[C++] Constexpr implementation for dynamic dispatch #49835

Describe the enhancement requested

Right now the DynamicDispatch makes no assumption on the DynamicFunction::implementations().
However in practice we can assume the implementations to be a compile-time sequence: each available must have been compiled so there is no possibility to add more at runtime (doing so would require a dlopen style plugin loading, which I don't believe is what we want to go towards).

With C++17 and CTAD, it is straightforward to use a static array without needing with compile-time array sizes.
For e.g. it is already done with:

There are actually few places to change such as:

This will be a small immediate yet negligible improvement (the std::vector is actually just parsed once).
The big win, is to use that information to skip dynamic dispatch altogether when there is only one implementation available, as done (verbosly) here:

But not done below:

Component(s)

C++