pkg/encoding: yaml.Validate doesn't handle disjunctions

[jpluscplusm]

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

$ cue version
cue version v0.7.0

go version go1.21.5
      -buildmode exe
       -compiler gc
       -trimpath true
     CGO_ENABLED 0
          GOARCH amd64
            GOOS linux
         GOAMD64 v1

Does this issue reproduce with the latest stable release?

Yes, 0.7.0.

What did you do?

As per https://cuelang.slack.com/archives/C012UU8B72M/p1703146006424709:

exec cue export regularFields.cue
exec cue export requiredFields.cue
exec cue export closedStructs.cue
-- regularFields.cue --
package p

import "encoding/yaml"

#OneOf: {a: string} | {b: string}

example: yaml.Validate(#OneOf)
example: "a: foo"
-- requiredFields.cue --
package p

import "encoding/yaml"

#OneOf: {a!: string} | {b!: string}

example: yaml.Validate(#OneOf)
example: "a: foo"
-- closedStructs.cue --
package p

import "encoding/yaml"

#a: a: string
#b: b: string
#OneOf: #a | #b

example: yaml.Validate(#OneOf)
example: "a: foo"

None of the 3 attempts in the repro validates the inline YAML.

What did you expect to see?

A passing test, or at least /one/ of the attempts succeeding (i.e. fewer than 3 failures in the testscript --continue output).

What did you see instead?

$ testscript --continue yamlValidate.txtar
> exec cue export regularFields.cue
[stderr]
example: unresolved disjunction {a:string} | {b:string} (type struct):
    ./regularFields.cue:7:10
[exit status 1]
FAIL: /tmp/testscript3854620968/yamlValidate.txtar/script.txtar:1: unexpected command failure
> exec cue export requiredFields.cue
[stderr]
example: unresolved disjunction {a!:string} | {b!:string} (type struct):
    ./requiredFields.cue:7:10
[exit status 1]
FAIL: /tmp/testscript3854620968/yamlValidate.txtar/script.txtar:2: unexpected command failure
> exec cue export closedStructs.cue
[stderr]
example: unresolved disjunction {a:string} | {b:string} (type struct):
    ./closedStructs.cue:9:10
[exit status 1]
FAIL: /tmp/testscript3854620968/yamlValidate.txtar/script.txtar:3: unexpected command failure
error running yamlValidate.txtar in /tmp/testscript3854620968/yamlValidate.txtar

[mvdan]

Verified that this is still a bug today, with and without the new evaluator experiment.

[myitcv]

Noting per a conversation offline that this this particular example (of oneofs) is intrinsically linked to https://github.com/cue-lang/cue/issues/3165.

[mvdan]

As of 9461bd296caec1abb7da62adfa436b469d4a86f2, the original testscript passes; the issue was resolved by https://review.gerrithub.io/c/cue-lang/cue/+/1194425.

That commit did not mark this issue as resolved because a closedness bug remained; however, that's unrelated to this issue, and it's already tracked in a TODO in the added tests in that commit.