As reported here:
http://mail.openjdk.java.net/pipermail/amber-dev/2019-March/004204.html

javac fails to compile this code:
---
enum MyEnum {
    X, Y;
     
    public static void main(String[] args) {
         MyEnum myEnum = MyEnum.X;
         int o;
         var f = switch(myEnum) {
             case X -> o = 5;
             case Y -> o = 10;
             // automatic exception
         };
         System.out.println(o);
     }
}
---

---
$ javac --enable-preview --source 13 /tmp/MyEnum.java
/tmp/MyEnum.java:12: error: variable o might not have been initialized
         System.out.println(o);
                            ^
Note: /tmp/MyEnum.java uses preview language features.
Note: Recompile with -Xlint:preview for details.
1 error
---