case null vs case dominance

[Remi Forax]

Hi all,
the first part of the message is about javac error message that could be improved,
the second part is about the current spec being not very logical.

With this code

    Object o = null;
    var value = switch(o) {
          //case null -> 0;
          case Object __ -> 0;
          case null -> 0;
    };
    System.out.println(value);

The error message is
  PatternMatching101.java:70: error: this case label is dominated by a preceding case label
          case null -> 0;
          ^

The error message is wrong here, because it's 'case null' and you can put a case null where you want but below a total pattern, so the error mesage should reflect that.

Here is an example that compiles showing that case null can be below a case String, which it dominates

    Object o = null;
    var value = switch(o) {
          case String s -> 0;
          case null -> 0;
          default -> 0;
    };

Also with default, the spec says that the code below should compile (and it works with javac),
because default and case null are disjoint in term of type, but it feels wrong to me.

    Object o = null;
    var value = switch(o) {
          default -> 0;
          case null -> 0;
    };
    System.out.println(value);

The problem is that sometimes 'default' acts as a total pattern sometimes it does not, if there is a case null. I wonder if it's not better to ask users to put the case null on top.

Rémi