The introduction of Sequenced collections is not a source compatible change

[Raffaello Giulietti]

Without changing the semantics at all, you could also write

	final List<Collection<String>> list = 
Stream.<Collection<String>>of(nestedDequeue, nestedList).toList();

to "help" type inference.




On 2023-05-03 15:12, forax at univ-mlv.fr wrote:
> Another example sent to me by a fellow French guy,
> 
>      final Deque<String> nestedDequeue = new ArrayDeque<>();
>      nestedDequeue.addFirst("C");
>      nestedDequeue.addFirst("B");
>      nestedDequeue.addFirst("A");
> 
>      final List<String> nestedList = new ArrayList<>();
>      nestedList.add("D");
>      nestedList.add("E");
>      nestedList.add("F");
> 
>      final List<Collection<String>> list = Stream.of(nestedDequeue, nestedList).toList();
> 
> This one is cool because no 'var' is involved and using collect(Collectors.toList()) instead of toList() solves the inference problem.
> 
> Rémi
> 
> ----- Original Message -----
>> From: "Stuart Marks" <stuart.marks at oracle.com>
>> To: "Remi Forax" <forax at univ-mlv.fr>
>> Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>
>> Sent: Tuesday, May 2, 2023 2:44:28 AM
>> Subject: Re: The introduction of Sequenced collections is not a source compatible change
> 
>> Hi Rémi,
>>
>> Thanks for trying out the latest build!
>>
>> I'll make sure this gets mentioned in the release note for Sequenced
>> Collections.
>> We'll also raise this issue when we talk about this feature in the Quality
>> Outreach
>> program.
>>
>> s'marks
>>
>> On 4/29/23 3:46 AM, Remi Forax wrote:
>>> I've several repositories that now fails to compile with the latest jdk21, which
>>> introduces sequence collections.
>>>
>>> The introduction of a common supertype to existing collections is *not* a source
>>> compatible change because of type inference.
>>>
>>> Here is a simplified example:
>>>
>>>     public static void m(List<Supplier<? extends Map<String, String>>> factories) {
>>>     }
>>>
>>>     public static void main(String[] args) {
>>>       Supplier<LinkedHashMap<String,String>> supplier1 = LinkedHashMap::new;
>>>       Supplier<SortedMap<String,String>> supplier2 = TreeMap::new;
>>>       var factories = List.of(supplier1, supplier2);
>>>       m(factories);
>>>     }
>>>
>>>
>>> This example compiles fine with Java 20 but report an error with Java 21:
>>>     SequencedCollectionBug.java:28: error: method m in class SequencedCollectionBug
>>>     cannot be applied to given types;
>>>       m(factories);
>>>       ^
>>>     required: List<Supplier<? extends Map<String,String>>>
>>>     found:    List<Supplier<? extends SequencedMap<String,String>>>
>>>     reason: argument mismatch; List<Supplier<? extends SequencedMap<String,String>>>
>>>     cannot be converted to List<Supplier<? extends Map<String,String>>>
>>>
>>>
>>>
>>> Apart from the example above, most of the failures I see are in the unit tests
>>> provided to the students, because we are using a lot of 'var' in them so they
>>> work whatever the name of the types chosen by the students.
>>>
>>> Discussing with a colleague, we also believe that this bug is not limited to
>>> Java, existing Kotlin codes will also fail to compile due to this bug.
>>>
>>> Regards,
>>> Rémi