[Bug libstdc++/11196] parse error before numeric constant <M_PI is defined in C++>

["jan at etpmod dot phys dot tue dot nl" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From jan at etpmod dot phys dot tue dot nl  2004-03-08 12:20 -------
The bug is that the C++ front end implicitly #defines _GNU_SOURCE. 
 
In glibc (I have version 2.3.2 here) this, amongst others, enables _BSD_SOURCE 
(see /usr/include/features.h). That in turn enables non-standard symbols like 
M_PI in math.h. Hence the failure. 
 
As a result one can argue that this is a C++ front end bug... however:   
 
gcc/config/linux.h tells us that: 
 
/* The GNU C++ standard library requires that these macros be defined.  */ 
#undef CPLUSPLUS_CPP_SPEC 
#define CPLUSPLUS_CPP_SPEC "-D_GNU_SOURCE %(cpp)" 
 
... so this may be a libstdc++ issue after all.  
 
The #define is also present for other configuration files in gcc/config/* and 
subdirectories. 
 
I googled and searched the libstdc++ archives and Changelog files, but did no 
find the reason why _GNU_SOURCE is in fact needed by gnu's libstdc++. I just 
removed the -D_GNU_SOURCE from gcc/config/linux.h (on i686-suse-linux) and 
bootstrapped gcc (HEAD) with languages=c,c++. That went just fine and 
(obviously) fixed the problem reported here. 
 
Could some libstdc++ guru explain why this define is actually needed? 

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11196