[GHC] #12041: GHC panics on "print_equality ~"

[GHC]

#12041: GHC panics on "print_equality ~"
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 Replying to [comment:1 Iceland_jack]:
 > Doesn't happen if
 >
 > {{{#!hs
 > data I (a :: Type) (b :: k)
 > }}}

 also doesn't happen if

 {{{#!hs
 data I (a :: k) (b :: Type)
 }}}

 ----

 Also test whether it panics on the `:kind!` command:

 {{{#!hs
 data I (a :: Type) (b :: Type)
 instance Category I where
   type Ob I = (~) Int
 }}}
 {{{
 ghci> :kind! Ob I
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.0.20160511 for x86_64-unknown-linux):
         print_equality ~

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

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12041#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler