[macOS] Build is broken on < 10.15: `vector_fwd.hpp: error: "unknown standard library"` etc.

[barracuda156]

In file included from In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/base64.hpp/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/format.hpp::97:
In file included from :
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/charconv.hpp:50:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/std_fwd.hpp:6:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/vector_fwd.hpp:29:2: error: "unknown standard library"
#error "unknown standard library"
 ^
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/charconv.hpp:50:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/std_fwd.hpp:6:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/vector_fwd.hpp:29:2: error: "unknown standard library"
#error "unknown standard library"
 ^
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/vector_fwd.hpp:38:/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/vector_fwd.hpp34:38:34: warning::  warningvariable templates are a C++14 extension [-Wc++14-extensions]
: variable templates are a C++14 extension [-Wc++14-extensions]
template<class Alloc> c4::substr to_substr(std::vector<char, Alloc> &vec);template<class Alloc> c4::substr to_substr(std::vector<char, Alloc> &vec);
                                 ^

                                 ^
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/vector_fwd.hpp/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/vector_fwd.hpp:38:34: error: redefinition of 'to_substr' as different kind of symbol
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/substr.hpp:2124:15: note: previous definition is here:38
:34: error: redefinition of 'to_substr' as different kind of symbol
inline substr to_substr(char *s)
              ^
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/substr.hpp:2124:15: note: previous definition is here
inline substr to_substr(char *s)
              ^
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/base64.cpp:1:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/base64.hpp:9In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/format.cpp:1:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/format.hpp:7:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/charconv.hpp:50:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/std_fwd.hpp:6:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_devel_c4core/c4core/work/c4core-0.1.11-src/src/c4/std/vector_fwd.hpp:38:49: error: no member named 'vector' in namespace 'std'
template<class Alloc> c4::substr to_substr(std::vector<char, Alloc> &vec);
                                           ~~~~~^
. . .

Examples of full logs:
https://build.macports.org/builders/ports-10.14_x86_64-builder/builds/198418/steps/install-port/logs/stdio
https://build.macports.org/builders/ports-10.8_x86_64-builder/builds/167564/steps/install-port/logs/stdio
https://build.macports.org/builders/ports-10.7_x86_64-builder/builds/180210/steps/install-port/logs/stdio

[ryandesign]

#error "unknown standard library"

It looks like this happens when _LIBCPP_ABI_NAMESPACE is not defined:

c4core/src/c4/std/vector_fwd.hpp

Lines 27 to 35 in e55d82c

	#elif defined(_LIBCPP_ABI_NAMESPACE)
	namespace std {
	inline namespace _LIBCPP_ABI_NAMESPACE {
	template<typename> class allocator;
	template<typename T, typename Alloc> class vector;
	} // namespace _LIBCPP_ABI_NAMESPACE
	} // namespace std
	#else
	#error "unknown standard library"

It looks like _LIBCPP_ABI_NAMESPACE was introducd in llvm/llvm-project@fe13c13 which first appeared in llvm.org clang 8 a.k.a. Apple clang 11.

[biojppm]

@ryandesign thanks for the input. I have no practical way to figure out what should go in the alternative branch. If you have the opportunity and are inclined, a PR with the proper fwd declaration would be very welcome.