With only <T: Something>: how does `anNSNumber as? T` know what the actual numeric type is to cast?

Using Swift
3

Okay, this is a better example showing where my mental blockage:

import Foundation
import SwiftUI

protocol Something { }
extension Double: Something { }

func convert<T: Something>(_ binding: Binding<T>) {
    print("convert, T is \(T.self)")
    // ??? How does it know T is what?
    if let value = NSNumber(value: 123) as? T {  // my example, how does it get Double here?
        binding.wrappedValue = value
    } else {
        fatalError("cannot convert type: \(T.self)")
    }
}

var abstractD: Something = 123.4
let abstractDBinding = Binding(get: { abstractD }, set: { abstractD = $0 })
convert(abstractDBinding)   // compile error: Protocol 'Something' as a type cannot conform to the protocol itself
print("abstractD = ", abstractD)

It's just strange to me it's this way, even though I've read protocol X does not conform to X.

I keep think convert<T: Something>, then T is only known to the compiler as Something. But it's really just "conformance" requirement. The real type is still used.