A DESCRIPTION OF THE PROBLEM :
This problem is about anonymous inner classes extending inner classes nested in generic classes when the outer instance has a wildcard in its type. It creates a class whose super class has a capture type as type parameter. That causes multiple problems.
For one, this can cause heap pollution (without unchecked warnings).
This also makes javac write out an incorrect type signature, causing a java.lang.reflect.GenericSignatureFormatError when using Class.getGenericSuperclass().

I originally wrote about this problem in this blog post: http://wouter.coekaerts.be/2018/java-type-system-broken


---------- BEGIN SOURCE ----------
public class CaptureInSuperClass {
  static class Box<A> {
    A a;
    Box(A a) { this.a = a; }

    class Inner {
      void setA(A newA) {
        a = newA;
      }

      A getA() {
        return a;
      }
    }
  }

  static Object prev;
  public static void main(String[] args) {
    Box<String> stringBox = new Box<>("1");
    for (Box<?> box : List.of(stringBox, new Box<>(1))) {
      box.new Inner() {{
        if (prev == null) prev = this;
        else this.getClass().cast(prev).setA(this.getA());
      }};
    }

    // ClassCastException
    String s = stringBox.a;

    // GenericSignatureFormatError
    ((Box<?>)stringBox).new Inner(){}.getClass().getGenericSuperclass();
  }
}
---------- END SOURCE ----------

FREQUENCY : always