On everything from the JVM to GHC to v8 there is a critical distinction between the semantics of a boxed type and the practical outcome on the gear. Every managed language runtime done by pros (and those all are, I should have said the CLR too) will quietly unbox in all the easy cases and some of the hard ones.

Parametric polymorphism is kind of orthogonal to that. It’s related I guess. It can involve some kind of runtime dispatch, but usually that gets JITed or inlined or whatever unless you really insult the compiler.

And even full frontal invokedynamic it’s still like, is it in the BTB and the I-cache? Ok well we’re still doing business.