In a message dated 2001-06-06 9:35:45 Pacific Daylight Time,
Peter_Constable@sil.org writes:
> we see that Unicode does not *exclude* D800 and DC00 from the
> codespace for the CCS, and therefore it would seem that that UTF-8 sequence
> would have to be interpreted (in the encoding form level of interpretation)
> as the code points < D800 DC00 >, which have *no* meaning *as codepoints*!!
But definition D29 says that a UTF must round-trip these invalid code points,
so we have no choice but to interpret them as <D800 DC00>. That is why
UTF-8s is ambiguous. The sequence <ED A0 80 ED B0 80> could be mapped as
either <D800 DC00>, because D29 says you have to allow for that, or as
<10000>, because that is the real intent.
Note that UTF-8 is not ambiguous in this regard, unless you permit these
so-called "lenient" processors, which I thought were made non-conformant by
the Corrigendum. The sequence <ED A0 80 ED B0 80> is every bit as much
"overlong" as is <C0 80>.
-Doug Ewell
Fullerton, California
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