#12671: enumFrom error thwarts checkOldIface's exception handling -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This bug needs a bit of jiggery-pokery to reproduce. First, we need to create a malformed hi file that we will feed `checkOldIface`. First, have a fresh build of GHC. Apply the following diff to your tree: {{{ diff --git a/compiler/main/HscTypes.hs b/compiler/main/HscTypes.hs index 7a585f3..4e41887 100644 --- a/compiler/main/HscTypes.hs +++ b/compiler/main/HscTypes.hs @@ -1033,7 +1033,8 @@ instance Binary ModIface where put_ bh vect_info put_ bh hpc_info put_ bh trust - put_ bh trust_pkg + putByte bh 3 + -- put_ bh trust_pkg get bh = do mod <- get bh }}} Next, rebuild ONLY the stage 2 compiler using `cd ghc; make stage=2 -j12 fast`. Now, use the new stage2 compiler to compile some module (it can be empty): {{{ [ezyang@hs01 ghc-backpack]$ inplace/bin/ghc-stage2 -c A.hs [ezyang@hs01 ghc-backpack]$ inplace/bin/ghc-stage2 -c A.hs ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20161006 for x86_64-unknown-linux): Prelude.Enum.Bool.toEnum: bad argument Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} With `-v3`, we see that this error occurred when checking the old interface: {{{ [ezyang@hs01 ghc-backpack]$ inplace/bin/ghc-stage2 -c A.hs -v3 Glasgow Haskell Compiler, Version 8.1.20161006, stage 2 booted by GHC version 8.0.1 Using binary package database: /home/hs01/ezyang/ghc- backpack/inplace/lib/package.conf.d/package.cache package flags [] loading package database /home/hs01/ezyang/ghc- backpack/inplace/lib/package.conf.d wired-in package ghc-prim mapped to ghc-prim-0.5.0.0 wired-in package integer-gmp mapped to integer-gmp-1.0.0.1 wired-in package base mapped to base-4.9.0.0 wired-in package rts mapped to rts wired-in package template-haskell mapped to template-haskell-2.11.0.0 wired-in package ghc mapped to ghc-8.1 wired-in package dph-seq not found. wired-in package dph-par not found. Hsc static flags: *** Checking old interface for A: }}} Dastardly. We've choked checking the old interface! The problem seems to be that the `toEnum` exception is not an `IOException`, so we don't catch it in `tryMost` (which `readIface` uses to catch errors). Funnily enough, the comment above says precisely, `TODO: I'm not entirely sure if this is catching what we really want to catch`. But to catch these, we'll have to catch all `UserError`s (because that is what is being thrown in this case.) Maybe a bridge too far? Another possibilty is to use some home-baked `toEnum` which catches errors of this sort. Anyway, the fix should be simple, we just have to decide what to do. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12671 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler