Union type interaction with overloads

[danielearwicker]

Where there are overloads of a method, the method cannot be directly called with a parameter of a union type that matches the available overloads.

Here's a cut-down example that highlights the apparent issue (can be pasted into TypeScript playground):

    interface JQuery {
        append(content1: JQuery): JQuery;
        append(content1: string): JQuery;
    }

    var messageElement: JQuery;

    function setMessage(message: string | JQuery) {

        messageElement.append(message); // error JQuery is not assignable to type string

        // Help the compiler out with a type guard, it's okay
        if (typeof message === "string") {
            messageElement.append(message);
        } else {
            messageElement.append(message);
        }
    }

It's clear why the heavily-redundant type-guard workaround makes it easier for the compiler: in the general case each overload is free to return a different type, e.g. hypothetically weird example:

    interface JQuery {
        append(content1: JQuery): boolean;
        append(content1: string): number;
    }

This is an example of overloads capturing a specific relation that would be obscured if we instead used union types:

    interface JQuery {
        append(content1: JQuery|string): boolean|number;
    }

By writing it with union types, we don't capture the fact that JQuery goes to boolean, whereas string goes to number, but string never goes to boolean, etc..

But what I'm hoping is that compiler could behave as if there was an additional "catch all" overload that accepted the union of the parameters and return the union of the return types, i.e.:

    interface JQuery {
        append(content1: JQuery): boolean;
        append(content1: string): number;

                // Therefore the compiler infers the existence of this extra overload:
        append(content1: JQuery|string): boolean|number;
    }

That way, union type parameters would go into overloads more smoothly.

[nycdotnet]

@danielearwicker I also reported this in #1805 - can you respond to @danquirk 's question there?

[danquirk]

Yeah, gonna dupe this and we can use that issue to discuss further.