GNU's libstdc++ or clang libc++

[viniciusferrao]

Hi @zauguin which standard library do you used to develop this library?

I have an error trying to compile it under macOS (again) but is seems to be an libc++ issue.

FAILED: merge/CMakeFiles/i18n-merge-pot.dir/merge.cpp.o 
/opt/local/bin/clang++ -DFMT_LOCALE -DFMT_SHARED -isystem /opt/local/include -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX12.1.sdk -std=c++20 -MD -MT merge/CMakeFiles/i18n-merge-pot.dir/merge.cpp.o -MF merge/CMakeFiles/i18n-merge-pot.dir/merge.cpp.o.d -o merge/CMakeFiles/i18n-merge-pot.dir/merge.cpp.o -c /Users/ferrao/Documents/Development/i18n++/merge/merge.cpp
/Users/ferrao/Documents/Development/i18n++/merge/merge.cpp:15:44: error: invalid operands to binary expression ('const std::optional<std::string>' (aka 'const optional<basic_string<char, char_traits<char>, allocator<char>>>') and 'const std::optional<std::string>')
    std::strong_ordering order = a.context <=> b.context;
                                 ~~~~~~~~~ ^   ~~~~~~~~~

The spaceship operator <=> is available on Clang but not on libc++.

From the <compare> library you see that LLVM does not implement the feature __cpp_lib_three_way_comparison

On libstdc++ it's properly defined here.

[zauguin]

As you seem to have guessed I'm mostly using libstdc++ and while I try to avoid any libstdc++ specific dependencies, I tend to assume that all standard libraries are at least as C++20 compliant as libstdc++. For libc++ this assumption is (sadly very) wrong.

Anyway, it should be fixed. Can you add an issue for this?

[viniciusferrao]

Opened #10