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  1. JDK
  2. JDK-4993221

Produce infinite types from lub as required by JLS


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      The main question I have is how type inference works in javac for
      cases where GJ-like type inference is actually undecidable. Unfortunately,
      with the introduction of variance annotations, the least upper bound
      of two types does not always exist. Naturally, this would lead to a
      looping compiler. javac doesn't loop but returns arbitrary results; so
      I wonder if this is intentional, and if it's intentional, I wonder
      what the correct behavior of a Java compiler is in such a case.
      Here is an example:

      class A<T> {}
      class B extends A<B> {}
      class C extends A<C> {}
      class D {
          <t> t choose(t x, t y) {
              return x;
          void foo() {
              B b = new B();
              C c = new C();
              A<? extends A<? extends A<?>>> x = choose(b, c);

      The polymorphic method 'choose' of class D expects two parameters of the
      same type. When this function is called, the compiler has to infer the
      "best" instantiation of the type parameter such that both actual
      parameters conform to this type. In my example, the compiler has to
      compute the least upper bound of B and C. A first approximation is
      A<? extends A<?>>, a better type would be A<? extends A<? extends A<?>>>,
      and so on. The problem now is that javac simply infers the type
      A<? extends A<?>>; this decision is arbitrary and if a programmer
      wants a better type (like in the program above), he has to enforce
      the instantiation manually by giving the desired type. I personally
      think that simply returning *a* solution is not a good idea; the compiler
      should try to construct a really big type and fail if the type required
      by the user is bigger (which never happens if the limit of the compiler
      is high enough). Then you could sell this as an implementation restriction,
      which is completely unrelevant for basically all hand-written Java


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              • Assignee:
                dlsmith Dan Smith
                gafter Neal Gafter
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