switch v := v.(type) {
	case T:
		return v
	default:
		var z T
		return z
	}
}

type Mystring string

func (Mystring) Foo() {
}

func Conv4(v interface{Foo()}) Mystring {
	return conv4[Mystring](v)
}

func conv4[T interface{Foo()}](v interface{Foo()}) T {
	switch v := v.(type) {
	case T:
		return v
	default:
		var z T
		return z
	}

	if !ok {
		panic("conversion failed")
	}
	if y != s {
		panic(fmt.Sprintf("got %s wanted %s", y, s))
	}
	z := a.Conv3(s)
	if z != s {
		panic(fmt.Sprintf("got %s wanted %s", z, s))
	}
	w := a.Conv4(a.Mystring(s))
	if w != a.Mystring(s) {
		panic(fmt.Sprintf("got %s wanted %s", w, s))
	}