type t = string | number

const test = (a: t, b: t) : t => {
  return a + b
}

我的本意是当test函数调用时，a和b的类型必须一样，即都为string或者都为number，但是这样写似乎不生效，应该如何实现呢

用重载。

const test = <T extends (string | number)>(a: T, b: T) => {
  return a + b;
};

测试地址

// const test = <T extends string | number>(x: T, y: T): T {
//   return (x as any) + (y as any);
// }
// test("a", "b"); // OK
// test(5, 3); // OK
// test(1,"2"); // error

function test<T extends string | number>(x: T, y: T): T;
function test(x: any, y: any) {
  return x + y;
}

let s = test("a", "b"); // fine
let n = test(1, 2); // fine
let n2 = test(1,"2"); // error

const test = <T extends (string | number)>(a: T, b: T): T => {
  return <any>a + <any>b;
};