field "TypeMeta" not allowed in closed struct

[cueckoo]

Originally opened by @errordeveloper in cuelang/cue#439

I have am using CUE in a Kubernetes operator, I am trying to pass my API type to CUE, and seeing this error:

cue: marshal error: template.items.0.metadata.name: field "TypeMeta" not allowed in closed struct

My template looks like this:

package basic

import "<path>/operator/api/v1alpha1"

MyTemplate :: {
	kind: "List"
	apiVersion: "v1"
	items: [{
		metadata: {
			name:      "\(resource.metadata.name)"
			namespace: "\(resource.metadata.namespace)"
		}
        }]
}

resource: v1alpha1.MyType

template: MyTemplate

The generated CUE types look like this:

MyType :: {
	metav1.TypeMeta
	metadata?: metav1.ObjectMeta    @go(ObjectMeta)
	spec?:     MyTypeSpec   @go(Spec)
	status?:   MyTypeStatus @go(Status)
}

I tried changing the type definition by removing metav1.TypeMeta, and the error I got was this:

cue: marshal error: template.items.3.spec.networkRef.name: field "name" is optional

I figured I'd try adding default values in my CUE tempate:

package basic

import "<path>/operator/api/v1alpha1"

MyTemplate :: {
	kind: "List"
	apiVersion: "v1"
	items: [{
		metadata: {
			name:      "\(resource.metadata.name)" | ""
			namespace: "\(resource.metadata.namespace)" | ""
		}
        }]
}

resource: v1alpha1.MyType

template: MyTemplate

That worked. I then added metav1.TypeMeta back and all works now.

I don't really understand why is this happening, could someone explain? What can I do to avoid having default values in the template?

I am pretty sure that TypeMeta error is a misleading actually, it only seems happens when an optional value has no default.

Here is a more elegant work-around:

package basic

import "<path>/operator/api/v1alpha1"

MyTemplate :: {
	kind: "List"
	apiVersion: "v1"
	items: [{
		metadata: {
			name:      "\(resource.metadata.name)" | "\(defaults.metadata.name)" 
			namespace: "\(resource.metadata.namespace)" | "\(defaults.metadata.namespace)" 
		}
        }]
}

resource: v1alpha1.MyType

template: MyTemplate

defaults: {}

I have a separate Fill call that sets defaults, it still uses v1alpha1.MyType, but somehow not having the type declared in the template makes it work. Albeit, as soon as one of the fields is not set in defaults, I get my TypeMeta error back again.