id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7079	Irrefutable pattern failed for pattern Data.Maybe.Just	lbolla		"Compiling the following piece of code fails badly.

{{{
{-# LANGUAGE Rank2Types #-}                                                        
                                                                                   
class Set s where                                                                  
    insert :: Ord a => a -> s a -> s a                                             
                                                                                   
data UnbalancedSet a = E | T (UnbalancedSet a) a (UnbalancedSet a)                 
                                                                                   
instance Ord a => forall a ∘ Set (UnbalancedSet a) where                           
    insert = undefined
}}}

Error message is:

{{{
$ ghc -v -dcore-lint ghc-bug.hs 
Glasgow Haskell Compiler, Version 7.4.2, stage 2 booted by GHC version 7.4.1
Using binary package database: /home/lbolla/src/junk/haskell/Okasaki/.hsenv_Okasaki/ghc_pkg_db/package.cache
hiding package QuickCheck-2.4.2 to avoid conflict with later version QuickCheck-2.5
wired-in package ghc-prim mapped to ghc-prim-0.2.0.0-23f345e1ec26a64d5ebc768bd0b2a5d9
wired-in package integer-gmp mapped to integer-gmp-0.4.0.0-c15e185526893c3119f809251aac8c5b
wired-in package base mapped to base-4.5.1.0-6909ea031307e047b8ba5b23968c534b
wired-in package rts mapped to builtin_rts
wired-in package template-haskell mapped to template-haskell-2.7.0.0-718c7a8346a48b195831957f3dba0eac
wired-in package dph-seq not found.
wired-in package dph-par not found.
Hsc static flags: -static
*** Chasing dependencies:
Chasing modules from: *ghc-bug.hs
Stable obj: []
Stable BCO: []
Ready for upsweep
  [NONREC
      ModSummary {
         ms_hs_date = Mon Jul 16 09:11:15 BST 2012
         ms_mod = main:Main,
         ms_textual_imps = [import (implicit) Prelude]
         ms_srcimps = []
      }]
*** Deleting temp files:
Deleting: 
compile: input file ghc-bug.hs
Created temporary directory: /tmp/ghc6514_0
*** Checking old interface for main:Main:
[1 of 1] Compiling Main             ( ghc-bug.hs, ghc-bug.o )
*** Parser:
*** Renamer/typechecker:
*** Deleting temp files:
Deleting: /tmp/ghc6514_0/ghc6514_0.s
Warning: deleting non-existent /tmp/ghc6514_0/ghc6514_0.s
*** Deleting temp dirs:
Deleting: /tmp/ghc6514_0
ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.2 for i386-unknown-linux):
	compiler/rename/RnSource.lhs:430:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars,
                                                        _,
                                                        SrcLoc.L _ cls,
                                                        _)


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

System used:

{{{
$ uname -a
Linux arch 3.4.4-3-ARCH #1 SMP PREEMPT Tue Jul 3 15:35:13 UTC 2012 i686 GNU/Linux

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: /build/src/gcc-4.7.1/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared --enable-threads=posix --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch --enable-libstdcxx-time --enable-gnu-unique-object --enable-linker-build-id --with-ppl --enable-cloog-backend=isl --disable-ppl-version-check --disable-cloog-version-check --enable-lto --enable-gold --enable-ld=default --enable-plugin --with-plugin-ld=ld.gold --with-linker-hash-style=gnu --disable-multilib --disable-libssp --disable-build-with-cxx --disable-build-poststage1-with-cxx --enable-checking=release
Thread model: posix
gcc version 4.7.1 (GCC) 
}}}
"	bug	closed	normal		Compiler	7.4.2	duplicate			Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown