Compilation error on macOSX v10.13.6

[olegded]

Describe the bug

There is a problem in vsg::ref_ptr< vsg::Object > AsciiInput::read() when compiling on macOS v10.13.6 using clang Apple LLVM version 10.0.0 (clang-1000.11.45.5):

/Users/.../dev/osg-vulkan/vsg/src/src/vsg/io/AsciiInput.cpp:132:30: error: call to unavailable member function 'value': introduced in macOS 10.14
        ObjectID id = result.value();
                      ~~~~~~~^~~~~
In file included from /Users/.../dev/osg-vulkan/vsg/src/src/vsg/io/AsciiInput.cpp:13:
In file included from /Users/.../dev/osg-vulkan/vsg/src/include/vsg/io/AsciiInput.h:21:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/optional:942:27: note: candidate function has been explicitly made unavailable
    constexpr value_type& value() &
                          ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/optional:933:33: note: candidate function has been explicitly made unavailable
    constexpr value_type const& value() const&
                                ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/optional:951:28: note: candidate function not viable: no known conversion from 'optional<...>' to 'optional<...>' for object argument
    constexpr value_type&& value() &&
                           ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/optional:960:34: note: candidate function not viable: no known conversion from 'optional<...>' to 'const optional<...>' for object argument
    constexpr value_type const&& value() const&&
                                 ^
1 error generated.

The definition is:

_LIBCPP_INLINE_VISIBILITY
_LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS
constexpr value_type const& value() const&
{
    if (!this->has_value())
        __throw_bad_optional_access();
    return this->__get();
}

Please see this Stackoverflow thread as well. It looks like that the corresponding method is only defined in standard library shipped with macOSX v10.14 and higher, at least using default clang compiler.

To Reproduce

• Run CMake
• Try to compile

Posible solution

1. Require macOSX v10.14 and higher in docs/CMake
2. Use std::optional::operator*

ObjectID id = *result;

which seems to work.

[vsg-dev]

Thanks for the suggested fix, unobtrusive enough a workaround. now committed.

2f4e117