pg_import_system_collations() refused to create any ICU collations if the current database's encoding didn't support ICU. This is wrongheaded: initdb must initialize pg_collation in an encoding-independent way since it might be used in other databases with different encodings. The reason for the restriction seems to be that get_icu_locale_comment() used icu_from_uchar() to convert the UChar-format display name, and that unsurprisingly doesn't know what to do in unsupported encodings. But by the same token that the initial catalog contents must be encoding-independent, we can't allow non-ASCII characters in the comment strings. So we don't really need icu_from_uchar() here: just check for Unicode codes outside the ASCII range, and if there are none, the format conversion is trivial. If there are some, we can simply not install the comment. (In my testing, this affects only Norwegian Bokmål, which has given us trouble before.)
For paranoia's sake, also check for non-ASCII characters in ICU locale names, and skip such locales, as we do for libc locales. I don't currently have a reason to believe that this will ever reject anything, but then again the libc maintainers should have known better too.
With just the import changes, ICU collations can be found in pg_collation in databases with unsupported encodings. This resulted in more or less clean failures at runtime, but that's not how things act for unsupported encodings with libc collations. Make it work the same as our traditional behavior for libc collations by having collation lookup take into account whether is_encoding_supported_by_icu().
Adjust documentation to match. Also, expand Table 23.1 to show which encodings are supported by ICU.
catversion bump because of likely change in pg_collation/pg_description initial contents in ICU-enabled builds.
ddb5fdc Further hacking on ICU collation creation and usage.
doc/src/sgml/charset.sgml | 92 +++++++++++++++++++++++--------
src/backend/catalog/namespace.c | 101 ++++++++++++++++++++++-------------
src/backend/commands/collationcmds.c | 86 +++++++++++++++++++----------
src/include/catalog/catversion.h | 2 +-
4 files changed, 194 insertions(+), 87 deletions(-)