ProtoObject and comparison for equality and ordering

[Adam D. Ruppe]

On Wednesday, 15 May 2019 at 06:19:19 UTC, Jonathan M Davis wrote:
> Even an interface is a problem, because then that locks in the 
> attributes.

That's not completely true in general, and only sort of true in 
this specific situation.

In general, attributes are semi-locked, accordance to the Liskov 
substitution principle: you can tighten constraints, but not 
loosen them on child classes.

(Like with current Object, which has no attributes, you are 
allowed to define a child class with `override @nogc @safe 
nothrow` etc. But once you do that, you can never remove them in 
child classes, since then you'd be breaking the promise of the 
parent class' interface)


In this specific situation, the attributes are only locked in 
after you define them - which is the same if you did it with or 
without the interface. The interface itself does not demand 
anything unless it specifically lists them.

But, note, they also are not inferred.

> By templatizing all of the relevant code in druntime

There should be NO code in druntime! All `a < b` is is syntax 
sugar for a method call. It does not need and should not use any 
other function. Just let the compiler rewrite the syntax to the 
call and then do its thing.

The ProtoObject version of __cmp should simply not exist.