evaluator: cycle-related infinite evaluation

[myitcv]

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

$ cue version
cue version +c942d0af linux/arm64

Does this issue reproduce with the latest release?

Yes

What did you do?

Follow up from https://github.com/cue-lang/cue/discussions/1679.

exec cue eval -H x.cue y.cue
cmp stdout stdout.x.golden

-- x.cue --
package y

#Secret: {
	$secret: id: string
}
#secrets: #Secret | {[string]: #secrets}

out: #secrets & {
	FOO: $secret: id: "100"
	ONE: TWO: THREE: $secret: id: "123"
}
-- y.cue --
package y

#Secret: {
	$secret: _id: string
}
#secrets: #Secret | {[string]: #secrets}

out: #secrets & {
	FOO: $secret: _id: "100"
	ONE: TWO: THREE: $secret: _id: "123"
}
-- stdout.golden --
FOO: {
    $secret: {
        id: "100"
        _id: "100"
    }
}
ONE: {
    TWO: {
        THREE: {
            $secret: {
                id: "123"
                _id: "123"
            }
        }
    }
}

What did you expect to see?

A passing test.

What did you see instead?

Indefinite hang with growing memory usage and no output.

[mpvl]

Upon closer inspection, this seems more like a disjunction problem than a cycle problem per se.