[Bug libstdc++/82417] New: Macros from <complex.h> defined in C++11

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82417

            Bug ID: 82417
           Summary: Macros from <complex.h> defined in C++11
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

The C++11 standard says that <complex.h> should just include the C++ <complex>
header and completely ignore the C library's header. We take some liberty and
include _both_ C++'s <complex> and C's <complex.h>, because historically
<complex.h> wasn't part of the C++ standard and so including it used the C
version and worked OK.

However this means that the following valid C++11 code is rejected:

#include <complex.h>

std::complex<double> I{0, 1};


In file included from /usr/include/c++/7/complex.h:36:0,
                 from I.cc:1:
I.cc:3:22: error: expected unqualified-id before ‘__extension__’
 std::complex<double> I{0, 1};
                      ^
I.cc:3:22: error: expected ‘)’ before ‘__extension__’


For C++11 mode we should #undef the macros that <complex.h> defines with
non-reserved names, and maybe consider not including <complex.h> at all for
-std=c++1* modes.