Bogus superclass dictionary with equality constraints

{-# LANGUAGE TypeFamilies, UndecidableInstances #-}
{-# OPTIONS -fglasgow-exts -Wnot #-}

module Main where

instance (Num a, Num b, a ~ b) => Num (a,b) where
     x * _ = x 

test1 = (1,1) * (2,2)

main = print test1

Running this program produces <<loop>>. Reason is that the Num (a,b) dictionary gets an Eq superclass dictionary that is essentially bottom. I'm not sure why, but it's a bad bug.

See http://article.gmane.org/gmane.comp.lang.haskell.general/16796

Simon

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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