為什麼我的Go介面方法回傳類型會導致編譯錯誤？

type IA interface {
    FB() IB
}

type IB interface {
    Bar() string
}

type A struct {
    b *B
}

func (a *A) FB() *B {
    return a.b
}

type B struct{}

func (b *B) Bar() string {
    return "Bar!"
}

cannot use a (type *A) as type IA in function argument:
    *A does not implement IA (wrong type for FB method)
        have FB() *B
        want FB() IB

func (a *A) FB() IB {
    return a.b
}

import (
    "foo" // Package containing IB interface
)

// Implementation in package bar
func (a *A) FB() foo.IB {
    return a.b
}