GHC inlines class method forever
irene-knapp showed me this over IRC, I refined the test case a bit:
{-# LANGUAGE FlexibleContexts, MultiParamTypeClasses #-}
module Bug () where
class C a b where
discord :: C a b => a b () -- Remove the constraint and it works
rhyme :: a b ()
instance C (,) b => C (,) [b] where
discord = discord
rhyme = discord
GHC 7.2.2 compiling this with -O loops forever. I've worked out:
- The
C a b
context in the class is completely superfluous but the bug isn't triggered without it. - The pattern of recursion in the two method signatures is fragile:
rhyme = rhyme
doesn't trigger the bug, for example. - Adding a
NOINLINE discord
stops the bug from triggering. Similarly, compiling without optimisations doesn't trigger the bug. - While it's looping, GHC slowly but steadily chews through all of your memory.
Trac metadata
Trac field | Value |
---|---|
Version | 7.2.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |