XML parsing error: undefined entity (revisited)

[RavanH]

After 10 years, bug #8464 seems to be back in 5.2.1
On a completely fresh install (no plugins, default theme) I added a tag with a ' (single quote) in the title. This converts to ’ in the feed title. This in turn, generates

This feed does not validate.

line 11, column 9: XML parsing error: <unknown>:11:9: undefined entity

for the tag's feed on ​https://validator.w3.org/feed/

See ​https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2F2607.fr%2Ftag%2Flapostrophe%2Ffeed%2F for example

[RavanH]

Strange, I tested on several other installations and on some the single quote gets converted to ’ (invalid) but on others to ’ (valid). Switching PHP version from 7.1 to 7.2 to 7.3, Nginx or Apache, I see no consistency in the behavior :/

[SergeyBiryukov]

Replying to RavanH:

Strange, I tested on several other installations and on some the single quote gets converted to ’ (invalid) but on others to ’ (valid).

’ is a translatable string​ in wptexturize(), so some locales may have translated it to ’.

For feeds, we generally run the content through ​ent2ncr and other filters​ to prevent this kind of issues. Looks like in addition to post titles we have to run it on term titles as well.