[phobos] Issue with opEquals(Object, Object)

[Andrei Alexandrescu]

On 04/27/2010 06:55 AM, Steve Schveighoffer wrote:
> Following the example in the book, let's say I make my own widget,
> called StevesTextWidget, which inherits Widget, not TextWidget.  It
> has some of the same functionality as TextWidget, but it's not a
> TextWidget.  If I want to be able to compare StevesTextWidget to a
> TextWidget, it's not possible, because the TextWidget will veto every
> time, even though StevesTextWidget has total understanding of
> TextWidget and can correctly do the comparison.  On the other hand,
> if || was used, comparing StevesTextWidget to a Widget will allow
> Widget's routine to override the opEquals of StevesTextWidget, and
> like the example in the book, could possibly cause an invalid equal
> response.

Let's suppose we allow the feature above. Then a StevesTextWidget can 
claim equality with a TextWidget in spite of the latter's opposition. 
But then a WaltersTextWidget could also claim equality with the same 
TextWidget, again in spite of the latter's opposition. But then 
StevesTextWidget may not compare equal with WaltersTextWidget, which 
would break transitivity.

Andrei