TSC incorrectly selects overloaded method signature

[chebum]

Hi,

I added a strongly typed jQuery.each method overload in order to perform type check when compiling my ts files:

each<T>(collection: { [key: string]: T }, callback: (indexInArray: string, valueOfElement: T) => any): any;   // Added by me
each(collection: any, callback: (indexInArray: any, valueOfElement: any) => any): any;

When using this method, tsc doesn't perform type check and allows you to write a code that will fail at runtime. For example:

/// <reference path="../../Lib/jquery.d.ts" />
var args: { [key: string]: number } = {};
$.each(args, (index: number, val) => {
    var chr = val.charAt(0);
});

There is no error at compile time, because val is treated as any. This happens because tsc chooses less specific overload with anys. However, I'd like to see a compilation error.

I do see compilation error when I write it as:

$.each(args, (index, val) => {
    var chr = val.charAt(0);
});

Why is that?

[danquirk]

The first overload cannot be picked given the type annotations you used in the first example. The callback says it is of type (string,T)=>any and you provided a (number, any)=>void so the first overload is skipped and the compiler looks for other matches in the overload set. The second overload does match since it all it requires is a value of any type and a function with 2 arguments, so val gets the type any and no error occurs.

The second example chooses the first overload because the args value you provided has a type that matches the first parameter and creates an inference of number for T. Since there are no type annotations on the function call the compiler will attempt to contextually type the arguments based on the overload it picked. So val is now treated as a number which of course does not have a charAt member.

The presence of an overload with any all over the place means that any calls to each will be extremely permissive since any combination of types will succeed as long as the arguments are a value plus a 2 argument callback function.