Record pattern matching with null gives wrong result

[datho7561]

Spotted by Jay in #1517 (comment)

Given the following code:

public class BooleanVariable {
	record Paper(int color) {}
	record Box<T>(T a) {}
	public static void main(String[] args) {
		Box<?> b = new Box<>(null);
		boolean res = b instanceof Box(Paper a);
		if (res) {
			System.out.println("res is true");
		} else {
			System.out.println("res is false");
		}
	}
}

ECJ prints "res is true" while javac prints "res is false".

[datho7561]

I'm planning on working on this

[datho7561]

If you instead do:

boolean res = b instanceof Box(Paper a) && a == null;

It will print "res is false", since a completely different method is used to generate boolean methods when a part of an && expression. I'll try to repurpose that method so that it will work without &&.

[datho7561]

This example from Jay's comment also doesn't work:

public class X {
	record Paper(int color) {}
	record Box<T>(T a) {}
	public static void main(String argv[]) {
		foo(null, null);
	}
	public static void foo(String abc, String def) {
		Box<?> p = new Box<>(new Paper(0));
		boolean b = false;
		switch (p) {
			case Box(Paper a) -> {
				b = true;
				break;
			}
			default -> {
				b = false;
				break;
			}
		}
		System.out.println(b);
	}
}

However, it gives a VerifyError in the generated bytecode instead of the wrong result.

[srikanth-sankaran]

@datho7561 - This is my plan for this ticket - I have assigned myself as a joint owner of this ticket. As mentioned in #1517 (comment) I am investigating and possibly undertaking a overhaul of code generation for patterns. If you don't mind, I would subsume this PR as a part of the larger work. Let us not rush this please. What do you say ?

[datho7561]

Okay. Just as a heads up, this issue is a blocker for code generation in lists of patterns in case statements: #1742

I've figured out how to get code generation working in eclipse-jdtls#9, but I'll probably need to rewrite the code after your overhaul of pattern code generation.

[srikanth-sankaran]

This example from Jay's comment also doesn't work:

public class X {
	record Paper(int color) {}
	record Box<T>(T a) {}
	public static void main(String argv[]) {
		foo(null, null);
	}
	public static void foo(String abc, String def) {
		Box<?> p = new Box<>(new Paper(0));
		boolean b = false;
		switch (p) {
			case Box(Paper a) -> {
				b = true;
				break;
			}
			default -> {
				b = false;
				break;
			}
		}
		System.out.println(b);
	}
}

However, it gives a VerifyError in the generated bytecode instead of the wrong result.

Happy to report that this one works fine on master, I'll push a regression test