为什么我会遇到这个一致性问题？

Question

protocol A {}
protocol B: A {}
protocol C { 
    var X: A { get } 
}
struct D: C { 
    let X: B // ERROR: does not conform to protocol "C"
} 

这应该没问题，因为 B 符合 A 吗？

我试图通过在协议 associatedtype 中使用 C 来补救它：

protocol A {}
protocol B: A {}
protocol C {
    associatedtype SomeType: A
    var X: SomeType { get }
}
struct D: C {
    let X: B // ERROR: does not conform to protocol C -- seemingly requiring a concrete type
}

Answer 1

如果你按照以下方式思考这个错误是有道理的。

/// It does not have any requirements
protocol A {}

/// It has to satisfy all of the requirements of A (whatever they are)
/// It does not have any of it's own requirements (currently)
/// It is expected to add some additional requirements of it's own
/// Otherwise it defeats the purpose of having this
protocol B: A {}

/// This has only one requirement, X that conforms to A
protocol C { 
    var X: A { get } 
}

/// This has only one requirement, X that conforms to B
/// The problem here is 
/// - A & B are different and can not be used interchangeably 
/// (at least in the current versions)
struct D: C { 
    let X: B // ERROR: does not conform to protocol "C"
} 

从逻辑上讲这是有道理的 - B 必须满足 A 的所有要求，因此应该被允许。

我认为 Swift Forums 可能是讨论这个更好的地方。