title: proguard: can't find referenced field 'java.lang.Boolean default'
body: <pre><code>interface Foo {
  
      Boolean getDefault();
  }
  
  
  class FooImpl implements Foo {
  
     private Boolean defaultValue;
  
     @Override
     public Boolean getDefault() {
        return defaultValue;
     }
  }
  </code></pre>
  
  <p>Proguard: can't find referenced field 'java.lang.Boolean default' in class FooImpl. What is wrong? </p>