Re: [PATCH] Fix __has_{cpp_}attribute with -traditional-cpp (PR preprocessor/65238)

[Dodji Seketeli <dodji at redhat dot com>]

Jason Merrill <jason@redhat.com> writes:

> On 03/20/2015 12:48 PM, Jakub Jelinek wrote:
>> On Fri, Mar 20, 2015 at 12:30:44PM -0400, Jason Merrill wrote:
>>> On 03/11/2015 03:10 PM, Jakub Jelinek wrote:
>>>> __has_{cpp_,}attribute builtin macros are effectively function-like macros
>>>> taking one argument (and the ISO preprocessor expands macros in the argument
>>>> which is IMHO desirable), but the traditional preprocessor has been crashing
>>>> on them or reporting errors.
>>>
>>> Why do we want ISO preprocessor behavior in this specific situation?
>>
>> You mean that we would handle
>> #define U unused
>> #if __has_attribute(U)
>> int u __attribute__((unused));
>> #endif
>> differently between ISO and traditional preprocessing?
>
>> That would be surprising to users.
>
> Why surprising?  Don't users of the traditional preprocessor expect
> traditional preprocessor behavior?

One of the reasons why I thought it'd be "nice" to have the traditionnal
mode support the macro-expansion of the arguments here is that there
already are cases where the traditionnal mode supports ISO behaviour.
For instance, the documentation of cpp says:

    10.3 Traditional miscellany
    ===========================

    Here are some things to be aware of when using the traditional
    preprocessor.

    [...]

       * A true traditional C preprocessor does not recognize '#error' or
         '#pragma', and may not recognize '#elif'.  CPP supports all the
         directives in traditional mode that it supports in ISO mode,
         including extensions, with the exception that the effects of
         '#pragma GCC poison' are undefined.

So I thought this useful particular use case of __has_attribute(U) might
well be another of such case even if it's not a directive.

Just my 2 cents.

-- 
		Dodji