Is this use of operator+= legitimate? #33
Description
I am Debian's maintainer for Boost packages.
In https://gitlab.com/hepcedar/rivet/-/blob/master/src/Tools/RivetPaths.cc#L58 Boost.Assign's operator+= is used on std::vector<std::string> and std::vector<std::string>. It used to compile with Boost 1.67, but it doesn't any more with Boost 1.71. The error is
In file included from /usr/include/boost/assign/std/vector.hpp:18,
from /usr/include/boost/assign/std.hpp:18,
from /usr/include/boost/assign.hpp:19,
from ../../include/Rivet/RivetBoost.hh:6,
from ../../include/Rivet/Math/MathUtils.hh:6,
from ../../include/Rivet/Rivet.hh:42,
from RivetPaths.cc:1:
/usr/include/boost/assign/list_inserter.hpp: In instantiation of 'void boost::assign_detail::call_push_back<C>::operator()(T&&) [with T = std::vector<std::__cxx11::basic_string<char> >; C = std::vector<std::__cxx11::basic_string<char> >]':
/usr/include/boost/assign/list_inserter.hpp:414:13: required from 'void boost::assign::list_inserter<Function, Argument>::insert(boost::assign::list_inserter<Function, Argument>::single_arg_type, T&&) [with T = std::vector<std::__cxx11::basic_string<char> >; Function = boost::assign_detail::call_push_back<std::vector<std::__cxx11::basic_string<char> > >; Argument = std::__cxx11::basic_string<char>]'
/usr/include/boost/assign/list_inserter.hpp:406:13: required from 'boost::assign::list_inserter<Function, Argument>& boost::assign::list_inserter<Function, Argument>::operator()(Ts&& ...) [with Ts = {std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >}; Function = boost::assign_detail::call_push_back<std::vector<std::__cxx11::basic_string<char> > >; Argument = std::__cxx11::basic_string<char>]'
/usr/include/boost/assign/std/vector.hpp:42:30: required from 'boost::assign::list_inserter<boost::assign_detail::call_push_back<std::vector<_Tp, _Alloc> >, V> boost::assign::operator+=(std::vector<_Tp, _Alloc>&, V2&&) [with V = std::__cxx11::basic_string<char>; A = std::allocator<std::__cxx11::basic_string<char> >; V2 = std::vector<std::__cxx11::basic_string<char> >]'
RivetPaths.cc:57:28: required from here
/usr/include/boost/assign/list_inserter.hpp:91:13: error: no matching function for call to 'std::vector<std::__cxx11::basic_string<char> >::push_back(std::vector<std::__cxx11::basic_string<char> >)'
91 | c_.push_back(boost::forward<T>(r));
| ^~
In file included from /usr/include/c++/9/vector:67,
from ../../include/Rivet/RivetSTL.hh:11,
from ../../include/Rivet/Rivet.hh:8,
from RivetPaths.cc:1:
/usr/include/c++/9/bits/stl_vector.h:1184:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; std::vector<_Tp, _Alloc>::value_type = std::__cxx11::basic_string<char>]'
1184 | push_back(const value_type& __x)
| ^~~~~~~~~
/usr/include/c++/9/bits/stl_vector.h:1184:35: note: no known conversion for argument 1 from 'std::vector<std::__cxx11::basic_string<char> >' to 'const value_type&' {aka 'const std::__cxx11::basic_string<char>&'}
1184 | push_back(const value_type& __x)
| ~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/9/bits/stl_vector.h:1200:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(std::vector<_Tp, _Alloc>::value_type&&) [with _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; std::vector<_Tp, _Alloc>::value_type = std::__cxx11::basic_string<char>]'
1200 | push_back(value_type&& __x)
| ^~~~~~~~~
/usr/include/c++/9/bits/stl_vector.h:1200:30: note: no known conversion for argument 1 from 'std::vector<std::__cxx11::basic_string<char> >' to 'std::vector<std::__cxx11::basic_string<char> >::value_type&&' {aka 'std::__cxx11::basic_string<char>&&'}
1200 | push_back(value_type&& __x)
| ~~~~~~~~~~~~~^~~
Is this use of operator+= legitimate? I can't find it in the documentation, but since it used to work I am wondering if the bug here is in Boost or in the using code.
Thanks.