A fresh look at comparisons

[Robert Fraser]

Janice Caron wrote:
> [...]

There's no reason why two user-defined types that are not typically 
castable to one another can't be defined as equal... as long as this is 
transitive (so they both know about each other). Your syntax arbitrarily 
limits that option. It may be questionable software design, but it has a 
very legitimate use: backwards compatibility. For example, version 1 of 
a library could have a certain thing as a struct, but to make it more 
extensible, version 2 introduces a class version of the same type. They 
both have overridden opEquals to make them evaluate as equal to one 
another (for example, so users can use both of them in  a hash and not 
have to switch their entire codebase over at once). For efficiency, both 
the class and struct versions may live on.