panic! simplCore/Simplify.lhs:1490

[mlauer]

When compiling the attached file you get the error
below. My System is SuSE Linux 7.2

prompt > ghc --version
The Glorious Glasgow Haskell Compilation System,
version 5.02.1

prompt > ghc -c A.hs
ghc-5.02.1: panic! (the `impossible' happened, GHC
version 5.02.1):
        simplCore/Simplify.lhs:1490: Non-exhaustive
patterns in function cat_evals


Please report it as a compiler bug to
glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.

[simonpj]

Logged In: YES 
user_id=50165

The program is invalid, but the compiler should not crash.

It's invalid because in
  data M m t a = Monad m => M { unM::(m (S m t a))}
the existential context does not introduce any fresh type 
variables; m is already in scope.

I've added new validity checking to the HEAD compiler.  I 
doubt it's worth fixing in 5.02.2