cmd/cue: gengotypes should be able to generate Go code for disjunctions

[rogpeppe]

Currently, if cue exp gengotypes is run on the following code:

package x

#T: #Foo | #Bar

#Foo: {
	type!: "foo"
	x?: bool
}

#Bar: {
	type!: "bar"
	x?: int
}

It will produce Go code like this:

package x

type T map[string]any

type Foo struct {
	Type string `json:"type"`

	X bool `json:"x,omitempty"`
}

type Bar struct {
	Type string `json:"type"`

	X int64 `json:"x,omitempty"`
}

Although the type T should be linked closely with the Foo and Bar types, it
is not.

It would be better if gengotypes could look at the schemas,
see that they're discriminated with the type field and generate Go code
that looks for the field at json umarshal time and creates the correct
struct type.

The cuediscrim package is a step in that direction.

[rogpeppe]

As one example of how a reasonably modern Go API does this, see the OpenAI Go API, for example the responses package. This appears to have been generated by https://www.stainless.com/.

See https://github.com/openai/openai-go?tab=readme-ov-file#response-unions for some general docs on how they deal with unions/disjunctions. Here's an example: http://pkg.go.dev/github.com/openai/openai-go/responses#ResponseCodeInterpreterToolCallResultUnion

Note the way that they use (generated) types to represent constant discriminator fields, such as https://pkg.go.dev/github.com/openai/openai-go/responses#FileSearch using https://pkg.go.dev/github.com/openai/[email protected]/shared/constant#FileSearch

As an aside, they also make extensive use of a generic Opt[T] type to represent optional fields, something which we could also choose to do.