Swift: Access to static var of a protocol implementation

I get a compiler error when I try to access a property of my protocol implementation and would like to understand why this is not working:

Static member 'foo' cannot be used on instance of type 'any MyProtocol'

protocol MyProtocol {
    static var foo: Int { get }
}

struct ProtocolImplementation: MyProtocol {
    static let foo: Int = 2
}


struct HoldsAProtocolImplementation {
    var bar: MyProtocol
}

let a = HoldsAProtocolImplementation(bar: ProtocolImplementation())
print(a.bar.foo)  // error: Static member 'foo' cannot be used on instance of type 'any MyProtocol

I tried to understand the error message. Any instance that conforms to "MyProtocol" should have a (static) var foo. So why am I not allowed to access it?

There are two basic issues.

The first is that a.bar is an instance and by definition you cannot use a static member on an instance. You have to specify the type. To invoke foo you need to write something like

    print(ProtocolImplementation.foo)
    //    ^^^^^^^^^^^^^^^^^^^^^^ this is a concrete type

This leads to the second problem, if all the compiler knows about an instance is that it conforms to a certain protocol, then it can't decide which static property on which conforming type to use. So you can't write

print(MyProtocol.foo)

because the compiler needs a concrete type from which to get the static property.

There are ways around this. One would be to extend the protocol with an instance property that just gets the static property

protocol MyProtocol 
{
	static var foo: Int { get }
}

extension MyProtocol
{
	var foo: Int { Self.foo }
}

print(a.bar.foo) // prints 2

Another would be to get the type at runtime

print(type(of: a.bar).foo)

There are probably other more clever methods of doing what you want.

A static property belongs to the type itself, not to an instance.

Therefore you have to call it on the type

print(ProtocolImplementation.foo)