import org.checkerframework.checker.nullness.qual.*;

public class Polymorphism {
  // Test parameters
  @PolyNull String identity(@PolyNull String s) {
    return s;
  }

  void testParam() {
    // Test without inference
    String nullable = null;
    @NonNull String nonNull = "m";

    // :: error: (assignment)
    nonNull = identity(nullable); // invalid
    nonNull = identity(nonNull);

    // test flow
    nullable = "m";
    nonNull = identity(nullable); // valid
  }

  // Test within a method
  @PolyNull String random(@PolyNull String m) {
    if (m == "d") {
      // :: error: (return)
      return null; // invalid
    }
    return "m"; // valid
  }

  public static @PolyNull Object staticIdentity(@PolyNull Object a) {
    return a;
  }

  void testStatic(@Nullable Object nullable, @NonNull Object nonnull) {
    @Nullable Object nullable2 = staticIdentity(nullable);
    @NonNull Object nonnull2 = staticIdentity(nonnull);
  }
}