ADDITIONAL SYSTEM INFORMATION :
openjdk version "20.0.2" 2023-07-18
OpenJDK Runtime Environment (build 20.0.2+9-78)
OpenJDK 64-Bit Server VM (build 20.0.2+9-78, mixed mode, sharing)

Run on Mac OS 13.4.1

A DESCRIPTION OF THE PROBLEM :
The jep 433 introduced the null case label. In previous versions (< java 20), the null case could be combined with other cases. Like this
switch (o) {
  case null, Integer i -> {}
  default -> {}
}
Now the compiler outputs an error: invalid case label combination

REGRESSION : Last worked in version 19

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Here's a minimal code sample. Run it with
java --enable-preview --source=20 Test.java

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The code is expected to compile
ACTUAL -
Test.java:6: error: invalid case label combination
            case null, A -> System.out.println("null or A");
                       ^
Note: Test.java uses preview features of Java SE 20.
Note: Recompile with -Xlint:preview for details.
1 error
error: compilation failed

---------- BEGIN SOURCE ----------
public class Test {
    public static void main(String[] args) {
        enum E {A, B}
        E e = E.A;
        switch (e) {
            case null, A -> System.out.println("null or A");
            case B -> System.out.println("B");
        }
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
The null case is actually combinable with the default clause. So this works

public class Test {
    public static void main(String[] args) {
        enum E {A, B}
        E e = E.A;
        switch (e) {
            case B -> System.out.println("B");
            case null, default -> System.out.println("null or A");
        }
    }
}

FREQUENCY : always