Make opCmp more flexible

[dlangBugzillaToGithub]

Anders Halager reported this on 2017-06-29T19:24:53Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17573

Description

The way custom comparisons are currently implemented force you to always return an integer and doesn't tell you what operation you are doing.

An example of how it can bite is that mir can't do elementwise comparisons similar to how it does addition or multiplication since it has to return an int for comparisons.
So a + a == [2, 2, 2] and a * a == [1, 1, 1] but you can't express a < a == [false, false, false].


I get the logic of only making the user create a single comparison function but maybe a possible fix would be to check if there is an opBinary for the relevant comparison operator before doing the usual opCmp logic.