The introduction of Sequenced collections is not a source compatible change

[Joseph D. Darcy]

PS And as a general policy, over the releases we've done more to 
preserve source compatibility when possible for language changes.

For example, there were many source compatibility breakages when 
"assert" was added as a keyword back in JDK 1.4; the current keyword 
management policies (https://openjdk.org/jeps/8223002), including 
contextual keywords (JLS §3.9) and hyphenated-keywords, mitigate the 
impact of analogous changes today.

-Joe

On 5/4/2023 7:47 PM, - wrote:
> In addition, in the CSR of sequenced collection, it already
> anticipated some other form of source incompatibility:
> If a class implements List and Deque at the same time, the return type
> of reversed() must extend both interfaces as well.
>
> This alone would be a greater source incompatibility than this type
> interference already; however, both are binarily compatible:
> Java doesn't care about generics at runtime, and for default
> reversed() overrides, reversed ()Ljava/util/List; and reversed
> ()Ljava/util/Deque; are distinct methods; code calling reversed via
> List or Deque interfaces will get the reversed version of respective
> interfaces, which are functionally correct as well.
>
> I personally would consider this type inference usage moot. It is same
> as adding another method addAll(List<String>) when there is already
> addAll(String[]): existing addAll(null) calls will break, but this is
> not a valid argument against adding a List<String> variant of addAll.
>
> On Thu, May 4, 2023 at 9:38 PM Joseph D. Darcy <joe.darcy at oracle.com> wrote:
>> A few comments on the general compatibility policy for the JDK. Compatibility is looked after by the Compatibility and Specification Review (CSR) process ( Compatibility & Specification Review). Summarizing the approach,
>>
>> The general compatibility policy for exported APIs implemented in the JDK is:
>>
>>      * Don't break binary compatibility (as defined in the Java Language Specification) without sufficient cause.
>>      * Avoid introducing source incompatibilities.
>>      * Manage behavioral compatibility changes.
>>
>> https://wiki.openjdk.org/display/csr/Main
>>
>> None of binary, source, and behavioral compatibly are absolutes and judgement is used to assess the cost/benefits of changes. For example, strict source compatibility would preclude, say, introducing new public types in the java.lang package since the implicit import of types in java.lang could conflict with a same-named type *-imported from another  package.
>>
>> When a proposed change is estimated to be sufficiently disruptive, we conduct a corpus experiment to evaluate the impact on the change on many public Java libraries. Back in Project Coin in JDK 7, that basic approach was used to help quantify various language design choices and the infrastructure to run such experiments has been built-out in the subsequent releases.
>>
>> HTH,
>>
>> -Joe
>> CSR Group Lead
>>
>> On 5/4/2023 6:32 AM, Ethan McCue wrote:
>>
>> I guess this a good time to ask, ignoring the benefit part of a cost benefit analysis, what mechanisms do we have to measure the number of codebases relying on type inference this will break?
>>
>> Iirc Adoptium built/ran the unit tests of a bunch of public repos, but it's also a bit shocking if the jtreg suite had nothing for this.
>>
>> On Thu, May 4, 2023, 9:27 AM Raffaello Giulietti <raffaello.giulietti at oracle.com> wrote:
>>> 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