Operator= ambiguous in C++1z and GCC 7.1.1

[nick-prat]

With the new GCC 7.1.1 there is a bug with reassigning to strings.

In this code

#include <string>
#include <iostream>
#include <json.hpp>

using json = nlohmann::json;

int main() {
    json js;
    std::string name = "";
    name = js["name"];
}

it produces the error

In file included from /usr/include/c++/7.1.1/string:52:0,
                 from json_error.cc:1:
/usr/include/c++/7.1.1/bits/basic_string.h: In instantiation of ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv<_Tp, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator=(_Tp) [with _Tp = nlohmann::basic_json<>; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv<_Tp, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&> = std::__cxx11::basic_string<char>&]’:
json_error.cc:10:21:   required from here
/usr/include/c++/7.1.1/bits/basic_string.h:764:28: error: call of overloaded ‘assign(nlohmann::basic_json<>&)’ is ambiguous
  { return this->assign(__sv); }
                            ^
/usr/include/c++/7.1.1/bits/basic_string.h:1313:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       assign(const basic_string& __str)
       ^~~~~~
/usr/include/c++/7.1.1/bits/basic_string.h:1329:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       assign(basic_string&& __str)
       ^~~~~~
/usr/include/c++/7.1.1/bits/basic_string.h:1440:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::__sv_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::__sv_type = std::basic_string_view<char>]
       assign(__sv_type __sv)
       ^~~~~~

when built with g++ -std=c++1z filename.cpp but it will compile fine with c++ -std=c++14 filename.cpp

this also occurs if using clang if the GCC libs version is 7.1.1

[nlohmann]

I cannot reproduce this on macOS with

• g++ (GCC) 8.0.0 20170726 (experimental)
• g++-7 (Homebrew GCC 7.1.0) 7.1.0

with -std=c++1z.

[nlohmann]

We further run GCC 7.1.0 with -std=c++17 without problems, see https://travis-ci.org/nlohmann/json/jobs/259074844.

[nlohmann]

May this be a bug in GCC?�

[nlohmann]

Maybe related: #667

[nlohmann]

Cannot reproduce.

[lieff]

Can reproduce this on CentOS 7.6 machine using gcc 7.2.1. Not reproduced on Ubuntu with gcc 5.5 and 8.1.

[nickaein]

It can be reproduced for version 3.5.0 of library using GCC 7.1.0 on Godbolt with -std=c++1z and even with -std=c++17 flag: https://godbolt.org/z/ff1t3Z. It compiles with -std=c++14 flag though.

Based on this documentation (under section Refereneces), while GCC support of C++17 core language has be complete since 7.1, the library support wasn't (and still isn't).

Therefore, this is either caused by a bug in GCC 7.1.0 (which seems unlikely as it cannot be reproduced in other environments) or an oversight in the libstdc++ that has been fixed in later versions.

Note on -std=c++1z

I'm not sure how far we should go to support it. The latest GCC documentation states that c++1z has been deprecated. Also, the documentation on GCC 7.1.0 states that (emphasis mine):

c++1z: The next revision of the ISO C++ standard, tentatively planned for 2017. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

So I believe c++1z should be abandoned in favor of -std=c++17.