Comment by leibnitz27

negative instanceof is a disaster

I posted a bit about it here http://www.benf.org/other/cfr/java14instanceof_pattern.html

it was first noted https://twitter.com/tagir_valeev/status/1210431331332689920 here

but the main thing is that if the 'taken' conditional is guaranteed to exit, then scope hiding happens, if not, not.

But in java

if (true) { throw new Exception(); }

is not guaranteed to exit.

So: https://github.com/leibnitz27/cfr_tests/blob/master/src_14/o...

In case you don't want to run, as of java (build 14-ea+34-1452) this prints:

Fred

WIBBLE

  public class InstanceOfPatternTest10 {
    static String s = "WIBBLE";

    public static void test(Object obj) {
        if (!(obj instanceof String s)) {
            throw new IllegalStateException();
        }
        System.out.println(s);
    }

    public static void test2(Object obj) {
        if (!(obj instanceof String s)) {
            if(true) {
                throw new IllegalStateException();
            }
        }
        System.out.println(s);
    }

    public static void main(String ... args) {
        test("Fred");
        test2("Fred");
    }
}