[GHC] #14969: panic on incorrect syntax

[GHC]

#14969: panic on incorrect syntax
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           Reporter:  johnleo        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 With a fresh build of 8.5:
 {{{
 GHCi, version 8.5.20180323: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /Users/leo/.ghci
 Prelude> 3 _ 4
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.5.20180323 for x86_64-apple-darwin):
         ASSERT failed!
   2
   1
   t_a4j6[tau:1]
   Maybe a_a4j7[tau:2]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1157:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1213:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/typecheck/TcMType.hs:720:54 in
 ghc:TcMType
 }}}

 I believe this fails in earlier versions as well, but I haven't tested it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14969>
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