Type families with class constraints: type-checker loops

[h.holtmann]

The following program:

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}

class (Eq (Depend s))=> Bug s where
  type Depend s 
  trans :: Depend s -> Depend s
  
instance Bug Int where
  type Depend Int = ()
  trans = (+1)
  
check :: (Bug s) => Depend s -> Bool
check d = d == trans d

runs into a type-checker loop

GHC-Bug.hs:1:0:
    Context reduction stack overflow; size = 20
    Use -fcontext-stack=N to increase stack size to N
        `ic :: {(Bug Int, Eq (Depend Int)) => Num ()}'
          arising from the type signature for `trans' at GHC-Bug.hs:24:2-13
        `ic :: {(Bug Int, Eq (Depend Int)) => Num ()}'
          arising from the type signature for `trans' at GHC-Bug.hs:24:2-13
    <snip>

Of course, this program should not compile, but the type-checker should not loop.

compiler: 6.9.20071105

[igloo]

Thanks for the report. Another one for you, Manuel?

[chak]

Works now in the head.