title: IntelliJ: cannot find java.util.Optional
body: <p>In a new project in IntelliJ I have set up a Java 8 JRE/JDK and language level 8.0.</p>
  
  <p>But for some reason, the IDE cannot find <code>java.util.Optional</code>. The project compiles and works just fine (from within the IDE, too) but code completion does not work.</p>
  
  <p>Any ideas? I'd like to ask a few other users before creating a ticket on their bugtracker...</p>