Thank you for the offer. It would be a first for me, but I would gladly contribute with a submission, time permitting.

– the skills of FunctionCallable and MethodCallable are easier to declare with a template skill I think, as it has access to the definition of the class itself (I assume that the included files do not add methods to the class)

– you’re right that the template type of NamedType is getting a bit lengthy as I’m adding features. I should do something about it (but it won’t get as short as a POD)

– maybe the POD approach makes it easier for the compiler to optimize? I remember that NamedType generated more code than weak typing with gcc in -O1. It would be nice to test if the POD does better here.

– I think a function should take a const LegthRef (however you define it, with std::reference_wrapper or something else) and you could pass it a Length. NamedType has a basic approach on that (I’m working to improve it), and I think there is something else to implement for the POD approach too.

– NamedType allows unit conversions (although it’s not its main feature)

In any case your approach is clearly very interesting if you don’t need some of the specific features of NamedType and don’t want external dependencies. I’m glad you shared this with me.
If you like to write, consider making a guest post submission on Fluent C++ to describe your approach and compare it with NamedType.