id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
2378	panic: lookupVers1, when standalone deriving Typeable and Data	guest		"When using !StandaloneDeriving and !DeriveDataTypeable on a rather large AST, occasionally GHC 6.8.2 and GHC 6.8.3 RC bail with

{{{
[13 of 24] Compiling Language.C.AST.Generic
...
ghc-6.8.2.20080527: panic! (the 'impossible' happened)
  (GHC version 6.8.2.20080527 for i386-apple-darwin):
        lookupVers1 language-c-test-0.2:Language.C.AST.Constants $cFlags{v}

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

This is probably related to ticket #2011 (marked as milestone 6.8.3), clean and rebuild resolves the problem temporarily.

I haven't been able to extract a small test case, but the problematic source is available online at 

http://code.haskell.org/language-c/src/Language/C/AST/Generic.hs

The bug is reproducible: (cabal) build, comment out the last 6 'deriving Data' clauses (line 96-101), build again, uncomment, build again.

Side notes: 
There is a warning (but no error !) when deriving a Typeable1 instance (maybe related to #1825)
{{{
[13 of 24] Compiling Language.C.AST.Generic
Var/Type length mismatch: 
    [f{tv i1lc} [tv]]
    []
... [as above]
}}}

Finally, in GHC 6.9 standalone deriving Data was unsupported as of 01042008:
{{{
[13 of 24] Compiling Language.C.AST.Generic
Illegal binding of built-in syntax: $cCHeader
}}}
"	merge	closed	normal	6.8 branch	Compiler	6.8.3	fixed		benedikt.huber@… claus	MacOS X	x86		Unknown	deriving/should_compile/T2378