I can't find any wording in the spec that would explain the legality or otherwise of
this program.  The question is whether isomorphic recursive interfaces are identical and
thus mutually assignable.   My intuition about interfaces would suggest that these types
ought to be identical.

http://play.golang.org/p/r92J5TY-WI

package main 

type I interface {
    f() I
}
type J interface {
    f() J
}

func main() {
    var i I
    var j J
    i = j
    j = i
}

gc and go/types both give an error.

gc:
prog.go:13: cannot use j (type J) as type I in assignment:
    J does not implement I (wrong type for f method)
        have f() J
        want f() I
prog.go:14: cannot use i (type I) as type J in assignment:
    I does not implement J (wrong type for f method)
        have f() I
        want f() J

go/types:
a.go:13:6: cannot assign j (variable of type J) to i (variable of type I)