An example design for goa v3 does not generate Validation function

design.go

package design

import . "goa.design/goa/v3/dsl"

var _ = API("api", func() {
	Title("sample api")
	Server("api", func() {
		Host("localhost", func() { URI("http://localhost:8088") })
	})
})

var _ = Service("calc", func() {
	Description("The calc service performs operations on numbers")
	Method("add", func() {
		Payload(func() {
			Attribute("a", Int, "Left operand")
			Attribute("b", Int, "Right operand")
			Required("a", "b")
		})
		Result(MainType)
		HTTP(func() {
			GET("/add/{a}/{b}")
			Response(StatusOK)
		})
	})
})

var MainType = ResultType("vnd.application/main+json", func() {
	ContentType("application/json")

	Attributes(func() {
		Attribute("version", String, "API version")
		Attribute("SubType", SubType, "sub type")
		Required("version")
	})

	View("default", func() {
		Attribute("version")
		Attribute("SubType")
	})
})

var SubType = Type("DSubTypeata", func() {
	Attribute("ID", String)
	Attribute("ThirdTypes", ThirdTypes)
	Required("ID")
})

var ThirdTypes = Type("ThirdTypes", func() {
	Attribute("ThirdType", ArrayOf(ThirdType))
})

var ThirdType = Type("ThirdType", func() {
	Attribute("Image", Image)
})

var Image = Type("Image", func() {
	Attribute("Width", Int, "width")
	Attribute("Height", Int, "height")
	Attribute("URL", String)

	Required("Width")
	Required("Height")
	Required("URL")
})