[patterns] Nullability in patterns, and pattern-aware constructs (again)

[Guy Steele]

> On Jan 10, 2020, at 11:03 AM, Brian Goetz <brian.goetz at oracle.com> wrote:
> 
> I don't want to dive into a design discussion on declared patterns, but I think we can resolve most of the nullability issues without doing so.  
> 
> Like methods, declared patterns are members.  Like methods, they can be static or instance.  
> 
> If they are instance, the target is the receiver, and that surely won't work if the receiver is null.  So by definition, instance patterns cannot match null.  Let's set that aside.  
> 
> (Example: an instance pattern is a pattern on a class, like:
> 
>     switch (aString) {
>         case palindrome(var firstHalf, boolean isOdd): ...
>     }
> 
> where palindrome is an instance pattern on String.)
> 
> Declared patterns have a target type (may want a better name), which for an instance pattern is the receiver type, and for a static pattern is part of the declaration.  (For example, `Optional.of(var x)` has a target type of `<T> Optional<T>`.)  
> 
> We won't even try to match the pattern unless the match target is of the target type for the pattern (it's not an Optional.of(anything) if its not an Optional to begin with.)  So we have to do some sort of test first -- either instanceof (in which case static pattern will never match null, period), or a type pattern match (which would be the more principled choice, and would only admit a null through to the user-written code if the target type of the static pattern were total on the operand of the switch/instanceof.)  
> 
> So it's already a pretty narrow case where a null would be admitted into the user code at all.  Now, the finishing blow: do we really want to ask the authors of the `Optional.of(var x)` and `Optional.empty()` patterns to have to check their target for null?  That would be creating a million new bugs to accommodate a handful of corner cases.  
> 
> Which brings us back to ...
> 
> Only the top (total) and bottom (null constant) patterns match null.  Guy is happy again.  (And the crowd goes wild.)

Nice bit of reasoning, and I am indeed happy again.  (I was hoping I would have to write “switch?” exactly once in my life.  Well, this makes twice.  But this isa much better solution.)

—Guy


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