std.stringbuffer

[Janice Caron]

Oh, sorry, I didn't read your whole post before replying. <embarrassed>.

OK, so private use characters might be a contrived exception. BUT,
nobody expects toUpper() to acknowledge private use characters. That
would require a run-time extensibility mechanism which is way beyond
what toUpper() does now, and likely beyond anything it's ever likely
to do any time soon. Maybe some future Unicode library with a
registerPrivateUseCharacters() function might cover that
functionality, but there are no plans for that on the table right now.
(And even then - as you say - it's a /very/ contrived case).