title: Why is the extension of my Swift class not visible outside the defining file?
body: <p>I have an Xcode-generated NSManagedObject class for my CoreData model.</p>
  
  <pre><code>@objc(SomeClass) class SomeClass : NSManagedObject { /* ... */ }
  </code></pre>
  
  <p>It is defined in a file named 'SomeClass.swift'. I would like to extend this class, so I created 'SomeClassExtension.swift'. I define the extension like this:</p>
  
  <pre><code>extension SomeClass {
      class func typeMethod1() {}
      func instanceMethod2() {}
  }
  </code></pre>
  
  <p>These extension methods can be used within this defining file, but they are not visible outside of it. What is causing this issue?</p>