id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
1255	UNPACKing a !Foo with newtype Foo = (Ptr Foo) causes the impossible	rashakil		"Here is the code that causes the error.  The strictness annotation and the unboxing of the field is necessary to cause the error.  Experimentation shows that the error happens regardless of the number of named fields.  I first found this when I tried using ""{{{:: {-# UNPACK #-} !X.GC}}}"" somewhere.

{{{
{-# OPTIONS_GHC -funbox-strict-fields #-}

module Foo where

import Foreign.Ptr


newtype  Bar        = Bar (Ptr Bar) -- Must be Bar inside.

data  Gah           = Gah { baaz  :: !Bar }

-- Alternately, disable -funbox-strict-fields and use {-# UNPACK #-},
-- and the impossible still happens.
}}}

Here's the error message.

{{{
$ ghc --make Foo -v
Glasgow Haskell Compiler, Version 6.6, for Haskell 98, compiled by GHC version 6.6
Using package config file: /usr/lib/ghc-6.6/package.conf
wired-in package base mapped to base-2.0
wired-in package rts mapped to rts-1.0
wired-in package haskell98 mapped to haskell98-1.0
wired-in package template-haskell mapped to template-haskell-2.0
Hsc static flags: -static
*** Chasing dependencies:
Chasing modules from: Foo.hs
Stable obj: []
Stable BCO: []
compile: input file Foo.hs
Created temporary directory: /tmp/ghc3424_0
*** Checking old interface for main:Foo:
[1 of 1] Compiling Foo              ( Foo.hs, Foo.o )
*** Parser:
*** Renamer/typechecker:
*** Desugar:
    Result size = 0
*** Simplify:
    Result size = 0
*** Tidy Core:
*** Deleting temp files:
Deleting: /tmp/ghc3424_0/ghc3424_0.s
Warning: deleting non-existent /tmp/ghc3424_0/ghc3424_0.s
*** Deleting temp dirs:
Deleting: /tmp/ghc3424_0
ghc-6.6: panic! (the 'impossible' happened)
  (GHC version 6.6 for i386-unknown-linux):
	dataConWorkId main:Foo.Bar{d rfa}

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
}}}"	bug	closed	normal	6.8.1	Compiler	6.6	fixed	UNPACK unbox newtype Ptr		Linux	x86		Unknown	tc226