[Bug lto/89358] New: Combining -std=c++14 and -std=c++17 objects gives ODR warnings

[rogero at howzatt dot demon.co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89358

            Bug ID: 89358
           Summary: Combining -std=c++14 and -std=c++17 objects gives ODR
                    warnings
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rogero at howzatt dot demon.co.uk
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

When object modules compiled with -std=c+14 and -std=c++17 are linked together
with -flto there can be warnings about ODR violations in standard library
headers

Example:
-- main.cpp --
#include <map>

extern void test();

int main()
{
        std::map<int, int> m;
        test();
}
-- test.cpp --
#include <map>

void test()
{
        std::map<int, int> m;
}
-- ends --

g++ -flto -std=c++17 -c main.cpp
g++ -flto -std=c++14 -c test.cpp
g++ -flto main.o test.o

-- output --

/usr/share/gcc-trunk/include/c++/9.0.0/bits/stl_function.h:381:12: note: type
'struct less' itself violates the C++ One Definition Rule
  381 |     struct less : public binary_function<_Tp, _Tp, bool>
      |            ^
/usr/share/gcc-trunk/include/c++/9.0.0/bits/stl_tree.h:684:9: note: type
'struct _Rb_tree_impl' itself violates the C++ One Definition Rule
  684 |  struct _Rb_tree_impl
      |         ^
/usr/share/gcc-trunk/include/c++/9.0.0/bits/stl_map.h:100:11: note: type
'struct _Rep_type' itself violates the C++ One Definition Rule
  100 |     class map
      |           ^

With gcc trunk from 2019-01-19 on Cygwin.
Almost identical messages appear on RHEL with devtools-7 and with gcc 7.30 on
WSL