Properly directly include optional and reference it in C++17 and beyond.
Properly directly include optional and reference it in C++17 and beyond.

ThePhD/sol2,ThePhD/sol2,billw2012/sol2,ThePhD/sol2,billw2012/sol2,ThePhD/sol2,OrfeasZ/sol2,OrfeasZ/sol2,billw2012/sol2,billw2012/sol2,nshcat/sol2,OrfeasZ/sol2,nshcat/sol2,OrfeasZ/sol2