[GHC] #12638: GHC panic when resolving Show instance

[GHC]

#12638: GHC panic when resolving Show instance
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           Reporter:  MichaelK       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:  PolyKinds,     |  Operating System:  Unknown/Multiple
  TypeFamilies                       |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I'm playing around with rolling my own type defaulting and found a panic.
 Note: adding a stub `Show (W a)` instance resolves this (i.e. `show _ =
 "test"`).

 {{{
 {- Test.hs -}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}

 module Test where

 import Data.Proxy

 data W (a :: k) = Wk | W (MkStar a)

 type family MkStar (a :: k) :: *

 main = print (W Proxy :: W (Proxy (~)))
 }}}

 {{{
 $ ghc Test.hs -o test
 [1 of 1] Compiling Test             ( Test.hs, Test.o )

 Test.hs:13:8: error:ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         print_equality ~

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 If `x = W Proxy :: W (Proxy (~))` is only defined, there is no issue. It
 seems to occur exactly when resolving a `Show` instance for `W (Proxy
 (~))` and one does not exist.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12638>
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