Interface that returns it's self (Cloneable)

Question

I'm trying to create a cloneable interface and am running into some problems getting structs to implement the interface. It appears this is a limit of go which isn't in many other langauges. I'm trying to understand the justification for this limit.

var _ Cloneable = test{}

type Cloneable interface {
    Clone() Cloneable
}

type test struct {
}

func (t *test) Clone() *test {
    c := *t
    return &c
}

Playground: https://play.golang.org/p/Kugatx3Zpw

Followup question since it still seems weird to me. This also does not compile

var _ Cloneable = &test{}

type Cloneable interface {
    Clone() Cloneable
}

type Cloneable2 interface {
    Clone() Cloneable2
}

type test struct {
}

func (t *test) Clone() Cloneable2 {
    c := *t
    return &c
}

Playground: https://play.golang.org/p/jlyMDPF1WB

Answer 1

To satisfy an interface method, the argument and return types must use the same types used in the interface declaration. The Clone method must return a Cloneable to satisfy the interface:

func (t *test) Clone() Cloneable {
  c := *t
  return &c
}

The Clone method cannot return a *test or Cloneable2 because these types are not the Cloneable type.

The pointer type implements the interface:

var _ Cloneable = &test{}

Because the test type must satisfy the Cloneable interface for the Clone method to compile, this compile time assertion is not needed.

playground example

(The question is a moving target. This is an answer to two previous edits of the question.)