Fix invalid unassignability check for lists of structs

[radiohead]

What

Fix list checking logic to include default values in list expression length check (i.e. check that there are two values when a default one is present), since the underlying issue in CUE has been resolved - calling .Expr no longer drops the default value.

Why

The reason is above, the underlying issue in CUE seems to have been resolved. This fixes a weird issue when using Thema with https://github.com/grafana/grafana-app-sdk and the following schema:

package kinds

test: {
	name:  "Test"
	group: "some"

	crd: {
		scope: "Namespaced"
	}

	codegen: {
		backend:  true
		frontend: false
	}

	lineage: {
		schemas: [{
			version: [0, 0]
			schema: {
				#Struct: {
					foo: string
				}

				#AnotherStruct: {
					foo: #Struct
				}

				spec: {
					foo: string
				}

				status: {
					foo: [...#Struct]
					bar: [...#AnotherStruct]
				}
			}
		}]
	}
}

Which results in a false-positive unassignability error:

panic: status.foo: schema is a complex disjunction of list types, may only correspond to interface{}/any
status.bar: schema is a complex disjunction of list types, may only correspond to interface{}/any

[joanlopez]

Could we add / extend the existing test suite to cover this, please?

[joanlopez]

...
...

Which results in a false-positive unassignability error:

panic: status.foo: schema is a complex disjunction of list types, may only correspond to interface{}/any
status.bar: schema is a complex disjunction of list types, may only correspond to interface{}/any

I don't see any default value on the example above, what I'm missing? 🤔 Thanks!

[radiohead]

...
...
Which results in a false-positive unassignability error:

panic: status.foo: schema is a complex disjunction of list types, may only correspond to interface{}/any
status.bar: schema is a complex disjunction of list types, may only correspond to interface{}/any

I don't see any default value on the example above, what I'm missing? 🤔 Thanks!

CUE assigns an implicit default value of an empty array to the expression, at least that's what it looks like in the code.

Could we add / extend the existing test suite to cover this, please?

I've tried adding an case to the tests in assignable_test.go, but it doesn't seem to support a use case with named structs. I'd appreciate any guidance / help.