title: Observable.combineLatest type inference in kotlin
body: <p>I'm using RxJava2, Kotlin-1.1 along with RxBindings in my project.</p>
  
  <p>I have simple login screen with 'login' button disabled by default, I want to enable the button only when username and password edittext fields are not empty.</p>
  
  <p><strong>LoginActivity.java</strong></p>
  
  <pre><code>Observable&lt;Boolean&gt; isFormEnabled =
      Observable.combineLatest(mUserNameObservable, mPasswordObservable,
          (userName, password) -&gt; userName.length() &gt; 0 &amp;&amp; password.length() &gt; 0)
          .distinctUntilChanged();
  </code></pre>
  
  <p>I'm unable to translate the above code from Java to Kotlin:</p>
  
  <p><strong>LoginActivity.kt</strong></p>
  
  <pre><code>class LoginActivity : AppCompatActivity() {
  
    val disposable = CompositeDisposable()
  
    private var userNameObservable: Observable&lt;CharSequence&gt;? = null
    private var passwordObservable: Observable&lt;CharSequence&gt;? = null
  
    override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_login)
      initialize()
    }
  
    fun initialize() {
      userNameObservable = RxTextView.textChanges(username).skip(1)
          .debounce(500, TimeUnit.MILLISECONDS)
      passwordObservable = RxTextView.textChanges(password).skip(1)
          .debounce(500, TimeUnit.MILLISECONDS) 
    }
  
    private fun setSignInButtonEnableListener() {
      val isSignInEnabled: Observable&lt;Boolean&gt; = Observable.combineLatest(userNameObservable,
          passwordObservable,
          { u: CharSequence, p: CharSequence -&gt; u.isNotEmpty() &amp;&amp; p.isNotEmpty() })
    }
  }
  </code></pre>
  
  <p>I assumed it's something related to type inference of the third argument in <code>combinelatest</code>, but I don't get the issue properly by reading the error message: 
  <a href="https://i.stack.imgur.com/obrG5.png" rel="noreferrer"><img src="https://i.stack.imgur.com/obrG5.png" alt="Type Inference issue"></a></p>