Protocol extension to be applied to several types

Question

I have a protocol, for instance:

protocol P {
    func foo()
}

It is possible to make default implementation for some type:

extension P where Self == Int {
    func foo() {
        // do something
    }
}

But how to have same default implementation for several types? Like this:

extension P where Self == Int or Self == Float {
    func foo() {
        // do something
    }
}

Answer 1

You can unite the types with another protocol (for example HasDefaultFoo). This allows the types to decide when adopting the protocol if they want the default implementation.

protocol P {
    func foo()
}

protocol HasDefaultFoo {
    func foo()
}

extension Int: P, HasDefaultFoo { }
extension Float: P, HasDefaultFoo { }

extension P where Self: HasDefaultFoo {
    func foo() {
        print(self)
    }
}

extension Double: P {
    func foo() {
        print("I provide my own foo()")
    }
}

Example:

5.foo()
(5.5 as Float).foo()
5.5.foo()

Output:

5
5.5
I provide my own foo()