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  1. JDK
  2. JDK-8054937

5.1.10: Fresh capture variables sometimes can't conform to bounds



      Given the following declarations:

      abstract class Animal<T extends Animal<T>> {
        public abstract T bestFriend();
        public abstract Iterable<T> friends();
      abstract class Dog extends Animal<Dog> {}

      The capture of the following is specified to trigger a compiler error, per 5.1.10:

      Animal<? extends Dog> a; // bound of capture var is glb(Dog, Animal<CAP>)

      "It is a compile-time error if, for any two classes (not interfaces) Vi and Vj, Vi is
      not a subclass of Vj or vice versa."

      Actual javac behavior is to allow it, producing (apparently) an intersection of two different class types, 'Dog & Animal<CAP>'. These leads to arbitrary implementation-specific type checking behavior when members are looked up, e.g., in:


      Two possible resolutions:

      1) Enhance capture so that it does not produce an intersection of different class types, for example by inferring that 'capture(Animal<? extends Dog>) = Animal<Dog>' (because we know that a type like Animal<Poodle> is not in bounds).

      2) Decide that the type Animal<? extends Dog> is malformed and require javac to enforce the restriction.

      (Whether the conclusion is (1) or (2), similar reasoning could apply in the case of a final class, like Class<? extends String>. However, in that case there's nothing wrong with simply allowing the type as is without any special accommodations.)


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              • Assignee:
                dlsmith Dan Smith
                dlsmith Dan Smith
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