[swift-users] How to dispatch on the size of Int?

[Martin R]

I wonder what the best way would be to call a specialized function dependent on the size of `Int`. Let's say that I have two implementations

    func foo_impl(value: Int32) { /* ... */ }
    func foo_impl(value: Int64) { /* ... */ }

and I want 

    func foo(value: Int)

to call the "right one" of them, according to the architecture (32-bit or 64-bit).

    func foo(value: Int) { foo_impl(value: value) }
   
does not compile. (I assume that is because `Int` is not a type alias to `Int32` or `Int64` but an independent type.)

This works: 

    func foo1(value: Int) {
        if MemoryLayout<Int>.size == 4 {
            foo_impl(value: Int32(value))
        } else {
            foo_impl(value: Int64(value))
        }
    }

or

    func foo2(value: Int) {
        switch MemoryLayout<Int>.size {
        case 4: foo_impl(value: Int32(value))
        case 8: foo_impl(value: Int64(value))
        default:
            abort()
        }
    }

But a typo in the constants would go unnoticed and just call the wrong function, or cause a runtime error instead of a compile-time error. And perhaps `Int` can be an 128-bit integer in the future? The compiler would not warn that the code needs to be updated.

This seems to be more promising:

    func foo3(value: Int) {
        switch (__WORDSIZE) {
        case 32: foo_impl(value: Int32(value)) // Warning: Will never be executed
        case 64: foo_impl(value: Int64(value))
        }
    }
    
Apparently the compiler "knows" which case will be executed, `foo3` does not compile if there is no case matching actual integer size. But there is always an annoying warning for the unused case. And I am not sure if it is guaranteed that __WORDSIZE is the number of bits in an `Int`.
 
So my question is: What would be the best way to dispatch dependent on the size of `Int`, such that

- The compiler checks the correctness.
- The compiler optimizes the code so that no runtime check is done.
- No warnings are produced.

If `Int` had a `native` property like `CGFloat` then I could simply call

    func foo(value: Int) { foo_impl(value: value.native) }

but I could not find such a property. (Would that be a reasonable thing to ask on swift-evolution?)
    
Background: I am asking this just out of curiosity, but the question came up when looking at the `hash_combine` function in the Boost library:

  http://www.boost.org/doc/libs/1_62_0/boost/functional/hash/hash.hpp 
  
with quite different implementations 

    inline void hash_combine_impl(boost::uint32_t& h1, boost::uint32_t k1)
    inline void hash_combine_impl(boost::uint64_t& h, boost::uint64_t k)
    
and I wondered how this would be done in Swift.

Regards, 
Martin