1、泛型定义本体有参量类型约束；
2、泛型扩展对参量类型约束；
3、函数参量约束；
泛型类型的访问控制：
1、与类型无关的通用函数，泛型的任何实例都可以访问；
2、与类型有关的函数（通过扩展约束实现），只有特定类型实例化的泛型实例才能访问；
由此得出结论：
再考虑泛型约束的情况下，泛型类型是一个代码复用家族；
1、与类型无关的部分为顶级泛型；
2、参量类型为继承（符合）关系的约束泛型为二级泛型；
3、参量类型为具体类型的泛型为具体泛型；
在考虑泛型约束的情况下，泛型函数的访问控制由泛型和参量类型共同决定；
不符合共同决定的情况，会被编译器否定（报错）。
https://docs.swift.org/swift-book/LanguageGuide/Generics.html#ID553
Extensions with a Generic Where Clause
You can also use a generic where clause as part of an extension. The example below extends the generic Stack structure from the previous examples to add an isTop(_:) method.
- extension Stack where Element: Equatable {
- func isTop(_ item: Element) -> Bool {
- guard let topItem = items.last else {
- return false
- }
- return topItem == item
- }
- }
This new isTop(_:) method first checks that the stack isn’t empty, and then compares the given item against the stack’s topmost item. If you tried to do this without a generic whereclause, you would have a problem: The implementation of isTop(_:) uses the == operator, but the definition of Stack doesn’t require its items to be equatable, so using the == operator results in a compile-time error. Using a generic where clause lets you add a new requirement to the extension, so that the extension adds the isTop(_:) method only when the items in the stack are equatable.
