failure to check completeness in instance declarations

[Ross Paterson]

On Fri, Mar 21, 2003 at 10:37:59AM -0800, Andrew Tolmach wrote:
> The following file is accepted by hugs (Nov.02 version on linux) without
> complaint.
> 
> data Foo a = Foo a
> 
> instance Eq (Foo a) where
> 
> z = Foo 1 == Foo 2

Incomplete instances are legal in Haskell 98, but this one isn't,
because (==) has a default definition:

	class  Eq a  where
	    (==), (/=) :: a -> a -> Bool
	    x /= y     =  not (x == y)
	    x == y     =  not (x /= y)

> Attempting to evaluate z causes a Segmentation fault (not unreasonably,
> perhaps!).

Segmentation fault is the Hugs implementation of stack overflow :-(