The only drawback I can see is if the UTF-8 bytes actually decode to a half surrogate. However, half surrogates should really only occur in UTF-16 (as I understand it), so they shouldn't be encoded in UTF-8 anyway!
Right: that's the rationale for UTF-8b. Encoding half surrogates violates parts of the Unicode spec, so UTF-8b is "safe".
As for handling this case, you could either:
1. Raise an exception (which is what you're trying to avoid)
or:
2. Treat it as invalid UTF-8 and map the bytes to half surrogates (encoding would produce the original bytes).
I'd prefer option 2.
I hadn't thought of this case, but you are right - they *are* illegal bytes, after all. Raising an exception would be useless since the whole point of this codec is to never raise unicode errors. Regards, Martin