cmd/compile: "missing method" diagnostic can be confusing when methods aren't promoted due to ambiguity #57352
Description
What version of Go are you using (go version)?
$ go version go version go1.19.4 linux/amd64
Does this issue reproduce with the latest release?
yes
What operating system and processor architecture are you using (go env)?
playground
What did you do?
package main
type A interface {
a()
}
type AB interface {
A
b()
}
type Foo struct {
A
AB
}
func main() {
var _ AB = Foo{}
}https://go.dev/play/p/_SQyOJ_n2qv
What did you expect to see?
Either no error or an error saying that Foo has 2 a methods.
What did you see instead?
./prog.go:18:13: cannot use Foo{} (value of type Foo) as type AB in variable declaration:
Foo does not implement AB (missing a method)
I didn't find this case in spec, but the message seems wrong.