encoding/openapi: struct.MinFields not satisfied when generating from CUE

[thorehusfeldt]

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

$ cue version

cue version v0.11.0

go version go1.23.3
      -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 amd64
            GOOS darwin
         GOAMD64 v1
cue.lang.version v0.11.0

Does this issue reproduce with the latest stable release?

It does.

What did you do?

I create

import "struct"

#foo: {
	[string]: string
} & struct.MinFields(1)

and run

> cue export *.cue --out openapi -d "#foo"

What did you expect to see?

{
    "openapi": "3.0.0",
    "info": {
        "title": "Generated by cue.",
        "version": "no version"
    },
    "paths": {},
    "components": {
        "schemas": {
            "foo": {
                "type": "object",
                "minProperties": 1,
                "additionalProperties": {
                    "type": "string"
                }
            }
        }
    }
}

In particular, the above does indeed happen for the more-or-less-symmetrical case with s/MinFields/MaxFields/ in the CUE schema and s/minProperties/maxProperties/ in the OpenAPI output.

What did you see instead?

#foo: openapi: #foo: invalid value {} (does not satisfy struct.MinFields(1)): len(fields) < MinFields(1) (0 < 1):
    ./test.cue:5:5

[myitcv]

Confirmed as of f99441a

exec cue export --out openapi -o - -d '#foo' x.cue

-- x.cue --
import "struct"

#foo: {
	[string]: string
} & struct.MinFields(1)
-- stdout.golden --
{
    "openapi": "3.0.0",
    "info": {
        "title": "Generated by cue.",
        "version": "no version"
    },
    "paths": {},
    "components": {
        "schemas": {
            "foo": {
                "type": "object",
                "minProperties": 1,
                "additionalProperties": {
                    "type": "string"
                }
            }
        }
    }
}

For some variation of stdout.golden, because CUE's interpretation of additionalProperties is not correct in all situations.