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
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