Cannot build superclass instance

[Johannes Waldmann]

The following program

import Complex

instance Ord a => Ord (Complex a) where 
    (x1 :+ y1) `compare` (x2 :+ y2)  = (x1, y1) `compare` (x2, y2)

gives the message

ERROR Bug.hs:3 - Cannot build superclass instance
*** Instance            : Ord (Complex a)
*** Context supplied    : Ord a
*** Required superclass : Eq (Complex a)

What is hugs' problem here? It compiles with ghc. 
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