cue: Iterator.Selector regression since v0.6 added required fields

[mvdan]

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

$ cue version
cue version v0.6.1-0.20231110172618-7a4ea8668a03

go version devel go1.22-8da6405e0d Sat Nov 11 02:02:35 2023 +0000
      -buildmode exe
       -compiler gc
        -ldflags -w -s
  DefaultGODEBUG httplaxcontentlength=1,httpmuxgo121=1,panicnil=1
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v3

Does this issue reproduce with the latest stable release?

Yes; verified with v0.6.0 as well as master as of 8f796bf.

What did you do?

go mod tidy

go run main.go
cmp stderr stderr.golden
-- main.go --
package main

import "cuelang.org/go/cue/cuecontext"

func main() {
	val := cuecontext.New().CompileString(`[{name: "Bob"}]`)
	iter, err := val.List()
	if err != nil {
		panic(err)
	}
	for iter.Next() {
		println(iter.Selector().Index())
	}
}
-- stderr.golden --
0
-- go.mod --
module test

go 1.21

require cuelang.org/go v0.6.0

What did you expect to see?

Success, like with v0.5.0.

What did you see instead?

> go run main.go
[stderr]
panic: Index called on non-index selector

goroutine 1 [running]:
panic({0x90d7e0?, 0xa80610?})
	/home/mvdan/tip/src/runtime/panic.go:772 +0x158 fp=0xc00002fdc8 sp=0xc00002fd18 pc=0x43a2f8
cuelang.org/go/cue.Selector.Index({{0xa86f50?, 0xc00000e7b0?}})
	$WORK/.gopath/pkg/mod/cuelang.org/[email protected]/cue/path.go:205 +0x45 fp=0xc00002fde8 sp=0xc00002fdc8 pc=0x6d2f65
main.main()
	$WORK/main.go:12 +0x10c fp=0xc00002ff50 sp=0xc00002fde8 pc=0x8d85ec
runtime.main()
	/home/mvdan/tip/src/runtime/proc.go:271 +0x29d fp=0xc00002ffe0 sp=0xc00002ff50 pc=0x43d91d
runtime.goexit({})
	/home/mvdan/tip/src/runtime/asm_amd64.s:1694 +0x1 fp=0xc00002ffe8 sp=0xc00002ffe0 pc=0x46cf21

I bisected this to https://review.gerrithub.io/c/cue-lang/cue/+/552171, which tweaked selectors for required fields.

Thanks to @alecholmez and @jack-bischoff for helping uncover this regression.