bad_optional_access on older macOs versions

[jukkae]

Technically unavailable before 10.14, but see https://stackoverflow.com/questions/52310835/xcode-10-call-to-unavailable-function-stdvisit for workaround

[jukkae]

https://stackoverflow.com/questions/52521388/stdvariantget-does-not-compile-with-apple-llvm-10-0?rq=1

[jukkae]

Actually, the error is bad_optional_access. Cause seems same. https://stackoverflow.com/questions/44217316/how-do-i-use-stdoptional-in-c

[jukkae]

Passing -mmacosx-version-min=10.12 to clang leads to error: call to unavailable function 'get': introduced in macOS 10.14 :(