FWIW, here's the example that doesn't work:

window.go

package window

// EventTarget interface
type EventTarget interface {
	DispatchEvent() Event
}

// Node interface
type Node interface {
	EventTarget

	ParentElement() HTMLElement
}

// Element interface
type Element interface {
	Node
}

// HTMLElement interface
type HTMLElement interface {
	Element
}

// Event interface
type Event interface {
	SrcElement() Element
}

input.go

package main

import "github.com/matthewmueller/deep-interface-bug/window"

func Element() window.Element {
	return nil
}

func main() {
	html := Element().(window.HTMLElement)
	html.DispatchEvent()
}

main.go

package main

import (
	"github.com/golang/tools/go/loader"
)

func main() {
	var conf loader.Config
	conf.Import("./input.go")

	program, err := conf.Load()
	if err != nil {
		panic(err)
	}

	println("got program!")
	_ = program
}

Here's the output when running main.go:

input.go:11:2: invalid operation: 
html (variable of type window.HTMLElement) has no field or method DispatchEvent

Full example: https://github.com/matthewmueller/deep-interface-bug

This is probably a dup of #18395.

CC @griesemer

@matthewmueller Could you please run your code using the latest (tip) version of go/types and report back if you see the same error? If it is indeed a duplicate of #18395 go/types will now complain with a respective error.

@matthewmueller I just confirmed myself that this is indeed a duplicate of #18395. A fix is in the works, but unfortunately it's too late to get that into Go 1.10. For:

package p

// EventTarget interface
type EventTarget interface {
	DispatchEvent() Event
}

// Node interface
type Node interface {
	EventTarget

	ParentElement() HTMLElement
}

// Element interface
type Element interface {
	Node
}

// HTMLElement interface
type HTMLElement interface {
	Element
}

// Event interface
type Event interface {
	SrcElement() Element
}

var _ = HTMLElement.DispatchEvent

gotype reports:

$ gotype x.go
x.go:22:2: internal error: incomplete embedded interface Element (issue #18395)
x.go:30:9: invalid operation: HTMLElement (type) has no field or method DispatchEvent

As a temporary work-around in this case you could manually inline the methods that the Element interface embeds via the Node type. This is always possible to do manually, and in fact the fix for go/types is to just do this automatically. But it's a larger fix which is why it's not going to make it anymore this round. Sorry.

Closing as duplicate of #18395.

Thanks for the response and workaround!

@griesemer I'll definitely give inlining a shot. It will definitely blow up the file sizes a lot because the DOM interfaces are huge – hopefully that doesn't slow down the compiler.

@matthewmueller Inlining all methods manually will always work but you may not have to go to that extreme to avoid the issue. You may be able to factor our a significant subset of the methods into a new interface that you then can embed.

Again, a fix is in the works ( https://go-review.googlesource.com/c/go/+/79575 ) and you could try the patch, but it's not the final version quite yet.

Thanks for the comments, I'm not sure factoring out by hand is an option because the DOM APIs are pretty massive and I'm not sure where all the cycles lie.

Again, a fix is in the works ( https://go-review.googlesource.com/c/go/+/79575 ) and you could try the patch, but it's not the final version quite yet.

Would this require a fork to Go itself or could I bring this in as a library?

Update Also a bit off topic. I'm not sure this would be helpful to the Go team, but I have a really massive generated 200kloc, working implementation (minus this bug) of the DOM in Go. Might be useful for testing + performance tuning, happy to share it if it'd be of value.

@matthewmueller You can just apply that patch ( https://go-review.googlesource.com/c/go/+/79575 ) to go/types (it's only affecting go/types) and use the updated library.

DOM implementation: Sure, let us know how to get it.