Wrong implementation of nested interfaces in Golang

Question

I'm struggling to understand why the following code cannot be compiled:

package main

import "fmt"

type A interface {
    Hello() B
}

type B interface {
    World()
}

type AStruct struct{}

func (a AStruct) Hello() BStruct { 
    return BStruct{}
}

type BStruct struct{}

func (b BStruct) World() {}

func build(a A) {
    fmt.Println("hello")
}

func main() {
    a := AStruct{}
    build(a)
}

The compilation error is: cannot use a (variable of type AStruct) as A value in argument to build: AStruct does not implement A (wrong type for method Hello) have Hello() BStruct want Hello() B

If I change func (a AStruct) Hello() B, the error is gone. Shouldn't BStruct implement the B interface?

<< UPDATED CODE >>

How shall I modify the code if the interface definition and the methods are in different packages.

packX.go

package packX

import "fmt"
import "packY"

type A interface {
    Hello() B
}

type B interface {
    World()
}

func build(a A) {
    fmt.Println("hello")
}

func read() {
    a := packY.AStruct{}
    build(a)
}

and the packY.go

package packY

type AStruct struct{}

func (a AStruct) Hello() BStruct { 
    return BStruct{}
}

type BStruct struct{}

func (b BStruct) World() {}

If I change func (a AStruct) Hello() BStruct to func (a AStruct) Hello() packX.B, then I have circular import.

Answer 1

Go type system uses strict function signature matching: if an interface method is declared to return B, then a type method implementing that interface must return B, and not a type that implements B.