[GHC] #2587: Optimiser bug with extistentials and GADT's

[GHC]

#2587: Optimiser bug with extistentials and GADT's
----------------------------------------------------------------------------------------+
    Reporter:  NeilMitchell                                                             |       Owner:          
        Type:  bug                                                                      |      Status:  new     
    Priority:  normal                                                                   |   Component:  Compiler
     Version:  6.9                                                                      |    Severity:  normal  
    Keywords:  neil.mitchell.2 at credit-suisse.com, ganesh.sittampalam at credit-suisse.com  |    Testcase:          
Architecture:  Unknown                                                                  |          Os:  Unknown 
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 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

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2587>
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