Exclude、Extract、ReturnType 是怎么实现的
How are Exclude, Extract and ReturnType implemented
一句话：三个都是条件类型（conditional type）—— 类型层面的三元表达式 T extends U ? X : Y。Exclude 和 Extract 靠它的分配律（distributive）过滤联合；ReturnType 再加上 infer，从函数类型里拆出返回值。
分配律：当 T 是裸类型参数、实参又是联合时，条件类型不把联合当整体判断，而是逐个成员代入、再把结果并起来。Exclude<"a" | "b" | "c", "a"> 因此展开成三次判断：命中的变成 never，而 never 是空联合，并进结果就消失了。「过滤」的本质，是「把不要的变成 never」。
infer：在 extends 右边的模式里挖一个洞，匹配成功时编译器负责把洞填上。T extends (...args: any[]) => infer R ? R : never里的 R 就是那个洞 —— T 匹配到函数类型时，R 被填成它的返回类型。同一招能拆数组元素、Promise 的值、函数参数（Parameters 就是把洞挖在参数位置）。
会追问：「怎么关掉分配律？」—— 两边包一层元组：[T] extends [U]，T 不再是裸的，联合就被当成整体。「Exclude<T, T> 是什么？」——never，每个成员都被自己命中。「never 传进去呢？」—— 还是 never：空联合循环零次。三问全是分配律的推论，吃透一条规则就全能答。
In one line: all three are conditional types — a ternary at the type level, T extends U ? X : Y. Exclude and Extract filter unions through its distributive behaviour; ReturnType adds infer to pull the return type out of a function type.
Distributivity: when T is a bare type parameter and the argument is a union, the conditional does not judge the union as a whole — it substitutes each member separately and unions the results. Exclude<"a" | "b" | "c", "a"> therefore expands into three checks: matching members become never, and never is the empty union, so it vanishes when merged back in. Filtering really means turning the unwanted members into never.
infer: it digs a hole in the pattern to the right of extends, and the compiler fills the hole when the match succeeds. In T extends (...args: any[]) => infer R ? R : never, R is that hole — when T matches a function type, R gets its return type. The same trick unpacks array elements, the value inside a Promise, or function parameters (Parameters is the same hole dug at the parameter position).
Follow-up: “How do you switch distributivity off?” — wrap both sides in a tuple: [T] extends [U], T is no longer bare, so the union is judged as one piece. “What is Exclude<T, T>?” — never: every member is matched by itself. “And feeding never in?” — still never: the empty union loops zero times. All three are corollaries of one rule — master distributivity and the whole set falls out.