RFR: 8331051: Add an `@since` checker test for `java.base` module

[Joe Darcy]

On Wed, 24 Apr 2024 14:10:29 GMT, Nizar Benalla <duke at openjdk.org> wrote:

> This checker checks the values of the `@since` tag found in the documentation comment for an element against the release in which the element first appeared.
> 
> Real since value of an API element is computed as the oldest release in which the given API element was introduced. That is:
> - for modules, classes and interfaces, the release in which the element with the given qualified name was introduced
> - for constructors, the release in which the constructor with the given VM descriptor was introduced
> - for methods and fields, the release in which the given method or field with the given VM descriptor became a member of its enclosing class or interface, whether direct or inherited
> 
> Effective since value of an API element is computed as follows:
> - if the given element has a `@since` tag in its javadoc, it is used
> - in all other cases, return the effective since value of the enclosing element
> 
> The since checker verifies that for every API element, the real since value and the effective since value are the same, and reports an error if they are not.
> 
> Preview method are handled as per JEP 12, if `@PreviewFeature` is used consistently going forward then the checker doesn't need to be updated with every release. The checker has explicit knowledge of preview elements that came before `JDK 14` because they weren't marked in a machine understandable way and preview elements that came before `JDK 17` that didn't have `@PreviewFeature`.
> 
> Important note : We only check code written since `JDK 9` as the releases used to determine the expected value of `@since` tags are taken from the historical data built into `javac` which only goes back that far
> 
> The intial comment at the beginning of `SinceChecker.java` holds more information into the program.
> 
> I already have filed issues and fixed some wrong tags like in #18640, #18032, #18030, #18055, #18373, #18954, #18972.

If not already planned, I recommend a much broader discussion/announcement of this test before it goes back. For example, a message to jdk-dev stating "The value of `@since` tags will be tested now by a tier $N test. This is how you run the test before a push. Here are rules for some tricky cases like introducing a covariant override." etc.

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PR Comment: https://git.openjdk.org/jdk/pull/18934#issuecomment-2083205402