Perform partial unmarshaling with yaml.DisallowUnknownField()

[ekoops]

Is your feature request related to a problem? Please describe.
I would like to use yaml.DisallowUnknownField() while still be able to perform partial unmarshaling.

Describe the solution you'd like

Describe alternatives you've considered

Additional context
The following snippet just try to perform unmarshaling in two steps:

• unmarshal the type
• unmarshal the remaining document content based on the type

type T struct {
	Type  string
	Value any
}

type A struct {
	Field1 string
}

type B struct {
	Field2 string
}

func (t *T) UnmarshalYAML(unmarshal func(interface{}) error) error {
	var typ struct{ Type string }
	if err := unmarshal(&typ); err != nil {
		return fmt.Errorf("error decoding type: %w", err)
	}

	decodedType := typ.Type
	var value any
	switch decodedType {
	case "a":
		value = &A{}
	case "b":
		value = &B{}
	default:
		return fmt.Errorf("unknown type %q", decodedType)
	}

	if err := unmarshal(value); err != nil {
		return fmt.Errorf("error decoding value: %w", err)
	}

	t.Type = decodedType
	t.Value = value
	return nil
}

var data = `
type: "a"
value:
    field1: "field1"
`

func main() {
	reader := strings.NewReader(data)
	dec := yaml.NewDecoder(reader, yaml.DisallowUnknownField())
	t := &T{}
	if err := dec.Decode(t); err != nil {
		log.Fatal(err)
	}
}

If I run this, unfortunately, I get the following error:

2009/11/10 23:00:00 error decoding type: [3:1] unknown field "value"
   2 | type: "a"
>  3 | value:
       ^
   4 |     field1: "field1"

The problem is that unmarshal(&typ) looks at the definition of the unnamed structure and see that there are not fields other than Type, so any other other field will be treated as unknown, as per yaml.DisallowUnknownField() specification. Is there any solution to this problem?