func TestCompleteSignup(t *testing.T) { var sentTo string mockedEmailSender = &EmailSenderMock{ SendFunc: func(to, subject, body string) error { sentTo = to // ← 引数アサーション return nil }, } CompleteSignUp("me@email.com", mockedEmailSender) callsToSend := len(mockedEmailSender.SendCalls()) if callsToSend != 1 { t.Errorf("Send was called %d times", callsToSend) } // ↓引数アサーション if sentTo != "me@email.com