[cfe-dev] rfc: winnt.h's UNREFERENCED_PARAMETER() vs clang's -Wunused-value

[Richard Smith via cfe-dev]

On Mon, Oct 12, 2015 at 4:21 PM, Nico Weber <thakis at chromium.org> wrote:

> On Mon, Oct 12, 2015 at 4:11 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> On Mon, Oct 12, 2015 at 3:04 PM, Nico Weber via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> On Mon, Oct 12, 2015 at 3:02 PM, David Majnemer <
>>> david.majnemer at gmail.com> wrote:
>>>
>>>> On Mon, Oct 12, 2015 at 2:18 PM, Reid Kleckner via cfe-dev <
>>>> cfe-dev at lists.llvm.org> wrote:
>>>>
>>>>> I'd really rather not do 2. Fighting with MSVC CRT headers over
>>>>> va_start has been pretty awkward. Users have reported issues with our
>>>>> header override fails. I'd rather not start fighting with winnt.h.
>>>>>
>>>>> I'm OK with 3, but I'm desensitized to having to do this kind of
>>>>> pattern matching for __uuidof, etc. Maybe we should wait for another +1. :)
>>>>>
>>>>
>>>> A +1 from me for option #3 as well.  I'd feel more comfortable doing #2
>>>> if it was for correctness (along the lines of our  va_start/va_arg hack).
>>>>
>>>> On what basis does MSVC decide that UNREFERENCED_PARAMETER should
>>>> silence an unused parameter?  IIRC, it expands to wrapping it's argument in
>>>> parenthesis.  Could we teach clang to do the same?  Sounds like it could be
>>>> a more general fix.
>>>>
>>>
>>> That sounds like a really good idea to me – teach sema that "(x);" has
>>> the same meaning as "(void)x;".
>>>
>>
>> Hold on a moment.
>>
>> `(x);` certainly seems like it should silence an "unreferenced parameter"
>> warning (which is what this macro is for in MSVC), but it seems much less
>> reasonable for it to suppress an "unused value" warning (which is the
>> warning we're getting with Clang).
>>
>
> The reason it seems kind of reasonable to me is that adding parentheses
> means "I really meant this" in a bunch of places. `if (a = b)` warns but
> `if ((a = b))` doesn't, for example. So `(x);` seems more natural than
> `(void)x;` to me. The latter admittedly does have going for it that it's an
> established convention.
>
> Can you think of an example where not warning on `(x);` would catch a bug?
> Hm, I suppose many people like to always write parens after return, and
> part of the motivation of -Wunused-value is catching forgetting "return"s.
> So that's an argument for warning on `(x);`.
>

Perhaps I meant to write `f(x);` and wrote `(x);` instead, or my `x` is a
callable and I meant to write `x();`. Using parens to mean "I am discarding
this value" is novel, counter-intuitive, and just not a good pattern,
whereas the very purpose of permitting casts to void is to allow explicitly
discarding a value.
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