evalv3: "field not allowed" regression with definition fields

[mvdan]

env CUE_EXPERIMENT=evalv3=0
exec cue export

env CUE_EXPERIMENT=evalv3=1
exec cue export

-- file.cue --
package p

#Schema: {
	schemaField?: {}
}

config: #Schema & {
	schemaField: {}
}

config: config2

config2: {
	#Extra: {}
}

As of 44cceca:

> env CUE_EXPERIMENT=evalv3=0
> exec cue export
[stdout]
{
    "config": {
        "schemaField": {}
    },
    "config2": {}
}
> env CUE_EXPERIMENT=evalv3=1
> exec cue export
[stderr]
config.#Extra: field not allowed:
    ./file.cue:11:9
[exit status 1]
FAIL: repro-evalv3.txtar:5: unexpected command failure

As far as I can tell, evalv2 is right in accepting this configuration. In fact, the error config.#Extra: field not allowed feels like it should be impossible; as far as my understanding of the spec goes, definitions and hidden fields should not be affected by closedness at all.

[mpvl]

This is related to this: #543

This behavior was proposed, but it should not be implemented by v3 yet or at all.

[mvdan]

Agreed. Even if we did want to do this soon, it should be as a separate experiment. We want to make the transition from v2 to v3 as smooth as possible first.