where_is_witness_table.swift

protocol Foo {
    func foo() -> String
}

protocol FooStandard {}

extension Foo where Self: FooStandard {
    func foo() -> String {
        return "hello"
    }
}


protocol FooFatal{}
extension Foo where Self: FooStandard, Self: FooFatal {
    func foo() -> String {
        return "fatal"
    }
}

class A: Foo, FooStandard {}
class B: A, FooFatal {}

A().foo() // hello
B().foo() // fatal

(A() as Foo).foo() // hello
(B() as Foo).foo() // hello

func callFoo<F: Foo>(_ f: F) -> String {
    return f.foo()
}

callFoo(A()) // hello
callFoo(B()) // hello