[swift-users] Synthesized Equatable Bug?

[Dmitri Gribenko]

On Mon, Jan 15, 2018 at 2:30 PM, Jon Shier via swift-users
<swift-users at swift.org> wrote:
> This is pretty straightforward code, so am I missing something here?

Your '<' function does not define a valid strict total order.

For

let p1 = Point(1, 1)
let p2 = Point(0, 2)

neither of 'p1 < p2', 'p2 < p1', 'p1 == p2' is true.  This is the root cause.

> static func <= (lhs: Point, rhs: Point) -> Bool {
>    return lhs < rhs || lhs == rhs
> }

A faster way to compute it (with only one call to a user-defined
comparison operator) is to return '!(rhs < lhs)', which is wat the
standard library does, which is why you see the behavior that you are
seeing.

Dmitri

-- 
main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/