[Bug libstdc++/69893] New: [6 Regression] Conflicting declarations in <math.h> and <parallel/algorithm>

[miyuki at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69893

            Bug ID: 69893
           Summary: [6 Regression] Conflicting declarations in <math.h>
                    and <parallel/algorithm>
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: miyuki at gcc dot gnu.org
                CC: redi at gcc dot gnu.org
  Target Milestone: ---

The following program used to work some time ago (with GCC 20160118), but with
current version it is rejected:

$ cat test.cc
#include <math.h>
#include <parallel/algorithm>

$ cc1plus test.cc
In file included from /opt/gcc-6.0.0/include/c++/6.0.0/tr1/random:46:0,
                 from
/opt/gcc-6.0.0/include/c++/6.0.0/parallel/random_number.h:36,
                 from /opt/gcc-6.0.0/include/c++/6.0.0/parallel/partition.h:38,
                 from /opt/gcc-6.0.0/include/c++/6.0.0/parallel/quicksort.h:36,
                 from /opt/gcc-6.0.0/include/c++/6.0.0/parallel/sort.h:48,
                 from /opt/gcc-6.0.0/include/c++/6.0.0/parallel/algo.h:45,
                 from /opt/gcc-6.0.0/include/c++/6.0.0/parallel/algorithm:37,
                 from test.cc:2:
/opt/gcc-6.0.0/include/c++/6.0.0/tr1/cmath: In function 'float
std::tr1::acosh(float)':
/opt/gcc-6.0.0/include/c++/6.0.0/tr1/cmath:424:18: error: 'float
std::tr1::acosh(float)' conflicts with a previous declaration
   acosh(float __x)
                  ^
In file included from /opt/gcc-6.0.0/include/c++/6.0.0/math.h:36:0,
                 from test.cc:1:
/opt/gcc-6.0.0/include/c++/6.0.0/cmath:1224:3: note: previous declaration
'constexpr float std::acosh(float)'
   acosh(float __x)
   ^~~~~
...

(more errors follow)