Feedback on nulls in switch

[Mark Staller]

Just throwing in my 2 cents on how I would expect `switch` to work with
pattern matching.

#1
switch (a) {  // <- NPE in case of `a == null`

     case Box(_type_or_var_ b): // We are here if `a instanceof Box`.
                                // `b` can be `null`

     case Integer i: // We are here if `a instanceof Integer`.
                     // `i` can't be `null`

     case Object o: // We are here if `a instanceof Object`.
                    // `o` can't be `null`
}

#2
switch (a) {  // <- NPE in case of `a == null`

     case Box(_type_or_var_ b): // We are here if `a instanceof Box`.
                                // `b` can be `null`

     case Integer i: // We are here if `a instanceof Integer`.
                     // `i` can't be `null`

     case Object o: // We are here if `a instanceof Object`.
                    //`o` can't be `null`

     default: // All other cases
}

#3
switch (a) {  // <- No NPE in case of `a == null`

     case null: // We are here if `a == null`

     case Integer i: // We are here if `a instanceof Integer`.
                     // `i` can't be `null`

     case Box(_type_or_var_ b): // We are here if `a instanceof Box`.
                                // `b` can be `null`

     case Object o: // We are here if `a instanceof Object`.
                    // `o` can't be `null`
}

#4
switch (a) {  // <- No NPE in case of `a == null`

     case null: // We are here if `a == null`

     case Box(_type_or_var_ b): // We are here if `a instanceof Box`.
                                // `b` can be `null`

     case Integer i: // We are here if `a instanceof Integer`.
                     // `i` can't be `null`

     case Object o: // We are here if `a instanceof Object`.
                    // `o` can't be `null`

     default: // All other cases
}

#1 and #2 keep the legacy behavior and compatibility.
Any `switch` that has a `case null:` does not throw NPE (signaling "new
null-friendly switch mode", kind of like how `->` signaled new
expression switch).
`default` does not catch nulls (this is debatable instead of `case
null:` if legacy behavior doesn't matter).
Deconstruction patterns are kept null agnostic, no matter if Box(var
b)/Box(Object b)/...

Mark