[the_mitsuhiko]

But it wouldn’t require a heap allocation.

[lmm]

Where is the proxy implementation going to go if not the heap?

[GolDDranks]

The proxy object would have statically known size (maximum of the size of the types it dispatches between, plus some metadata such as a vtable pointer or an enum discriminant). Now, because you know the size statically, you can store it in the stack.