[patterns] Unchecked cast warning is absent?

[Maurizio Cimadamore]

On 03/10/17 09:16, Remi Forax wrote:
> Maurizio,
> __matches <=> instanceof + cast,
> the instanceof part is currently illegal, so the implication is that __matches should be illegal too.
 From a code gen perspective I agree with you. From a type checking 
perspective I don't otherwise you would make pattern matching 
incompatible with generics:

List<String> ls = ...
if (x matches ArrayList<String> a) {
   ...
} else if (x matches LinkedList<String> ll) {
   ...
}

I think accepting programs such as these is useful (and actually the 
above is not even an unsafe program!). But, using your strict 
interpretation, I can't do an instanceof whose target type is 
ArrayList<String> - so this should be rejected?

I think instanceof makes sense to describe what the generated code 
should do - but I believe applying the same type checking rules you have 
for instanceof would result in overly restrictive semantics.

Maurizio
>
> Rémi
>
> ----- Mail original -----
>> De: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
>> À: "Tagir Valeev" <amaembo at gmail.com>, "amber-dev" <amber-dev at openjdk.java.net>
>> Envoyé: Mardi 3 Octobre 2017 10:12:11
>> Objet: Re: [patterns] Unchecked cast warning is absent?
>> I don't think you want to disallow. It's same as cast - you can do
>>
>> (T)obj
>>
>> Which is a no-op, and you get an unchecked warning for it. If T has some
>> more interesting bound, the cast is not a no-op.
>>
>> Maurizio
>>
>>
>> On 03/10/17 05:28, Tagir Valeev wrote:
>>> It's even more interesting that method type parameter is allowed in __matches:
>>>
>>>     static <T> void matches(Object obj) {
>>>       if(obj __matches T t) {
>>>         System.out.println("Matches!");
>>>       }
>>>     }
>>>
>>> Bytecode shows that "instanceof Object" is generated which is pretty
>>> useless. I think, such construct should be disabled and result in
>>> compilation error (similarly to instanceof).
>>>
>>> With best regards,
>>> Tagir Valeev.
>>>
>>> On Mon, Oct 2, 2017 at 4:45 PM, Tagir Valeev <amaembo at gmail.com> wrote:
>>>> Hello!
>>>>
>>>> Just tried to play with pattern matching implementation. Seems that
>>>> the matching against a non-reifiable type is unsafe, which is
>>>> expected. Should the warning be displayed in this case?
>>>>
>>>> E.g.:
>>>>
>>>> import java.util.*;
>>>>
>>>> public class ListTest {
>>>>
>>>>     static void matches(Object obj) {
>>>>       if(obj __matches List<String> list) {
>>>>         System.out.println(list.get(0));
>>>>       }
>>>>     }
>>>>
>>>>     public static void main(String[] args) {
>>>>       matches(Collections.singletonList(1));
>>>>     }
>>>> }
>>>>
>>>> $ ~/j/bin/javac -Xlint:all ListTest.java
>>>> (compilation successful, no warning)
>>>> $ ~/j/bin/java ListTest
>>>> Exception in thread "main" java.lang.ClassCastException:
>>>> java.base/java.lang.Integer cannot be cast to
>>>> java.base/java.lang.String
>>>> at ListTest.matches(ListTest.java:7)
>>>> at ListTest.main(ListTest.java:12)
>>>>
>>>> With best regards,
>>>> Tagir Valeev.