Improve typing of arguments with a function (with respect to overloads)

Search Terms

function overload arguments narrowing

Suggestion

The example is probably the best illustration I can give... It's also much clearer (I think) that the description in text.

Currently, the types of the arguments of a function with overloads cannot be forwarded to another function with the same overloads (1) because the raw typing of the argument doesn't match any of the overloads. But, at the same time, this restriction is not carried out in the relationship between the arguments' types is not used once in the function's body.

We currently have to either assert types (using as) or do if (...) throw with conditions that will never be met runtime.

That requires to sacrifice either type safety or performance.

I've seen #13225. However, my suggestion is more regarding the arguments than the return type. It shouldn't impose any additional restrictions that currently exist (actually, remove some).

Moreover, one of the arguments that was given at the time ("Pass-through overloads are extremely common and would require writing "artificial" code to satisfy the checker") is not true anymore. (See point (1) in the example). I'm not sure if this particular point should be considered a bug in itself or not...

Having the type-system aware of the overloads when typing the arguments could actually restore this.

Use Cases

Simplify overloads implementation and make them more readable.

Examples

let baz: string;

function fn2(foo: 'a', bar: string): void;
function fn2(foo: 'b', bar: number): void;
function fn2(foo: 'a' | 'b', bar: string | number): void
{
    fn(foo, bar); // (1) If this doesn't work, ...
}

function fn(foo: 'a', bar: string): void;
function fn(foo: 'b', bar: number): void;
function fn(foo: 'a' | 'b', bar: string | number): void
{
    if (foo == 'a')
    {
        baz = bar; // (2) ... This should!
    }
}

https://www.typescriptlang.org/play/index.html#src=let%20baz%3A%20string%3B%0D%0A%0D%0Afunction%20fn2(foo%3A%20'a'%2C%20bar%3A%20string)%3A%20void%3B%0D%0Afunction%20fn2(foo%3A%20'b'%2C%20bar%3A%20number)%3A%20void%3B%0D%0Afunction%20fn2(foo%3A%20'a'%20%7C%20'b'%2C%20bar%3A%20string%20%7C%20number)%3A%20void%0D%0A%7B%0D%0A%20%20%20%20fn(foo%2C%20bar)%3B%20%2F%2F%20(1)%20If%20this%20doesn't%20work%2C%20...%0D%0A%7D%0D%0A%0D%0Afunction%20fn(foo%3A%20'a'%2C%20bar%3A%20string)%3A%20void%3B%0D%0Afunction%20fn(foo%3A%20'b'%2C%20bar%3A%20number)%3A%20void%3B%0D%0Afunction%20fn(foo%3A%20'a'%20%7C%20'b'%2C%20bar%3A%20string%20%7C%20number)%3A%20void%0D%0A%7B%0D%0A%20%20%20%20if%20(foo%20%3D%3D%20'a')%0D%0A%20%20%20%20%7B%0D%0A%20%20%20%20%20%20%20%20baz%20%3D%20bar%3B%20%2F%2F%20(2)%20...%20This%20should!%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

該 issue 提供了一個 TypeScript playground 範例，但沒有指明任何儲存庫檔案或測試。先重現 overload 實作和 forwarding call，然後調查 checker 對 overload 引數的行為；完成的標準是 forwarding call 被接受，並且 conditional branch 在不使用 assertions 或不可達的 runtime checks 的情況下將 bar 窄化為 string。