GADT code from RepLib causes panic!

[ben.kavanagh]

While compiling RepLib? generics library I encountered a ghc panic. I have isolated the failure to very small example that does some simple computation over a heterogenous list. I have attached the following files

RepMinimal?.hs. This is the problem reduced to small example. The function that seems to cause the problem is "badFunction". If I remove this function the file compiles.

RepSmall?.hs. This is a trimmed down core of RepLib? where the original problem occurs. I only include it if someone looking at the bug wants to know what this code was used for.

below is the output of ghc run.

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[ben@lapdog RepLib?]$ ghc -v RepCombined?3.hs Glasgow Haskell Compiler, Version 6.10.1, for Haskell 98, stage 2 booted by GHC version 6.10.1 Using package config file: /usr/lib/ghc-6.10.1/./package.conf hiding package base-3.0.3.0 to avoid conflict with later version base-4.0.0.0 wired-in package ghc-prim mapped to ghc-prim-0.1.0.0 wired-in package integer mapped to integer-0.1.0.0 wired-in package base mapped to base-4.0.0.0 wired-in package rts mapped to rts-1.0 wired-in package haskell98 mapped to haskell98-1.0.1.0 wired-in package syb mapped to syb-0.1.0.0 wired-in package template-haskell mapped to template-haskell-2.3.0.0 wired-in package dph-seq mapped to dph-seq-0.3 wired-in package dph-par mapped to dph-par-0.3 Hsc static flags: -static Created temporary directory: /tmp/ghc3284_0 *** Checking old interface for main:Data.RepLib.RepCombined: *** Parser: *** Renamer/typechecker: *** Desugar:

Result size = 176

*** Simplify:

Result size = 283 Result size = 283

*** Tidy Core:

Result size = 283

*** CorePrep?:

Result size = 465

*** Stg2Stg: *** CodeGen?: *** CodeOutput?: *** Deleting temp files: Deleting: /tmp/ghc3284_0/ghc3284_0.s *** Deleting temp dirs: Deleting: /tmp/ghc3284_0 ghc: panic! (the 'impossible' happened)

(GHC version 6.10.1 for i386-unknown-linux):

initC: srt_lbl

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

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[ben.kavanagh]

Minimal example of failure. Failure seems to be related to compilation of 'badFunction'

[ben.kavanagh]

Cut down version of RepLib? library core. This is to show where the problem code was used originally.

[simonpj]

Thank you for boiling this down. It turns out to be a dup of #2799, reported a few days ago, which I fixed yesterday.

Simon