Why a context reduction stack overflow?

[Mike Gunter]

Compilation, with ghc 5.02, of the following program:

  data Zero   = Zero      deriving Show
  data Succ a = Succ a    deriving Show
  
  -- Compilations succeeds with the following line uncommented.
  -- t :: Succ Zero	--      <=============================
  t = Zero `p` (Succ Zero)
  
  class P a b c | a b -> c where
    p :: a -> b -> c
  instance            P Zero  a   a   where p Zero a = a
  instance P a b c => P b     a   c   where p b    a = p a b
  
  main = putStrLn $ "Hello."

succeeds with the indicated line uncommented.  With it commented-out,
the compilation fails with:

  Flip.hs:6:
      Context reduction stack overflow; size = 21
      Use -fcontext-stack20 to increase stack size to (e.g.) 20
      `P (Succ Zero) Zero c' arising from use of `p' at Flip.hs:6
      `P Zero (Succ Zero) c' arising from use of `p' at Flip.hs:6
      `P (Succ Zero) Zero c' arising from use of `p' at Flip.hs:6
      `P Zero (Succ Zero) c' arising from use of `p' at Flip.hs:6 

       ...

      `P (Succ Zero) Zero c' arising from use of `p' at Flip.hs:6
      `P Zero (Succ Zero) c' arising from use of `p' at Flip.hs:6
      `p at [Zero Succ Zero c]' arising from use of `p' at Flip.hs:6
      When generalising the type(s) for t
  
.  Why?

        thanks,
        mike gunter