Pretty neat!

Beware that put must handle duplicate insertion.

Also, cast's signature seems overkill if you aim for exact type equality. In this case, it can be simply bool(std::type_index). The void* signature is only needed if you want to keep the TES' ability to downcast multiple types into T*.

I was actually thinking of specializing cast_ to allow conversion to base... not sure if that can be done yet ;-) And yes, put is far too simple, but then again, this is a proof-of-principle working store in less than 100 lines of code...

@sponce, @clemenci -- FYI...

@rmatev, @nnolte -- FYI...

@raaij: FYI