proposal: Go 2: Move cmp.Ordered to builtin.orderable

[djordje200179]

Go 1.20 added comparable type constraint, Go 1.21 added cmp.Ordered type constraint.
It is probably late now, but wouldn't it be better, for the sake of consistency, to name this interface orderable, place it in builtin package, and use it in same manner as comparable interface, because both are related to comparison operators and built-in types.

Because of backwards compatibility, we could patch cmp.Ordered to be alias of orderable type constraint that would be placed besides comparable and any type constraint.

Changes:

File builtin.go:

// already existing type constraints
type any = interface{}
type comparable interface{ comparable }

type orderable interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 |
		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
		~float32 | ~float64 |
		~string
}

File cmp.go:

type Ordered = orderable

[seankhliao]

see #59488 (comment)

[djordje200179]

@seankhliao But why immediately closing it? Wouldn't it be better to let others have a discussion about this? :/

[seankhliao]

this was a recent discussion, we've made the decision, and there is no new information to justify revisiting the decision at this point in time.