What version of Go are you using (go version)?

$ go version
go version go1.14.10 windows/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

$ go env
set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\x00371158\AppData\Local\go-build
set GOENV=C:\Users\x00371158\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS= -mod=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=D:\goproject
set GOPRIVATE=
set GOPROXY=https://goproxy.io,direct
set GOROOT=D:\go1.14
set GOSUMDB=off
set GOTMPDIR=
set GOTOOLDIR=D:\go1.14\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=D:\xuxinhao\gana\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\X00371~1\AppData\Local\Temp\go-build498719130=/tmp/go-build -gno-record-gcc-switches

What did you do?

old package

type i interface {
	b()
}

type I interface {
	A()
	i
}

type X struct {

}

func (x X) b() {
}

if we change the I definition to this(adding a B() method), new package:

type i interface {
	b()
}

type I interface {
	A()
	B()
	i
}

type X struct {

}

func (x X) b() {
}

client code

type Impl struct {
	apidiff.X
}

func (i Impl) A() {
}

func TestAssert(t *testing.T)  {
	var a apidiff.I = Impl{}
	fmt.Println(a)
}

What did you expect to see?

Incompatible changes

What did you see instead?

Compatible changes:
- I.B: added

in the readme of this package, here, it says:

Interfaces
A new interface is compatible with an old one if and only if:

1. The old interface does not have an unexported method, and it corresponds to the new interfaces (i.e. they have the same method set), or
2. The old interface has an unexported method and the new exported method set is a superset of the old.

Rule 2 is based on the observation that if an interface has an unexported method, the only way a client can implement it is to embed it. Adding a method is compatible in this case, because the embedding struct will continue to implement the interface. Adding a method also cannot break any call sites, since no program that compiles could have any such call sites.

in my example, this assumption is wrong.