHC] #14175: Panic repSplitTyConApp_maybe

[GHC]

#14175: Panic repSplitTyConApp_maybe
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.2
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           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:
-------------------------------------+-------------------------------------
 This definition panics!

 {{{#!hs
 {-# LANGUAGE TypeFamilies, TypeInType #-}

 module Whoops where
 import Data.Kind

 type family PComp (k :: j -> Type) (x :: k) :: ()
 }}}

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.3.20170828 for x86_64-unknown-linux):
         repSplitTyConApp_maybe
   j_aon[sk:1]
   *
   *
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1138:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:1123:5 in ghc:Type
 }}}

 If I make it a type synonym instead, I get a proper error as expected.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14175>
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