函数的泛型约束是函数签名的一部分，不符合约束的初始调用将不能查找到函数（报错）

1. Type constraints, as described in Type Constraints, enable you to define requirements on the type parameters associated with a generic function, subscript, or type.

1. func allItemsMatch<C1: Container, C2: Container>
2. (_ someContainer: C1, _ anotherContainer: C2) -> Bool
3. where C1.Item == C2.Item, C1.Item: Equatable {
5. // Check that both containers contain the same number of items.
6. if someContainer.count != anotherContainer.count {
7. return false
8. }
10. // Check each pair of items to see if they're equivalent.
11. for i in 0..<someContainer.count {
12. if someContainer[i] != anotherContainer[i] {
13. return false
14. }
15. }
17. // All items match, so return true.
18. return true
19. }

The following requirements are placed on the function’s two type parameters:

• C1 must conform to the Container protocol (written as C1: Container).
• C2 must also conform to the Container protocol (written as C2: Container).
• The Item for C1 must be the same as the Item for C2 (written as C1.Item == C2.Item).
• The Item for C1 must conform to the Equatable protocol (written as C1.Item: Equatable).

The first and second requirements are defined in the function’s type parameter list, and the third and fourth requirements are defined in the function’s generic where clause.