evalv3: lookup failure even when field apparently exists

[rogpeppe]

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

v0.11.0-rc.1

Does this issue reproduce with the latest stable release?

Yes (v0.10.0)

What did you do?

exec go mod tidy
exec go test

env CUE_EXPERIMENT=evalv3

exec go test

-- go.mod --
module test

go 1.24

require (
	cuelang.org/go v0.11.0-rc.1
	github.com/go-quicktest/qt v1.101.0
)
replace cuelang.org/go => /home/rogpeppe/src/cuelabs/cue
-- main.go --
package main

func main() {}
-- main_test.go --
package main

import (
	"os"
	"slices"
	"testing"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/cuecontext"
	"github.com/go-quicktest/qt"
)

func TestLoad(t *testing.T) {
	evalVersion := cuecontext.EvalV2
	if os.Getenv("CUE_EXPERIMENT") == "evalv3" {
		// Earlier CUE versions don't recognise CUE_EXPERIMENT
		// by default, so simulate it so bisection works OK.
		evalVersion = cuecontext.EvalV3
	}
	ctx := cuecontext.New(cuecontext.EvaluatorVersion(evalVersion))
	schemasv := ctx.CompileString(`pkg: x, x: y: "hello"`)

	pkgv := schemasv.LookupPath(cue.MakePath(cue.Str("pkg")))
	if !pkgv.Exists() {
		t.Fatalf("cannot find package in generated schemas")
	}

	// We can see the field "y" in the resulting value when we iterate through its fields.
	var fields []string
	iter, err := pkgv.Fields(cue.All())
	qt.Assert(t, qt.IsNil(err))
	for iter.Next() {
		fields = append(fields, iter.Selector().String())
	}
	if !slices.Contains(fields, "y") {
		t.Fatalf("pkg value does not contain field y when iterating")
	}

	// ... but it fails on lookup (with v3 only)!
	schemav := pkgv.LookupPath(cue.MakePath(cue.Str("y")))
	if !schemav.Exists() {
		t.Fatalf("schema not found")
	}
}

What did you expect to see?

A passing test.

What did you see instead?

> exec go mod tidy
> exec go test
[stdout]
PASS
ok  	test	0.004s
> env CUE_EXPERIMENT=evalv3
> exec go test
[stdout]
--- FAIL: TestLoad (0.00s)
    main_test.go:42: schema not found
FAIL
exit status 1
FAIL	test	0.004s
[exit status 1]
FAIL: y.txtar:6: unexpected command failure

[rogpeppe]

I dug into the difference between Value.Fields and Value.LookupPath here and the difference seems to be that Fields obtains a value from Value.Default before iteration, where LookupPath does not.

[mvdan]

Retitled this as it seems to only happen with evalv3.