[cfe-dev] -Wheader-guard emits warning after #undef of guard macro

[Prathamesh Kulkarni]

On Tue, Jan 14, 2014 at 11:54 PM, Argyrios Kyrtzidis <akyrtzi at gmail.com> wrote:
> On Jan 13, 2014, at 1:24 PM, Richard Trieu <rtrieu at google.com> wrote:
>
> On Mon, Jan 13, 2014 at 3:06 AM, Prathamesh Kulkarni
> <bilbotheelffriend at gmail.com> wrote:
>>
>> For the following test-case:
>> #ifndef FOO
>> #define FOO2
>>
>> #undef FOO2
>> #endif
>>
>> -Wheader-guard emits warning:
>> ./foo.h:4:9: warning: 'FOO' is used as a header guard here, followed
>> by #define of a different macro [-Wheader-guard]
>> #ifndef FOO
>>         ^~~
>> ./foo.h:5:9: note: 'FOO2' is defined here; did you mean 'FOO'?
>> #define FOO2
>>         ^~~~
>>
>> Is this appropriate or should it suppress warning in this case
>> since it's equivalent to the following: ?
>> #ifndef FOO
>> #endif
>>
>> Thanks and Regards,
>> Prathamesh
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>
>
> Oops, forgot to include the mailing list.  Sorry for the double email,
> Prathamesh.
>
> Prathamesh,
>
> Can you tell me more about your use case?  Is this a common pattern you have
> encountered?
>
> The header guard warning triggers on files with the following format:
>
> // Start of file
> #ifndef Macro1
> #define Macro2
> ...
> #endif // matches the #ifndef on the first line.
> // End of file
>
> and warns when Macro1 and Macro2 are close, but not exactly the same.  The
> warning already ignores cases where Macro1 and Macro2 are very different
> (edit distance is over half the length).  A similar exemption maybe be
> warranted in this case too.
>
>
> Could we maybe not warn if the the #ifndef and #define are separated by at
> least one empty line ?

Also would it be a good idea, if clang warns on #undef of defined
control macro ?
example:
#ifndef FOO
#define FOO

#undef FOO
#endif

Warn that, control macro FOO gets undefined ?

>
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>