[PATCH] Warning on unused results in an unevaluated context

[Aaron Ballman]

On Thu, Oct 16, 2014 at 4:05 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> It would be better to have the warning and no-warning cases for typeid next
> to each other. Otherwise, LGTM, thanks!

Fixed up and committed in r219954

> Does this also fix the same issue for -Wunused-comparison?

It does, so I added another test for it.

Thanks!

~Aaron

>
> On Thu, Oct 16, 2014 at 11:58 AM, Aaron Ballman <aaron at aaronballman.com>
> wrote:
>>
>> Patch attached with more test cases; the typeid case you were
>> concerned is also addressed in this patch.
>>
>> ~Aaron
>>
>> On Thu, Oct 16, 2014 at 2:27 PM, Aaron Ballman <aaron at aaronballman.com>
>> wrote:
>> > On Thu, Oct 16, 2014 at 11:28 AM, Aaron Ballman <aaron at aaronballman.com>
>> > wrote:
>> >> On Thu, Oct 16, 2014 at 10:23 AM, Aaron Ballman
>> >> <aaron at aaronballman.com> wrote:
>> >>> On Thu, Oct 16, 2014 at 9:05 AM, Aaron Ballman
>> >>> <aaron at aaronballman.com> wrote:
>> >>>> On Wed, Oct 15, 2014 at 10:33 PM, Richard Smith
>> >>>> <richard at metafoo.co.uk> wrote:
>> >>>>> Does this still warn on cases like:
>> >>>>>
>> >>>>>   typeid(f(), g());
>> >>>>>
>> >>>>> where we don't know we've got an evaluated operand until after we've
>> >>>>> finished parsing it?
>> >>>>>
>> >>>>> ... Hmm, we don't seem to get any warning on that case right now. =(
>> >>>>> Might
>> >>>>> be worth someone looking into that, maybe there are other cases
>> >>>>> we're
>> >>>>> missing.
>> >>>
>> >>> I'd like to make sure my understanding is correct. Given:
>> >>>
>> >>> #include <typeinfo>
>> >>>
>> >>> struct B {
>> >>>   virtual int f();
>> >>> };
>> >>>
>> >>> struct D : B {
>> >>>   virtual int f();
>> >>> };
>> >>>
>> >>> B *h() __attribute__((warn_unused_result));
>> >>> B *h() { return new D; }
>> >>>
>> >>> int f() __attribute__((warn_unused_result));
>> >>> int g();
>> >>>
>> >>> int main() {
>> >>>   const auto &t1 = typeid(f(), g()); // Should not warn; unevaluated
>> >>> context?
>> >>>   const auto &t2 = typeid(h(), g()); // Should warn; evaluated
>> >>> context?
>> >>> }
>> >>>
>> >>> Is that the test case we're looking at for the typeid portion of this?
>> >>
>> >> I don't think it is. The result of h(), g() is still int, which is not
>> >> a glvalue of polymorphic type. I'm struggling to think of a way in
>> >> which I should trigger the unused result warning in the potentially
>> >> evaluated case that wouldn't count as a use.
>> >
>> > Okay, I just needed to eat lunch to figure this out. ;-)
>> >
>> > #include <typeinfo>
>> >
>> > struct B {
>> >   virtual void f();
>> > };
>> >
>> > struct D : B {
>> >   void f() override;
>> > };
>> >
>> > struct C {};
>> >
>> > int f() __attribute__((warn_unused_result));
>> >
>> > int main() {
>> >   D d;
>> >   C c;
>> >   (void)typeid(f(), d); // Warn about unused result because d is a
>> > glvalue of polymorphic type
>> >   (void)typeid(f(), c); // Do not warn about unused result, because c
>> > is not a polymorphic type
>> > }
>> >
>> > ~Aaron
>
>