evaluator: phase out and deprecate arithmetic on lists

[mvdan]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.5.0-beta.3

      -buildmode exe
       -compiler gc
        -ldflags -w -s
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v3

Does this issue reproduce with the latest release?

Yes.

What did you do?

! exec cue export root.cue

-- root.cue --
import "list"

foo: [1, 2] + [3]
bar: list.Concat([[1, 2], [3]])

What did you expect to see?

testscript -v repro-cmd.txtar should succeed; arithmetic on lists hasn't been part of the spec since 2021, per https://cue-review.googlesource.com/c/cue/+/8063. Right now, that feature continues to work, and I didn't even notice that I shouldn't be using it until @myitcv reminded me.

I expect cue export to fail on arithmetic on lists, or at least for a command like cue fmt to rewrite + with list.Concat as a transition step. For example, we could make CUE 0.6 apply this cue fmt rewrite, and a future version (0.8? 0.9?) reject the old form entirely.

What did you see instead?

An old and removed spec feature continues to work without hiccups.

[myitcv]

Marking this a v0.6.0 pending discussion with @mpvl

[myitcv]

Moving forward to v0.6.x to implement a "fix" to rewrite to use list.Concat. Later deprecation etc to follow in a later release.

[4ad]

This should also remove multiplication on lists.

[mvdan]

We closed this prematurely; the only merged commit does the cue mod fix step, but the step to actually remove the feature is https://review.gerrithub.io/c/cue-lang/cue/+/1200221, yet to be merged.