Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

ping https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00161.html

Thanks,
Prathamesh

On 3 May 2017 at 11:30, Prathamesh Kulkarni
<prathamesh.kulkarni@linaro.org> wrote:
> On 3 May 2017 at 03:28, Martin Sebor <msebor@gmail.com> wrote:
>> On 05/02/2017 11:11 AM, Prathamesh Kulkarni wrote:
>>>
>>> Hi,
>>> The attached patch attempts to add option -Wenum-conversion for C and
>>> objective-C similar to clang, which warns when an enum value of a type
>>> is implicitly converted to enum value of another type and is enabled
>>> by Wall.
>>
>>
>> It seems quite useful.  My only high-level concern is with
>> the growing number of specialized warnings and options for each
>> and their interaction.
>>
>> I've been working on -Wenum-assign patch that complains about
>> assigning to an enum variables an integer constants that doesn't
>> match any of the enumerators of the type.  Testing revealed that
>> the -Wenum-assign duplicated a subset of warnings already issued
>> by -Wconversion enabled with -Wpedantic.  I'm debating whether
>> to suppress that part of -Wenum-assign altogether or only when
>> -Wconversion and -Wpedantic are enabled.
>>
>> My point is that these dependencies tend to be hard to discover
>> and understand, and the interactions tricky to get right (e.g.,
>> avoid duplicate warnings for similar but distinct problems).
>>
>> This is not meant to be a negative comment on your patch, but
>> rather a comment about a general problem that might be worth
>> starting to think about.
>>
>> One comment on the patch itself:
>>
>> +         warning_at_rich_loc (&loc, 0, "implicit conversion from"
>> +                              " enum type of %qT to %qT", checktype, type);
>>
>> Unlike C++, the C front end formats an enumerated type E using
>> %qT as 'enum E' so the warning prints 'enum type of 'enum E'),
>> duplicating the "enum" part.
>>
>> I would suggest to simplify that to:
>>
>>   warning_at_rich_loc (&loc, 0, "implicit conversion from "
>>                        "%qT to %qT", checktype, ...
>>
> Thanks for the suggestions. I have updated the patch accordingly.
> Hmm the issue you pointed out of warnings interaction is indeed of concern.
> I was wondering then if we should merge this warning with -Wconversion
> instead of having a separate option -Wenum-conversion ? Although that will not
> really help with your example below.
>> Martin
>>
>> PS As an example to illustrate my concern above, consider this:
>>
>>   enum __attribute__ ((packed)) E { e1 = 1 };
>>   enum F { f256 = 256 };
>>
>>   enum E e = f256;
>>
>> It triggers -Woverflow:
>>
>> warning: large integer implicitly truncated to unsigned type [-Woverflow]
>>    enum E e = f256;
>>               ^~~~
>>
>> also my -Wenum-assign:
>>
>> warning: integer constant ‘256’ converted to ‘0’ due to limited range [0,
>> 255] of type ‘‘enum E’’ [-Wassign-enum]
>>    enum E e = f256;
>>               ^~~~
>>
>> and (IIUC) will trigger your new -Wenum-conversion.
> Yep, on my branch it triggered -Woverflow and -Wenum-conversion.
> Running the example on clang shows a single warning, which they call
> as -Wconstant-conversion, which
> I suppose is similar to your -Wassign-enum.
>
> test-eg.c:3:12: warning: implicit conversion from 'int' to 'enum E'
> changes value from 256 to 0 [-Wconstant-conversion]
> enum E e = f256;
>        ~   ^~~~
>
> Thanks,
> Prathamesh
>>
>> Martin