go/types: "invalid AST" error for syntactically correct (but otherwise invalid) program

[griesemer]

x.go:

package p

type T = interface {
	m() interface{ T }
}

gotype x.go produces

$ gotype x.go
x.go:4:17: invalid AST: T is not a named type
x.go:4:17: invalid AST: T is not a named type

For reference:
cmd.compile:

x.go:3:6: invalid recursive type alias T
	x.go:3:6: T uses <T>
	x.go:3:10: <T> uses <T>
	x.go:4:3: <T> uses <T>
	x.go:4:6: <T> uses T

gccgo accepts this code w/o error.

[griesemer]

gp/types incorrectly assumes (and asserts) that the embedded type name denotes a defined type (or what we called a "named" type before the introduction of type aliases).

[griesemer]

The harder issue is that this property is exposed via the API, through the NewInterface and Embedded methods that create/access interfaces. Fixing this may require new API surface.

[griesemer]

PS: Both cmd/compile and gccgo accept this code:

package main

type (
	A = interface {
		M()
	}
	T interface {
		A
	}
	S struct{}
)

func (S) M() { println("m") }

func main() {
	var t T = S{}
	t.M()
}

gccgo also accepts the example of the issue:

I = interface {
	m() interface{ I }
}

[gopherbot]

Change https://golang.org/cl/114317 mentions this issue: go/types: permit embedding of non-defined interfaces via alias type names

[zchee]

@griesemer Hi,
I used tip, and sometimes use github.com/google/go-cmp package for testing.

Currently tip is can’t building go-cmp package because you merged
c730a93
and maybe caused by following line:

• https://github.com/google/go-cmp/blob/master/cmp/options.go#L53-L55
• https://github.com/google/go-cmp/blob/master/cmp/options.go#L50

Do you have any idea? We should fix go-cmp package?

[griesemer]

@zchee Can you please describe the problem you're seeing in more detail. It's a bit unclear from your message.

Also, there was a bug in https://golang.org/cl/114317 which has been fixed as well. Please make sure all your code is at the latest tip.