[GHC] #14591: HEAD panic: tcSyntaxOp "noSyntaxExpr"

[GHC]

#14591: HEAD panic: tcSyntaxOp  "noSyntaxExpr"
-------------------------------------+-------------------------------------
           Reporter:  tianxiaogu     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Linux
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following program triggers a bug in 8.2.2 and 8.5.20171215.

 {{{#!hs
 -- Checks that the ordering constraint on the groupWith function is
 respected
 {-# OPTIONS_GHC -XMonadComprehensions -XTransformListComp #-}
 module ShouldFail where
 import GHC.Exts (groupWith)
 data Unorderable = Gnorf | Pinky | Brain
 foo = [ ()
       | Gnorf <- [Gnorf, Brain]
       , then group by x using groupWith
       ]
 }}}

 Reproduce:

 {{{
 ghc bug.hs
 }}}

 Output:
 {{{
 [1 of 1] Compiling ShouldFail       ( bug.hs, bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.5.20171215 for x86_64-unknown-linux):
         tcSyntaxOp
   "noSyntaxExpr"
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/TcExpr.hs:1436:31 in
 ghc:TcExpr

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

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14591>
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