thing-at-point's meaning of current sexp vs. up-list's: which is correct?

[Kelly Dean]

If point is on a closing delimiter, then thing-at-point says the delimiter is 
part of the current sexp, but up-list says the delimiter is part of the sexp 
that contains the current sexp.
The result is that kill-backward-up-list, which assumes that thing-at-point and 
up-list have the same meaning for current sexp, fails when point is on a 
closing delimiter: type "((a))" and put point on the first closing parenthesis, 
then do kill-backward-up-list, and it reinserts the same sexp that it kills, 
leaving the text unchanged.
I'll file a bug report, but should I file it for thing-at-point, or for 
up-list? Surely not for kill-backward-up-list.