Opened 3 years ago

#12671 new bug

enumFrom error thwarts checkOldIface's exception handling

Reported by: ezyang Owned by:
Priority: normal Milestone:
Component: Compiler Version: 8.1
Keywords: Cc: bgamari
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown 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:

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-
wired-in package integer-gmp mapped to integer-gmp-
wired-in package base mapped to base-
wired-in package rts mapped to rts
wired-in package template-haskell mapped to template-haskell-
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 UserErrors (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.

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