> Oh, I forgot to give a little bit more details.
> 
> b'abc\xff-'.decode('punycode', 'ignore') and
> b'abc\xff-'.decode('punycode', 'replace') raise a UnicodeDecodeError:
> the error handler is just useless (ignored) here.

That's not my point:

b"foo".decode("punycode","replace")

currently succeeds, but raises an UnicodeError under the patch.