[GHC] #14174: GHC panic with TypeInType and type family

[GHC]

#14174: GHC panic with TypeInType and type family
-------------------------------------+-------------------------------------
           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 rather simple type family,

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

 module GenWhoops where
 import GHC.Generics

 type family GenComp k (x :: k) (y :: k) :: Ordering where
   GenComp ((x :+: y) p) ('L1 x) ('L1 y) = GenComp (x p) x y
 }}}

 produces the following panic:

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.3.20170828 for x86_64-unknown-linux):
         piResultTy
   k_a1LK[tau:1]
   p_a1Lz[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:949:35 in ghc:Type
 }}}

 This happens with both GHC 8.2.1 and something very close to HEAD.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14174>
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