If a invalid utf8 or utf16 code need to be converted to utf32, then it should be converted to an invalid utf32. that's why D800~DFFF are marked as invalid points in unicode standard.
== Quote from spir (denis.s...@gmail.com)'s article > This is not a good idea, imo. Surrogate values /are/ invalid code points. (For > the ones who guess, there are a range of /code unit/ values used to code in > utf16 code points > 0xFFFF.) They should never appear in a string of dchar[]; > and a string of char[] code units should never encode a non-code point in the
