language: incomplete value because of optional field

[loisch]

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

$ cue version
cue version v0.12.0

go version go1.23.5
      -buildmode exe
       -compiler gc
       -trimpath true
  DefaultGODEBUG asynctimerchan=1,gotypesalias=0,httpservecontentkeepheaders=1,tls3des=1,tlskyber=0,x509keypairleaf=0,x509negativeserial=1
     CGO_ENABLED 1
          GOARCH arm64
            GOOS darwin
         GOARM64 v8.0
cue.lang.version v0.12.0

Does this issue reproduce with the latest stable release?

Yes

What did you do?

exec cue export test1.cue
cmp stdout golden.json
exec cue export test2.cue
cmp stdout golden.json

-- test1.cue --
"Psycho-Onko-Screening": {
	SingleForm: {
		formId:                 "psychoOnkoScreening"
		showNumberOfInstances?: bool
	}
}

-- test2.cue --
"Psycho-Onko-Screening": {
	SingleForm: {
		formId:                 "psychoOnkoScreening"
		showNumberOfInstances?: bool
	}
} | {
	SingleForm: {
		formId: "psychoOnkoScreening"
	}
}
-- golden.json --
{
    "Psycho-Onko-Screening": {
        "SingleForm": {
            "formId": "psychoOnkoScreening"
        }
    }
}

What did you expect to see?

A passing test.

What did you see instead?

The test fails with the following message

"Psycho-Onko-Screening": incomplete value {SingleForm:{formId:"psychoOnkoScreening",showNumberOfInstances?:bool}} | {SingleForm:{formId:"psychoOnkoScreening"}}

but showNumberOfInstances?:bool shouldn't cause an incomplete value error, because the field is optional. As the test shows this only happens when an alternative is involved. I'm not sure if it might be correct with respect to the language definition. It seems to also cause performance problems in a larger example because these alternatives aren't resolved / merged. I don't see how {a?: X} would be different from {} | {a?: X} because {a?: X} "includes" {}.

[mpvl]

This is a language change, but I agree we should do this! This will be critical in making closeness more orthogonal, among other things.