[GHC] #15108: Panic: collectNBinders

[GHC]

#15108: Panic: collectNBinders
-------------------------------------+-------------------------------------
        Reporter:  cdisselkoen       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Profiling         |              Version:  8.4.2
      Resolution:                    |             Keywords:  panic
                                     |  collectNBinders
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:                    |
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Comment (by nomeata):

 Thanks for the nice test case, I added it to the test suite.

 JFTR, this can be easily reproduced without stack or GHC:
 {{{
 ~/build/haskell/ghc-master $ ./inplace/bin/ghc-stage2
 testsuite/tests/profiling/should_compile/T15108.hs  -prof -fprof-auto
 -dcore-lint
 [1 of 1] Compiling Main             (
 testsuite/tests/profiling/should_compile/T15108.hs,
 testsuite/tests/profiling/should_compile/T15108.o )
 *** Core Lint errors : in result of Desugar (after optimization) ***
 testsuite/tests/profiling/should_compile/T15108.hs:22:7: warning:
     [RHS of myInt_aVZ :: Int -> MyInt]
     Join point has too few lambdas
     Join var: myInt_aVZ
     Join arity: 1
     Number of lambdas: 0
     Rhs =  scctick<getInt.myInt> MyInt
 *** Offending Program ***
 getInt :: Shape -> MyInt
 [LclIdX]
 getIntghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.5.20180410 for x86_64-unknown-linux):
         collectNBinders
   1
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1162:37 in
 ghc:Outputable
         pprPanic, called at compiler/coreSyn/CoreSyn.hs:2056:25 in
 ghc:CoreSyn

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

 As you can see, it does not require any optimization level, and the bug is
 in place right after the desugarer.

 Maybe the optimizer copies the join arity from `MyInt` to `myInt`, under
 the assumption that `myInt` will be inlined, but the `scctick` prevents
 that from happening?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15108#comment:5>
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