making tasks handle types with no default constructor

[julienlopez]

Hello,

Working on a small toy project, I could use support for types with no default constructor in tasks.

From what I've seen in the code, that could be handled pretty easily and with no major drawback with an std::optional if c++17 is activated.

Would it be possible to include something like this ? (I can work on a PR if it's ok)

What would be the prefered way to manage this? add a compile option? do it by default when c++17 is activated? something else?