旧代码

function foo() {
    // no return
}

// x = undefined
let x = foo();

//  fine - we inferred that 'f1' returns 'void'
function f1() {
    // no returns
}

//  fine - 'void' doesn't need a return statement
function f2(): void {
    // no returns
}

//  fine - 'any' doesn't need a return statement
function f3(): any {
    // no returns
}

//  error!
// A function whose declared type is neither 'void' nor 'any' must return a value.
function f4(): undefined {
    // no returns
}

新代码

//  Works in TypeScript 5.1!
function f4(): undefined {
    // no returns
}

//  Works in TypeScript 5.1!
takesFunction((): undefined => {
    // no returns
});

//  Works in TypeScript 5.1!
takesFunction(function f() {
    //                 ^ return type is undefined

    // no returns
});

//  Works in TypeScript 5.1!
takesFunction(function f() {
    //                 ^ return type is undefined

    return;
});

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