relatively simple test case for "internal error: PAP object entered!" on GHC 6.12.3

[carlhowells]

So, here's a test case

-- bug.cabal
Name:                bug
Version:             0.1
Build-type:          Simple
Cabal-version:       >=1.2

Executable bug
  Main-is: Bug.hs

  Build-depends:
    base > 4 && < 5,
    HUnit >= 1.2.2.1 && < 1.3,
    monads-fd >= 0.1 && < 0.2

-- Bug.hs
module Main where

import Control.Monad.Trans
import Control.Monad.State.Strict

import Test.HUnit

runMyTest :: StateT () IO a -> IO a
runMyTest x = evalStateT x ()

explode :: StateT () IO ()
explode = when ("" == "") . liftIO . assertFailure $ "X"

main :: IO Counts
main = runTestTT . TestCase . runMyTest $ explode

---

$ cabal build && ./dist/build/bug/bug
Preprocessing executables for bug-0.1...
Building bug-0.1...
[1 of 1] Compiling Main             ( Bug.hs, dist/build/bug/bug-tmp/Main.o )
Linking dist/build/bug/bug ...
Cases: 1  Tried: 0  Errors: 0  Failures: 0bug: internal error: PAP object entered!
    (GHC version 6.12.3 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Aborted

I couldn't figure out how to get the error to happen with HUnit or a monad transformer. But here are some additional notes:

Compiling with -O0 fixes it.

Using mtl instead of monads-fd doesn't change anything.

Changing the ("" == "") to True results in not getting a GHC internal error, but the test run reports 0 failures, which is incorrect. Changing the import of Control.Monad.State.Strict to Control.Monad.State also results in no crash, but 0 failures reported.

None of these issues occurs in GHC 6.10.4. I haven't tested on versions other than 6.10.4 and 6.12.3.

[carlhowells]

Whoops, let me format that code properly.

-- Bug.hs
module Main where

import Control.Monad.Trans
import Control.Monad.State.Strict

import Test.HUnit

runMyTest :: StateT () IO a -> IO a
runMyTest x = evalStateT x ()

explode :: StateT () IO ()
explode = when ("" == "") . liftIO . assertFailure $ "X"

main :: IO Counts
main = runTestTT . TestCase . runMyTest $ explode

-- bug.cabal
Name:                bug
Version:             0.1
Build-type:          Simple
Cabal-version:       >=1.2

Executable bug
  Main-is: Bug.hs

  Build-depends:
    base > 4 && < 5,
    HUnit >= 1.2.2.1 && < 1.3,
    monads-fd >= 0.1 && < 0.2

[igloo]

Thanks for the report

[lilac]

STG lint failure

[lilac]

This also occurs on GHC version 6.12.1 for x86_64_unknown_linux, in case that helps narrow it down... The code passes core lint but fails STG lint.

[simonpj]

I believe I've fixed this: see #3959. Could you see if it works for you now?

Thanks

Simon

[michalt]

It seems to work for me. All three cases: the default code, one with ("" == "") changed to True and one with Lazy State monad, result in:

### Failure:
X
Cases: 1  Tried: 1  Errors: 0  Failures: 1

Tested with

The Glorious Glasgow Haskell Compilation System, version 7.1.20101015

[simonpj]

Great. So I'll close the ticket. We have other tests for this.

Simon