opCmp / opEquals do not actually support partial orders

[Dominikus Dittes Scherkl]

On Wednesday, 18 July 2018 at 17:49:19 UTC, H. S. Teoh wrote:
> On Wed, Jul 18, 2018 at 11:30:21AM -0600, Jonathan M Davis via 
> On the other hand, if opCmp is allowed to return a user-defined 
> type, it would solve the problem in a neat way: just define a 
> quaternary type that encapsulates the values -1, 0, 1, NaN, and 
> have opCmp return the equivalent of NaN for non-comparable 
> arguments.  Then we could support partial orders correctly.
>
> But I have a hard time seeing this actually work in practice, 
> because a user-defined return type for opCmp leads to recursion:
It already works with float, no recursion. A lot of the types I 
use depend on this.

But having a language supported quarterny type would be good for 
its improved speed.