在expressiblebyxxxliteral类型上传递/设置变量不起作用

ExpressibleByStringLiteral

struct Foo: ExpressibleByStringLiteral {
    var raw: String

    init(stringLiteral value: String) {
        self.raw = value
    }
}

现在使用这个可以像这些一样简单

func bar(foo: Foo) {}

let foo1: Foo = "example"
let foo2 = "example" as Foo

bar(foo: "example")
bar(foo: foo1)
bar(foo: foo2)

但这样做是行不通的

let string: String = "example"

bar(foo: string) // Cannot convert value of type 'String' to expected argument type 'Foo'
let foo: Foo = string // Cannot convert value of type 'String' to specified type 'Foo'
bar(foo: string as Foo) // Cannot convert value of type 'String' to type 'Foo' in coercion

// Even string interpolation doesn't work which is weird because it's a string
bar(foo: "\(string)" // Cannot convert value of type 'String' to expected argument type 'Foo'

不 String 也符合 ExpressibleByStringLiteral表达式 ExpressibleBy 类型和这似乎是各地的行为。

这里有什么解决办法吗?