Swift is able to avoid monomorphizing even when the type arguments have different GC shapes and allocation strategies. To do that, the dictionary (what Swift calls a "witness table") has entries for the functions the memory manager needs to allocate and trace a value of type argument's type.

Go's notion of dictionary and Swift's witness tables share a lot of DNA with how Haskell compiles generic code using type classes.

C#/.NET does an interesting hybrid approach. When source code is compiled to .NET bytecode, the generic code is only compiled once. Then at load time, the JIT will take that bytecode and monomorphize it to native code for each instantiation using a value type (primitive or struct). But instantiations of reference types all share a single JITTed implementation.

Besides the call indirection, the compiler also loses optimization opportunities.

With the stenciling design many generic functions found in the C++ stl have to pay significant abstraction costs.

Anyway, I like the go design.