[patterns] Several patterns and guards

[Tagir Valeev]

Hello!

Currently, when the switch label contains several patterns, only one
guard could be declared, which is applied to all the patterns at once.
In other words, the following code is not possible:

void test(Object obj) {
  switch (obj) {
    case Integer _ when ((Integer) obj) > 0,
         String _ when !((String) obj).isEmpty()
            -> System.out.println("Positive number or non-empty string");
    default -> System.out.println("other");
  }
}

Does it make sense to lift this restriction? Probably it could be
useful to declare separate guards? Ideally it should be possible to be
able to declare a pattern variable, which is visible inside the
pattern-specific guard only (but not inside the rule body).

Another confusing thing here:

void test(Object obj) {
  switch (obj) {
    case Integer _,
         String _ when !((String) obj).isEmpty()
            -> System.out.println("Number or non-empty string");
    default -> System.out.println("other");
  }
}

Now, the guard is applied even if obj is Integer (resulting in
ClassCastException). This is not quite evident from the code. We may
say that 'when' precedence is lower than ',' precedence, but people
may expect the opposite. Should not we reconsider this and make guard
a part of the lebel element, rather than the part of the whole label?

With best regards,
Tagir Valeev.