Is this use of operator+= legitimate?

[giomasce]

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.

[jeking3]

@giomasce is this still an issue / blocking anything?

[giomasce]

Hi, the issue was happening in a Debian package (rivet) that was in the meantime removed for other reasons, so it's not blocking anything any more on my side. I don't know if the issue was actually due to a Boost bug and, if so, whether it is still present.