Add known problems with `class` feature to perldelta

[johannessen]

When the experimental class feature was added in v5.38, it had some known problems that could cause segmentation faults, including #20956, #20947, and #20890 (duplicate: #21221).

As these issues persist to this day, I think they should be mentioned somewhere in Perl’s documentation. This change adds them to perl5380delta, and includes a brief erratum in the new v5.40 perldelta.

[Leont]

I'm not sure perldelta is really the right place for this.

[rwp0]

Good to know about those specific cases.

Since there's no perlbugs, I'd go to perldelta by default to check the status for v5.38 updates, as a user.

[johannessen]

The description of the ”Known Problems” section in perldelta_template.pod is as follows:

XXX Descriptions of platform agnostic bugs we know we can't fix go here. Any tests that had to be TODOed for the release would be noted here. Unfixed platform specific bugs also go here.

There is certainly room to split some hairs about what exactly this language means. But I think it’s not meant to be exclusive.

The way I see it, the intention is that any unfixed platform-agnostic bugs which simply happen to not have TODO tests yet should indeed be listed as “known problems,” too.

[Leont]

We don't usually update perldelta like that (except minor edits like typos and broken links). Perhaps perlexperimental is a more logical place.

[johannessen]

I see your point, but a “KNOWN PROBLEMS” section in perlexperiment might imply to readers that all known issues for experiments are listed. I’m not in a position to produce such a comprehensive list.

I suppose adding a “BUGS” section to perlclass might work, plus a pointer in the new 5.40 perldelta for “updated documentation.” Would that be more appropriate?

[Leont]

@leonerd probably some ideas about where the best place would be

[leonerd]

I imagine a "KNOWN BUGS" section in perlclass.pod would be handy.

[johannessen]

Thank you all for your guidance! PR updated.

I’m not sure how much detail I should go into when describing the issues in perlclass.pod. I’ve opted for just a brief summary plus a GH link. Is that appropriate here?