TypeScript-函数兼容性

let fn1 = (x: number, y: number) => {
};
let fn2 = (x: number) => {
};

fn1 = fn2;

let fn1 = (x: number, y: number) => {
};
let fn2 = (x: number) => {
};

fn2 = fn1;

let fn1 = (x: number) => {
};
let fn2 = (x: number) => {
};
let fn3 = (x: string) => {
};
fn1 = fn2;
fn2 = fn1;

let fn1 = (x: number) => {
};
let fn2 = (x: number) => {
};
let fn3 = (x: string) => {
};

fn1 = fn3;
fn3 = fn1;

let fn1 = (): number => 123;
let fn2 = (): number => 456;
let fn3 = (): string => 'abc';
fn1 = fn2;
fn2 = fn1;

let fn1 = (): number => 123;
let fn2 = (): number => 456;
let fn3 = (): string => 'abc';

fn1 = fn3;
fn3 = fn1;

let fn1 = (x: (number | string)) => {
};
let fn2 = (x: number) => {
};
fn1 = fn2;
fn2 = fn1;

let fn1 = (x: boolean): (number | string) => x ? 123 : 'abc';
let fn2 = (x: boolean): number => 456;
fn1 = fn2;

let fn1 = (x: boolean): (number | string) => x ? 123 : 'abc';
let fn2 = (x: boolean): number => 456;
fn2 = fn1;

function add(x: number, y: number): number;
function add(x: string, y: string): string;
function add(x, y) {
    return x + y;
}

function sub(x: number, y: number): number;
function sub(x, y) {
    return x - y;
}

function add(x: number, y: number): number;
function add(x: string, y: string): string;
function add(x, y) {
    return x + y;
}

function sub(x: number, y: number): number;
function sub(x, y) {
    return x - y;
}

let fn = add;
fn = sub;

function add(x: number, y: number): number;
function add(x: string, y: string): string;
function add(x, y) {
    return x + y;
}

function sub(x: number, y: number): number;
function sub(x, y) {
    return x - y;
}

let fn = sub;
fn = add;