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

[simonpj]

Excellent example, thank you. Fixed by

Sun Sep 14 04:34:34 PDT 2008  simonpj@microsoft.com
  * Fix Trac #2587: take account of type lets

Simon