id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
2587,Optimiser bug with extistentials and GADT's,NeilMitchell,,"Given the program:

{{{
{-# LANGUAGE GADTs, ExistentialQuantification #-}
module GadtBug(bug) where

data Existential = forall a . Existential (Gadt a)

data Gadt a where Value :: Gadt Double


bug = [ match undefined | ps <- undefined, _ <- ps ]
  where
        match (Existential _) = undefined
        match (Existential _) = undefined
}}}

Using GHC Head from yesterday, compiling without optimisations I get:

{{{
C:\Neil>ghc --make GadtBug.hs
[1 of 1] Compiling GadtBug          ( GadtBug.hs, GadtBug.o )

GadtBug.hs:11:8:
    Warning: Pattern match(es) are overlapped
             In the definition of `match': match (Existential _) = ...
}}}

With optimisations (-O or -O2) I get:

{{{
C:\Neil\paradise-bug-csdg\alone>ghc --make GadtBug.hs -O
[1 of 1] Compiling GadtBug          ( GadtBug.hs, GadtBug.o )

GadtBug.hs:11:8:
    Warning: Pattern match(es) are overlapped
             In the definition of `match': match (Existential _) = ...
ghc: panic! (the 'impossible' happened)
  (GHC version 6.9.20080905 for i386-unknown-mingw32):
        idInfo a{tv ag4} [sk]

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

-- Neil",bug,closed,normal,,Compiler,6.9,fixed,"neil.mitchell.2@credit-suisse.com, ganesh.sittampalam@credit-suisse.com",,Unknown/Multiple,Unknown/Multiple,,Unknown,gadt/T2587,,,