RFR: 8199318: add idempotent copy operation for Map.Entry

[forax at univ-mlv.fr]

> De: "John Rose" <john. r.rose at oracle.com >
> À: "Remi Forax" < forax at univ-mlv.fr >
> Cc: "Peter Levart" < peter.levart at gmail.com >, "Rémi Forax" <
> forax at openjdk.java.net >, "core-libs-dev" < core-libs-dev at openjdk.java.net >
> Envoyé: Jeudi 3 Juin 2021 22:51:28
> Objet: Re: RFR: 8199318: add idempotent copy operation for Map.Entry

> On Jun 3, 2021, at 12:46 PM, Remi Forax < [ mailto:forax at univ-mlv.fr |
> forax at univ-mlv.fr ] > wrote:

>> I kind of regret that the compiler does not provide automatically an
>> implementation of compareTo if the record implements Comparable.
>> People sucks at writing compareTo and the resulting bugs are hard to
>> find/reproduce.

> That’s a slippery slope. IIRC we consciously stopped
> before that step.

> That said, there are other ways to fix this. We should
> have utilities (maybe in the JDK but not the JLS) which
> build such methods and make it easy for users to grab onto
> them. Maybe something like this:

> interface ComparableRecord<T extends Record & ComparableRecord<T>>
> extends Comparable<T> { … }

> record Foo(int x, String y) implements ComparableRecord<Foo> { … }

> [ http://cr.openjdk.java.net/~jrose/draft/ComparableRecord.java |
> http://cr.openjdk.java.net/~jrose/draft/ComparableRecord.java ]

[repost with a link] 

The main issue with this kind of code is that the JIT does not see through the ClassValue. 

Tweaking a little bit your code, I get 
https://gist.github.com/forax/e76367e1a90bf011692ee9bec65ff0f8 

(It's a PITA that we have to use a raw type to workaround circularly defined parameter type) 

> — John

Rémi