r312750 - [Sema] -Wtautological-compare: handle comparison of unsigned with 0S.

[Friedman, Eli via cfe-commits]

On 9/8/2017 2:18 PM, Hans Wennborg via cfe-commits wrote:
> On Fri, Sep 8, 2017 at 2:09 PM, Roman Lebedev <lebedev.ri at gmail.com> wrote:
>>
>> Interesting. My first thought was to explicitly specify enum as signed:
>>
>> enum MediaDeviceType : signed int {
>> MEDIA_DEVICE_TYPE_AUDIO_INPUT = 0,
>> MEDIA_DEVICE_TYPE_VIDEO_INPUT,
>> MEDIA_DEVICE_TYPE_AUDIO_OUTPUT,
>> NUM_MEDIA_DEVICE_TYPES,
>> };
>>
>> inline bool IsValidMediaDeviceType(MediaDeviceType type) {
>> return type >= 0U && type < NUM_MEDIA_DEVICE_TYPES;
>> }
>>
>> But it still seem to warn, and *that* sounds like a bug.
>> Please open a new bugreport.
> I'm reporting it here :-)
>
>> As for different default signedness, i'm not sure what is there to do. Does
>> not sound like a problem for this diagnostic to intentionally avoid to
>> be honest.
> I think it is a problem for this warning. If a user sees the warning
> and removes the "type >= 0" check, thinking that it was unnecessary
> because the compiler told them so, they have now introduced a bug in
> the code.

Even if you declare the enum type as signed, it still isn't allowed to 
contain a negative number; that's undefined behavior.  You can check for 
this using ubsan's -fsanitize=enum.

-Eli

-- 
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