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  1. JDK
  2. JDK-7045341

Left subtyping argument must be captured



    • generic
    • generic


      Per JLS 4.10.2, a subtyping test that involves indentifying the direct supertypes of a wildcard-parameterized type must capture the type before performing a substitution:

      "The direct supertypes of the type C<T1,...,Tn>, where Ti (1 ≤ i ≤ n) is a type, are D<U1 θ,...,Uk θ>, where:
      • D<U1,...,Uk> is a direct supertype of C<F1,...,Fn>, and θ is the substitution [F1:=T1,...,Fn:=Tn].
      • C<S1,...,Sn> where Si contains Ti (§4.5.1) for 1 ≤ i ≤ n.

      "The direct supertypes of the type C<R1,...,Rn>, where at least one of the Ri (1 ≤ i ≤ n) is a wildcard type argument, are the direct supertypes of C<X1,...,Xn>, where C<X1,...,Xn> is the result of applying capture conversion (§5.1.10) to C<R1,...,Rn>."

      javac does not do this; as a result, unsound results are permitted. The following program compiles without error and crashes at runtime because the subtyping implementation allows C<?> <: A<Box<?>>.

      public class WildSubstitution {

      static class Box<T> {
       private T val;
       public Box(T val) { this.val = val; }
       public T get() { return val; }
       public void set(T val) { this.val = val; }

      interface A<T> { T get(); void set(T arg); }
      interface B<T> extends A<T> {}
      interface C<S> extends A<Box<S>> {}

      static class D implements B<String> {
       String s;
       public String get() { return s; }
       public void set(String s) { this.s = s; }

      static class E implements C<String> {
       Box<String> b;
       public Box<String> get() { return b; }
       public void set(Box<String> arg) { b = arg; }

      public static void main(String... args) {
       // B<?> <: A<?> is true
       Box<D> b1 = new Box<D>(new D());
       Box<? extends B<?>> b2 = b1;
       Box<? extends A<?>> b3 = b2;

       // C<?> <: A<Box<?>> is false
       Box<E> b4 = new Box<E>(new E());
       Box<? extends C<?>> b5 = b4;
       Box<? extends A<Box<?>>> b6 = b5;

       b6.get().set(new Box<Integer>(10));
       String s = b4.get().get().get();



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              dlsmith Dan Smith
              dlsmith Dan Smith
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