$ cat q.go
package main

type (
	a interface { m() interface { a; b } }
	b interface { m() interface { a; b } }
)

var _ = new(interface{ a }) == new(interface{ b })

func main() {}

$ gccgo q.go
q.go:4:39: error: duplicate method ‘m’
    4 |         a interface { m() interface { a; b } }
      |                                       ^