Binary operator '+' cannot be applied to two 'T' operands

In Swift 4 / Xcode 9+ you can take advantage of the Numeric protocol.

func add<T: Numeric>(num1: T, num2: T) -> T {
    return num1 + num2
}

print(add(num1: 3.7, num2: 44.9)) // 48.6
print(add(num1: 27, num2: 100))  // 127

Using this means you won't have to create a special protocol yourself.

This will only work in the cases where you need the functionality provided by the Numeric protocol. You may need to do something similar to @adam's answer for % and other operators, or you can leverage other protocols provided by Apple in the Xcode 9 SDK.

Swift doesn't know that the generic type T has a '+' operator. You can't use + on any type: e.g. on two view controllers + doesn't make too much sense

You can use protocol conformance to let swift know some things about your type!

I had a go in a playground and this is probably what you are looking for :)

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

func add<T: Addable>(num1: T, _ num2: T) -> T {
    return num1 + num2
}

extension Int: Addable {}
extension Double: Addable {}
extension Float: Addable {}

add(3, 0.2)

Let me know if you need any of the concepts demonstrated here explained

For those who wish to use comparison such as < , > etc, simply tell Swift that your generic type adopt to comperable protocol like so: -

func genericComparison<T: Comparable>(left: LinkedList<T>, right: LinkedList<T>) -> LinkedList<T>