[Bug lto/89358] [7/8/9 Regression] Combining -std=c++14 and -std=c++17 objects gives ODR warnings

[redi at gcc dot gnu.org]

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

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This shows it has nothing to do with r243379 at all:


template<typename _Arg1, typename _Arg2, typename _Result>
  struct binary_function
  {
    typedef _Arg1 first_argument_type;
    typedef _Arg2 second_argument_type;
    typedef _Result result_type;
  };

template<typename _Tp>
  struct less : public binary_function<_Tp, _Tp, bool>
  {
    constexpr
    bool
    operator()(const _Tp& __x, const _Tp& __y) const
    { return __x < __y; }
  };

int f(int i, int j);

#if __cplusplus < 201703L

int f(int i, int j)
{
  return less<int>()(i, j);
}

#else

int main()
{
  return less<int>()(3, 4) == f(2, 1);
}

#endif


g++ -flto -c src.cc
g++ -flto -c src.cc -std=c++17 -o src17.o
g++ -flto src.o src17.o
src.cc:10:10: warning: type ‘struct less’ violates the C++ One Definition Rule
[-Wodr]
   struct less : public binary_function<_Tp, _Tp, bool>
          ^
src.cc:10:10: note: a type with different bases is defined in another
translation unit
   struct less : public binary_function<_Tp, _Tp, bool>
          ^
src.cc:14:5: warning: ‘operator()’ violates the C++ One Definition Rule 
[-Wodr]
     operator()(const _Tp& __x, const _Tp& __y) const
     ^
src.cc:14:5: note: ‘operator()’ was previously declared here
     operator()(const _Tp& __x, const _Tp& __y) const
     ^


This shows the exact same source code, compiled with -std=c++14 and -std=c++17,
and bogus warnings about different bases.