ListenableFuture<?>[] futures = new ListenableFuture<?>[0];
    Callable<String> combiner =
        new Callable<String>() {
          @Override
          public String call() throws Exception {
            return "hi";
          }
        };

    // We'd like for all the following to compile.
    ListenableFuture<String> unused;

    // Compiles:

    ListenableFuture<?>[] futures = new ListenableFuture<?>[0];
    Callable<String> combiner =
        new Callable<String>() {
          @Override
          public String call() throws Exception {
            return "hi";
          }
        };

    // We'd like for all the following to compile.
    ListenableFuture<String> unused;

    // Compiles: