1.1.4.2. nullability.NullReturnedFromNonnull (ObjC)
Warns when a null pointer is returned from a function that has _Nonnull return type.

Examples:

- (nonnull id)firstChild {
  id result = nil;
  if ([_children count] > 0)
    result = _children[0];

  // Warning: nil returned from a method that is expected
  // to return a non-null value
  return result;
}