[GHC] #14904: Compiler panic (piResultTy)

[GHC]

#14904: Compiler panic (piResultTy)
-------------------------------------+-------------------------------------
           Reporter:  kcsongor       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  14873
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Type-checking the following type family

 {{{#!hs
 type family F (f :: forall a. g a) :: Type where
   F (f :: forall a. g a) = Int
 }}}

 panics with the message:

 {{{#!txt
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.4.0.20180118 for x86_64-apple-darwin):
         piResultTy
   k_aVM[tau:1]
   a_aVF[sk:1]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:950:35 in ghc:Type

 }}}

 The panic happens with HEAD, 8.4 and 8.2. 8.0 rejects the program with an
 error message, but even it panics on the following version:

 {{{#!hs
 type family F f :: Type where
   F ((f :: forall a. g a) :: forall a. g a) = Int
 }}}

 #14873 seemed somewhat related, so I tried with the patch implemented in
 3d252037234ce48f9bdada7d5c9b1d8eba470829, but that fails with the same
 panic too.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14904>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler