r173952 - [preprocessor] Don't warn about "disabled expansion of recursive macro"

[Richard Smith]

On Wed, Jan 30, 2013 at 1:39 PM, Douglas Gregor <dgregor at apple.com> wrote:
>
> On Jan 30, 2013, at 1:23 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
>> On Wed, Jan 30, 2013 at 12:44 PM, Argyrios Kyrtzidis <akyrtzi at gmail.com> wrote:
>>> On Jan 30, 2013, at 11:54 AM, Abramo Bagnara <abramo.bagnara at gmail.com> wrote:
>>>
>>>> Il 30/01/2013 19:55, Argyrios Kyrtzidis ha scritto:
>>>>> Author: akirtzidis
>>>>> Date: Wed Jan 30 12:55:52 2013
>>>>> New Revision: 173952
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=173952&view=rev
>>>>> Log:
>>>>> [preprocessor] Don't warn about "disabled expansion of recursive macro"
>>>>> for "#define X X".
>>>>>
>>>>> This is a pattern that, for example, stdbool.h uses.
>>>>> rdar://12435773
>>>>
>>>> Please revert this commit: this has already been discussed in detail in
>>>> past and we decided otherwise. Last time it has been discussed in this
>>>> thread:
>>>>
>>>> http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20121029/067098.html
>>>
>>> Reverted in r173970.
>>>
>>>>
>>>>>
>>>>> Modified:
>>>>>   cfe/trunk/lib/Lex/PPMacroExpansion.cpp
>>>>>   cfe/trunk/test/Headers/stdbool.cpp
>>>>>   cfe/trunk/test/Preprocessor/warn-disabled-macro-expansion.c
>>>>>
>>>>> Modified: cfe/trunk/lib/Lex/PPMacroExpansion.cpp
>>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Lex/PPMacroExpansion.cpp?rev=173952&r1=173951&r2=173952&view=diff
>>>>> ==============================================================================
>>>>> --- cfe/trunk/lib/Lex/PPMacroExpansion.cpp (original)
>>>>> +++ cfe/trunk/lib/Lex/PPMacroExpansion.cpp Wed Jan 30 12:55:52 2013
>>>>> @@ -459,7 +459,10 @@ bool Preprocessor::HandleMacroExpandedId
>>>>>      if (MacroInfo *NewMI = getMacroInfo(NewII))
>>>>>        if (!NewMI->isEnabled() || NewMI == MI) {
>>>>>          Identifier.setFlag(Token::DisableExpand);
>>>>> -          Diag(Identifier, diag::pp_disabled_macro_expansion);
>>>>> +          // Don't warn for "#define X X" like "#define bool bool" from
>>>>> +          // stdbool.h.
>>>>
>>>> Would you be so kind to add a comment here to remember that the choice
>>>> to have this warning for thar case (although disabled by default) is
>>>> deliberate?
>>>
>>> Just to make sure, are you of the opinion that we should warn when <stdbool.h> is included and its macros are used ?
>>
>> As discussed on that thread, we should modify <stdbool.h> to suppress
>> the warning.
>
> How do you propose to do that? #pragmas don't help, because the diagnostic comes from the expansion location, not the definition location. Changing #define bool bool to something that involves _Pragma is very likely to break code elsewhere.

To my reading of C11 6.10.3.4, that's a bug in Clang's preprocessor.
_Pragmas are supposed to take effect (and, per 6.10.9/1, be removed
from the token stream) during macro rescanning, so I think this should
work:

#define true _Pragma("clang diagnostic push") _Pragma("clang
diagnostic ignored \"-Wdisabled-macro-expansion\"") true
_Pragma("clang diagnostic pop")

#define CAT2(x,y) x##y
#define CAT(x,y) CAT2(x,y)
bool b = CAT(true,n);

... because the pragma should take effect during the rescan of the
expansion of 'true', before we perform the token paste.

Another example we seem to get wrong:

#define foo _Pragma("GCC diagnostic ignored \"-Wuninitialized\"")
#define str(x) #x
#define STR(x) str(x)
const char *p = STR(foo);
int n() { int k; return k; }

This should produce an empty string and no warning, as far as I can tell.