[GHC] #12072: GHC panic on type wildcard in left-hand side of data family

[GHC]

#12072: GHC panic on type wildcard in left-hand side of data family
-------------------------------------+-------------------------------------
           Reporter:  andreash       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While playing around with data families I received the following output by
 ghc/ghci:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for x86_64-unknown-linux):
         rnHsTyKi HsWildcardTy

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

 I had accidentally left a wildcard `_` on the left-hand side of a data
 instance definition.

 To reproduce enter the following code into a file and try to load that
 file into ghci, or try to compile it with ghc:

 {{{#!hs
 data family Bug x
 data instance Bug _ = Bug
 }}}

 The expected behavior would be a (non-panic) error message of some sort.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12072>
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