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

[David Majnemer via cfe-dev]

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.


>
> On Sat, Oct 10, 2015 at 2:50 PM, Nico Weber via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> clang has a useful warning -Wunused-value that warns on things such as
>> `4;` (this probably meant to say `return 4;`).
>>
>> winnt.h has a macro UNREFERENCED_PARAMETER(x) that's used to mark unused
>> function parameters. It's somewhat commonly used on Windows, for example
>> the default project generated by MSVC uses it a few times. The macro is
>> defined as `#define UNREFERENCED_PARAMETER(x) (x)`, which means if you use
>> it like it's intended:
>>
>>   void f(int x) {
>>     UNREFERENCED_PARAMETER(x);
>>   }
>>
>> then clang will emit a -Wunused-value warning for it. How do we want to
>> fix this? Ideas:
>>
>> 1. Don't. Tell people to not use UNREFERENCED_PARAMETER().
>> 2. Have a winnt.h wrapper in lib/Headers that undefines this macro,
>> include_next's regular winnt.h, undefines it again, and then defines it to
>> something that doesn't trigger -Wunused-value (say, `(void)p`, or `do {
>> (void) P; } while (0)` or similar).
>> 3. Change clang's Wunused-value to check if the unused value is expanded
>> from a macro that was defined in a system header that's spelled
>> UNREFERENCED_PARAMETER.
>>
>> 2 sounds like it has the highest chance of unintended side effects. 3 is
>> a bit hacky, but seems nicest from a user's perspective and isn't worse
>> than other things we do really.
>>
>> Other ideas? Opinions?
>>
>> Thanks,
>> Nico
>>
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>>
>
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