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,,,