what to do about Comparable

After #7395, I changed Comparable to be invariant.

@jvasileff points out that a bunch of our functions including sort() and max() don’t work as well when you have things that indirectly implement comparable. It is possible to rescue all these functions by adding a new type parameter C and having parameters of type, for example, Element & Comparable<C> or {Element & Comparable<C>*}.

However, that’s pretty messy and I don’t see it as acceptable. There’s two reasonable solutions:

1. Make Comparable contravariant and remove its self type.

2. Taking advantage of #7408, define Comparable as follows:

   shared interface Comparable<Other> of Other 
           given Other satisfies Comparable<in Other> { ... }
   

   Then functions like sort() and max() just use the same type constraint given Other satisfies Comparable<in Other>.

I like solution 2, assuming that #7408 doesn’t run into some terrible snag.

[gavinking]

After sleeping on this, I think the cleanest solution is just to:

1. Make Comparable contravariant and remove its self type.

So I’ve done that. Thanks to @jvasileff for pointing out the problems with making Comparable invariant.

[gavinking]

Right.