Improve the interactions between java.lang.Comparable and scala.math.Ordered

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A few years ago, Ordered was made a sub-class of Comparable.

One issue remained: Ordered used an abstract compare, while Comparable had an abstract compareTo. This was approached by implementing compareTo in Ordered in terms of compare.

Some issues remain with that solution:

• It is impossible for a class to implement Ordered if that class made compareTo final. (Example: java.lang.Enum)
• It is necessary to implement Ordered's compare again, even if a super-class has a perfectly fine implementation of Comparable's compareTo, which would be sufficient for Ordered.

Proposed solution:

Ordered implements compare in terms of Comparable's compareTo. A detailed overview of the existing usages will be necessary to figure out the least disruptive way of fixing this issue.

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Imported From: https://issues.scala-lang.org/browse/SI-7017?orig=1 Reporter: @soc Affected Versions: 2.11.0-M1

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@Ichoran said: Want me to take this one, Simon? Finding existing usages is one of my primary tasks these days. I'm not sure anything can be done, but when I've got the tooling in place at least I can assess.

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@soc said: I think we should have a look whether we can alias Ordered to Comparable on the JVM, and add some warning, and let the compiler move the implementation from compare to compareTo in people's code.