[cfe-dev] -Wsign-compare warns on different signs of operands for ?:

[Richard Trieu]

On Thu, Jul 14, 2011 at 5:17 PM, John McCall <rjmccall at apple.com> wrote:

> On Jul 14, 2011, at 4:57 PM, Richard Trieu wrote:
>
> On Thu, Jul 14, 2011 at 1:39 PM, John McCall <rjmccall at apple.com> wrote:
>
>> On Jul 14, 2011, at 1:32 PM, Richard Trieu wrote:
>>
>> On Thu, Jul 14, 2011 at 1:11 PM, John McCall <rjmccall at apple.com> wrote:
>>>
>>> On Jul 14, 2011, at 12:58 PM, Richard Trieu wrote:
>>>
>>> On Wed, Jul 13, 2011 at 10:08 PM, John McCall <rjmccall at apple.com>wrote:
>>>
>>>>
>>>> On Jul 13, 2011, at 5:15 PM, Richard Trieu wrote:
>>>>
>>>> > For code such as:
>>>> >
>>>> > void f(unsigned int u, int i) {
>>>> >   (void) (true ? u : i);
>>>> > }
>>>> >
>>>> > When run with -Wsign-compare, Clang gives the following warning.
>>>> >
>>>> > warning: operands of ? are integers of different signs:
>>>> >       'unsigned int' and 'int' [-Wsign-compare]
>>>> >   (void) (true ? u : i);
>>>> >                ^ ~   ~
>>>> >
>>>> > Yet, no comparison is going on between u and i, so the warning seems
>>>> out of place for grouping with -Wsign-compare.  I think that it would be
>>>> better is this warning was pulled out of the -Wsign-compare diagnostic
>>>> group.  Also, gcc's -Wsign-compare does not warn on this code, so this
>>>> change will make Clang's sign compare closer to gcc's.  Does this seem
>>>> reasonable?
>>>>
>>>> gcc's -Wsign-compare does warn on this code if you make the condition
>>>> non-trivial.  Clang is just missing that special case.
>>>>
>>>> John.
>>>>
>>>
>>> Could you give an example of a non-trivial case that gcc does warn on?
>>>  I've tried:
>>>
>>> void f(bool b, unsigned int u, int i) {
>>>   (void) (b ? u : i);
>>> }
>>>
>>> void f(unsigned int u, int i) {
>>>   (void) ((i > 5) ? u : i);
>>> }
>>>
>>> but both don't give an warning from gcc -Wsign-compare.  Clang's
>>> -Wsign-compare warns on both.
>>>
>>>
>>> Aha.  From 'bool' I take it that you're talking about C++.  You're
>>> correct that G++'s -Wsign-compare does not warn about conditional operators.
>>>  GCC's does.  In clang, we decided that that inconsistency wasn't worth
>>> emulating.
>>>
>>> John.
>>>
>>
>> Sorry for the confusion.  If we don't care about consistency in this case,
>> can we remove this warning so that our -Wsign-compare will only be about
>> signed comparisons?
>>
>>
>> I have no problem with that;  just don't tell me it's about consistency
>> with what GCC does. :)
>>
>> John.
>>
>
> Originally, I was going to move this warning (operands of different signs)
> out to its own flag.  However, a similar warning (? operand conversion)
> exists in -Wsign-conversion which will warn on the same case as this
> warning (operands of different signs) does.  Since this case is covered
> under (? operand conversion), should this warning (operands of different
> signs) be completely removed from Clang?
>
>
> I think that was just a hack so that it would appear under both flags.
>
> John.
>

Guess I'll go remove that hack.
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