In Swift, how to generically limit function to types that understand T + T

You may have a similar need for other functions too and implementing them for all integer and floating point types (or other "summable" things) would cause massive code duplication.

One partial solution, specifically for +, -, *, /, % is to require conformance to IntegerArithmeticType protocol:

func example<T: IntegerArithmeticType>(x: T, y: T) -> T { return x + y }
println(example(40, 2)) // --> 42

This does not apply to floating point types because they do not implement overflow operations defined in _IntegerArithmeticType protocol, which IntegerArithmeticType inherits from.

However, extending types for a specific globally defined operator function conformance is not as "annoying" as you might think:

protocol Summable { func +(lhs: Self, rhs: Self) -> Self }

extension Int: Summable {}
extension Double: Summable {}
extension String: Summable {}
// extension for any other types... you are in full control!

You do this once, then forevermore you can use Summable in you code:

func example<T: Summable>(x: T, y: T) -> T { return x + y }

println( example("4", "2") ) // --> 42

Indeed, and as @connor pointed out, this is equivalent to @Jean-PhilippePellet's answer you mentioned.

You should do as that answer suggested. Create a protocol for your use and extend all of the classes that you want to use it. Then make sure T implements that protocol in your method signature.

You can't use an "addable" protocol because there isn't one in swift. The swift library actually creates a function + for each valid addition operations. Command-click on something like Int to see where all of these are defined. It will look something like this:

//....
func +(lhs: Int8, rhs: Int8) -> Int8
func +(lhs: UInt16, rhs: UInt16) -> UInt16
func +(lhs: Int16, rhs: Int16) -> Int16
func +(lhs: UInt32, rhs: UInt32) -> UInt32
func +(lhs: Int32, rhs: Int32) -> Int32
//etc...

Have you tried using the protocol AdditiveArithmetic?

https://developer.apple.com/documentation/swift/additivearithmetic

Looks like thats exactly what you are looking for. That protocol has the method:

static func + (Self, Self) -> Self

Using that protocol your method becomes:

class funccalc {
    func doAdd<T: AdditiveArithmetic>(x:T,y:T) -> T {
        return x + y
    }
}