cmd/go2go: design corner cases when passing interface type arguments

[tdakkota]

What version of Go are you using (go version)?

$ go version
go version devel +08b9fd164c Tue Jun 30 19:00:27 2020 +0000 windows/amd64

Does this issue reproduce with the latest release?

n/a

What operating system and processor architecture are you using (go env)?

• windows/amd64
• Go2go playground

What did you do?

This code compiles https://go2goplay.golang.org/p/-5nF8t38Ebr - it's incorrect.

But this code does not compile https://go2goplay.golang.org/p/MdJl77uLB-g.

package main

import (
	"fmt"
)

type AwesomeMap(type V interface {
	type interface{}
}) map[string]V

type A interface {
	type interface{}
	Method()
}

func main() {
	var i AwesomeMap(A)
	fmt.Println(i)
}

What did you expect to see?

Compilation error in both cases.
type interface{} can't satisfy interface {Method();}

What did you see instead?

Successful compilation.

[ianlancetaylor]

CC @griesemer

Sorry, I'm not sure what you are saying. You have two different examples. One compiles, one doesn't (because AwesomeMap is not defined). Under "what did you see instead" you listed both "successful compilation" and a type checking error.

Can you clarify the bug report? Thanks.

[tdakkota]

Updated.

[griesemer]

There's a couple of issues here. In

type AwesomeMap(type V interface {
	type interface{}
}) map[string]V

var _ AwesomeMap(interface {
	type interface{}
	Method()
})
...

it's not clear if we should permit a type argument that itself contains a type list (this is a design question). Also, should it be possible to pass a type argument that is an interface with Method to a type parameter that must be an interface (but doesn't have any methods)? I think that's another open design question. It's not clear to me that this should be legal or illegal for that matter.

[griesemer]

The 2nd example behaves more sane (error reported), but I think the current design draft is unclear about exactly should happen in these cases.