如何修复涉及Protocol与类型别名的Swift循环引用编译错误？

问题代码与报错 #

以下代码在Xcode 14.1中无法编译：

enum GenericState<Mode>: Hashable where Mode: Hashable {}

// 错误：循环引用
protocol P {
  associatedtype Mode: Hashable
  // （虚假错误）- Type 'Self.Mode' does not conform to protocol 'Hashable'
  typealias SomeState = GenericState<Mode>

  typealias Foo = [CustomKeyPath<Self, String>]
}

struct Interface<Base> where Base: P {}
typealias CustomKeyPath<Base, T> = KeyPath<Interface<Base>, T> where Base: P

触发的编译错误：

• 协议P内部存在循环引用
• 针对SomeState的虚假报错：编译器错误提示Self.Mode不符合Hashable协议，但实际上associatedtype Mode已经被约束为Hashable

修复方案 #

1. 打破循环引用 #

原代码中CustomKeyPath的where子句约束与协议P的Foo类型别名形成循环依赖。将Base: P的约束直接整合到CustomKeyPath的泛型参数定义中，替代原有的where子句即可解决：

typealias CustomKeyPath<Base: P, T> = KeyPath<Interface<Base>, T>

2. 消除虚假协议一致性错误 #

尽管associatedtype Mode已经声明了Hashable约束，但编译器未正确识别。在SomeState的类型别名中显式添加约束，即可让编译器正确解析：

typealias SomeState = GenericState<Mode> where Mode: Hashable

修复后的完整代码 #

enum GenericState<Mode>: Hashable where Mode: Hashable {}

protocol P {
  associatedtype Mode: Hashable
  typealias SomeState = GenericState<Mode> where Mode: Hashable

  typealias Foo = [CustomKeyPath<Self, String>]
}

struct Interface<Base> where Base: P {}
typealias CustomKeyPath<Base: P, T> = KeyPath<Interface<Base>, T>

内容的提问来源于stack exchange，提问作者Awais Fayyaz