1.79: Build error with gcc 8 in C++17 mode: ambiguous conversion to string_view

[Lastique]

Version of Boost

Boost 1.79.0 release with Boost.JSON patch from release notes applied.

Steps necessary to reproduce the problem

Build Boost.JSON:

b2 -j16 -q -d2  --layout=system --ignore-site-config --user-config=/build/boost/boost_1_79_0/user-config.jam variant=release debug-symbols=on cxxstd=17-gnu numa=on link=static,shared  --without-python

This results in compiler errors like this:

gcc.compile.c++ bin.v2/libs/json/build/gcc-8/release/cxxstd-17-gnu/debug-symbols-on/link-static/numa-on/threading-multi/visibility-hidden/src.o

    "g++"   -std=gnu++17 -fvisibility-inlines-hidden -g -O2 -fdebug-prefix-map=/build/boost/boost_1_79_0=. -fstack-protector-strong -Wformat -Werror=format-security -O3 -fno-math-errno -fno-rounding-math -fno-signaling-nans -fno-trapping-math -ftree-vectorize -fgcse-sm -fgcse-las -fgcse-after-reload -ftree-loop-im -ftree-loop-distribution -ftree-loop-if-convert -funswitch-loops -frename-registers -fivopts -fvisibility-inlines-hidden -fno-plt -faligned-new -march=nehalem -mtune=skylake -Wno-unused-local-typedefs -Wno-register -Wno-deprecated-declarations -Wno-register -Wno-deprecated-declarations -pthread -O3 -finline-functions -Wno-inline -Wall -g -fvisibility=hidden -Wdate-time -D_FORTIFY_SOURCE=2  -DBOOST_ALL_NO_LIB=1 -DBOOST_CONTAINER_STATIC_LINK=1 -DBOOST_JSON_SOURCE -DBOOST_JSON_STATIC_LINK=1 -DBOOST_USE_NUMA -DNDEBUG  -I"."  -c -o "bin.v2/libs/json/build/gcc-8/release/cxxstd-17-gnu/debug-symbols-on/link-static/numa-on/threading-multi/visibility-hidden/src.o" "libs/json/src/src.cpp"

In file included from ./boost/json/value.hpp:20,
                 from ./boost/json/array.hpp:1756,
                 from ./boost/json.hpp:15,
                 from ./boost/json/src.hpp:27,
                 from libs/json/src/src.cpp:10:
./boost/json/string.hpp: In member function 'int boost::json::string::compare(boost::json::string_view) const':
./boost/json/string.hpp:1948:33: error: call of overloaded 'basic_string_view(const boost::json::string&)' is ambiguous
         return string_view(*this).compare(sv);
                                 ^
In file included from ./boost/json/string_view.hpp:14,
                 from ./boost/json/kind.hpp:14,
                 from ./boost/json/array.hpp:14,
                 from ./boost/json.hpp:15,
                 from ./boost/json/src.hpp:27,
                 from libs/json/src/src.cpp:10:
./boost/core/detail/string_view.hpp:391:5: note: candidate: 'boost::core::basic_string_view<Ch>::basic_string_view(const std::basic_string_view<Ch, std::char_traits<_CharT> >&) [with Ch = char]'
     basic_string_view( std::basic_string_view<Ch, std::char_traits<Ch> > const& str ) BOOST_NOEXCEPT: p_( str.data() ), n_( str.size() )
     ^~~~~~~~~~~~~~~~~
./boost/core/detail/string_view.hpp:335:26: note: candidate: 'constexpr boost::core::basic_string_view<char>::basic_string_view(const boost::core::basic_string_view<char>&)'
 template<class Ch> class basic_string_view
                          ^~~~~~~~~~~~~~~~~
./boost/core/detail/string_view.hpp:335:26: note: candidate: 'constexpr boost::core::basic_string_view<char>::basic_string_view(boost::core::basic_string_view<char>&&)'
In file included from ./boost/json/value.hpp:20,
                 from ./boost/json/array.hpp:1756,
                 from ./boost/json.hpp:15,
                 from ./boost/json/src.hpp:27,
                 from libs/json/src/src.cpp:10:

Full build log attached:
boost-build.log

All relevant compiler information

This happens with gcc 8.3 that is shipped with Debian 10 Buster, when building in C++17 mode with GNU extensions.

[Lastique]

I suspect, this is because the compiler considers two conversion paths:

1. boost::json::string -> boost::core::string_view (via boost::json::string::operator string_view) -> boost::core::string_view (via boost::core::string_view(boost::core::string_view&&) constructor)
2. boost::json::string -> std::string_view (via boost::json::string::operator std::string_view) -> boost::core::string_view (via boost::core::string_view(std::string_view const&) constructor)

I realize that the last step in the first chain is redundant and should not be needed (this is just my interpretation of the compiler error), and I agree that this is probably a compiler issue. But still, gcc 8 is fairly recent and it would be nice if it was supported.

[Lastique]

@pdimov FYI, just in case if boost::core::string_view needs to be fixed.

[vinniefalco]

It looks like gcc 8.3 fails: https://godbolt.org/z/bnd9eMEx9

But gcc 8.4 (and 8.5) works: https://godbolt.org/z/TGj1exv1v

[vinniefalco]

This might fix it #709

[pdimov]

All the string_view(*this) occurrences need to be changed to subview() though.