In F# (.NET) primitives aren't boxed unless they are stored in a variable/field of type System.Object. Generic methods are specialized by the JIT compiler when their generic arguments are primitive types.
Upd: I noticed that someone mentioned upthread that OCaml uses this to fit sum types that are either an int or a pointer into a single 64-bit value.
F# doesn't do that and always stores a separate tag field for sum types. This is marginally less efficient, but doesn't make interop with 99% of languages and data formats in existence awkward.
