------- Comment #20 from jakub at gcc dot gnu dot org 2009-11-25 18:28 ------- That's on purpose. b is actually used by assignment to a in both cases, c is used in the first case by assignment to b. If the user decides to remove the a variable (resp. c in the second case) based on this warning, he'll get further warnings and can tweak it, but not counting references assigned to variables warned about is neither possible in this implementation (you don't know until the end whether something is going to be just set and not used) nor desirable (in some cases you don't want to fix code by removing the set but not used variable, but instead actually use it somewhere, and in that case you could be removing the other set but not used vars even when they are now actually used by some used variable).
-- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18624
