Virgil doesn't have reflection, so not a lot of metadata needed at runtime. It does full monomorphization (like MLTon), so you can't have polymorphic recursion. Of course that could go exponential, but in practice I see something like 20%-30% space overhead. I have a tendency to use polymorphism for really generic datastructures, like lists, vectors, maps, but I use tuples a ton and now I added algebraic datatypes. It's a lot of fun and the compiler generates pretty good code and compiles fast--full optimized bootstrap in < 400ms).

Of course everyone worries about exponential code blowup. I have a slightly broken implementation of specialization-up-to-machine-rep, but that's not turned on because of some bugs. I think that's the way I'll go in the future.

It is certainly possible to do a type-passing scheme. The built-in interpreter can interpret polymorphic code using dynamic type environments, but can also run monomorphized code. The interpreter is slow.