[Haskell-cafe] Non-termination due to context

Hello all! Consider the following program:

{-# LANGUAGE FlexibleInstances, OverlappingInstances, UndecidableInstances #-}

class B a => A a

instance A Int

class Eq a => B a

instance (A a, Eq a) => B a

eq :: B a => a -> a -> Bool eq = (==)

test = 1 `eq` (2::Int)

(This is a condensed version of a much larger program that I've been debugging.) It compiles just fine, but `test` doesn't terminate (GHCi 6.10.4). If I change the context `B a` to `Eq a` for the function `eq`, it terminates. Although I don't know all the details of the class system, it seems unintuitive that I can make a program non-terminating just by changing the context of a function (regardless of UndecidableInstances etc.). Is this a bug or a feature? / Emil