constraints: move to x/exp for Go 1.18

[rsc]

There are still questions about the the constraints package. To start with, although many people are happy with the name, many are not. On top of that, it is unclear exactly which interfaces are important and should be present and which should be not. More generally, all the considerations that led us to move slices and maps to x/exp apply to constraints as well.

We left constraints behind in the standard library because we believed it was fundamental to using generics, but in practice that hasn't proven to be the case. In particular, most code uses any or comparable. If those are the only common constraints, maybe we don't need the package. Or if constraints.Ordered is the only other commonly used constraint, maybe that should be a predeclared identifier next to any and comparable. The ability to abbreviate simple constraints let us remove constraints.Chan, constraints.Map, and constraints.Slice, which probably would have been commonly used, but they're gone.

Unlike other interfaces like, say, context.Context, there is no compatibility issue with having a constraint interface defined in multiple packages. The problems that happen with duplicate interfaces involve other types built using that type, such as func(context.Context) vs func(othercontext.Context). But that cannot happen with constraints, because they can only appear as type parameters, and they are irrelevant to type equality for a particular substitution. So having x/exp/constraints and later having constraints does not cause any kind of migration problem at all, unlike what happened with context.

For all these reasons, it probably makes sense to move constraints to x/exp along with slices and maps for Go 1.18 and then revisit it in the Go 1.19 or maybe Go 1.20 cycle. (Realistically, we may not know much more for Go 1.19 than we do now.)

Discussed with @robpike, @griesemer, and @ianlancetaylor, who all agree.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://golang.org/cl/382535 mentions this issue: constraints: new package

[gopherbot]

Change https://golang.org/cl/382460 mentions this issue: constraints: remove package

[bcmills]

The tests for x/exp/constraints had apparently imported the wrong package, and were broken by this removal.
I'll send a CL to update them. (Reopening until the test cleanup is complete.)

[gopherbot]

Change https://golang.org/cl/382834 mentions this issue: constraints: test against the constraints package from x/exp instead of std

[gopherbot]

Change https://golang.org/cl/383254 mentions this issue: _content: change generic min example to index

[gopherbot]

Change https://golang.org/cl/383694 mentions this issue: _content/doc: Update generics tutorial to not mention constraints package