Finalizing in JDK 16 - Pattern matching for instanceof

[Guy Steele]

> On Aug 26, 2020, at 2:46 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
> 
> 
>>> 
>>> Proposed: An `instanceof` expression must be able to evaluate to both true and false, otherwise it is invalid.  This rules out strongly total patterns on the RHS.  If you have a strongly total pattern, use pattern assignment instead.  
>> 
>> Makes sense to me, but one question: would this restriction "must be able to evaluate to both true and false” be applied to _every_ `instanceof` expression, or only those that have a pattern to right of the `instanceof` keyword?  I ask because if it is applied to _every_ `instanceof` expression, this would represent an incompatible change to the behavior of `x instanceof Object`, among others.  Is it indeed the intent to make an incompatible change to the language?
> 
> Well, it's more of an aspiration than a rule -- based on what we already do.  We already use some form of this to rule out bad casts:
> 
>     String s = ...
>     if (s instanceof Integer)  { ... }
>     // Error, incompatible types
> 
> So here, we have a candidate instanceof that would always be false, which the compiler can derive by type analysis, and was always rejected.  It would be joined by 
> 
>     if (s instanceof Object o) { ... }
> 
> because Object o is total in this case, but not
> 
>     if (s instanceof Object)
> 
> because this _can_ yield either true (if s is not null) or false (if it is.)  

Thank you: I had missed this point about null.  Duh.


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