In ICU 54 and earlier, if ucol_open() is unable to find a matching locale, it will fall back to the *environment*.
Using ICU 54: initdb -D data -N --locale="en_US.UTF-8" pg_ctl -D data -l logfile start psql postgres -c "create collation asdf(provider=icu, locale='asdf')" # returns true psql postgres -c "select 'abc' collate asdf < 'ABC' collate asdf" psql postgres -c "alter system set lc_messages='C'" pg_ctl -D data -l logfile restart # returns false and warns about collation version mismatch psql postgres -c "select 'abc' collate asdf < 'ABC' collate asdf" This was fixed in ICU 55 to fall back to the root locale instead[1], which is stable, has a collator version, and is not dependent on the environment. As far as I can tell, 55 and later never fall back to the environment when opening a collator (unless you explicitly pass NULL to ucol_open(), which is documented). It would be nice if we could detect when this fallback-to-environment happens, so that we could just refuse to create the bogus collation. But I didn't find a good way. There are non-error return codes from ucol_open() that seem promising[2], but they aren't actually very useful to distinguish the fallback-to-environment case as far as I can tell. Unless someone has a better idea, I think we need to bump the minimum required ICU version to 55. That would solve the issue in v16 and later, but those using old versions of ICU and old versions of postgres would still be vulnerable to these kinds of typos. Regards, Jeff Davis [1] https://icu.unicode.org/download/55m1 [2] https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utypes_8h.html#a3343c1c8a8377277046774691c98d78c