Improved union type guards

[ahejlsberg]

Fixes #1587.

This PR improves handing of union types in type guards. Specifically, when a type guard of the form x instanceof C is true, the type of x is narrowed as follows:

• If C is a subtype of the type of x, the type of x is narrowed to C.
• Otherwise, if x is of a union type, all types that are not subtypes of C are removed from the type of x.

In the example

class Message {
    value: string;
}

function saySize(message: Message | Message[]) {
    if (message instanceof Array) {
        return message.length;  // message of type Message[] here
    }
}

the Message type is removed under the type guard because it is not a subtype of Array.

[JsonFreeman]

👍

[danielearwicker]

Note that (x instanceof Array) !== Array.isArray(x) in cross-window situations, though not that common these days.

[battmanz]

If I modify the code as follows and add an else block, why does the compiler think that the type of message inside that else block is Message | Message[]? Shouldn't it only be Message?

class Message {
    value: string;
}

interface MessageHandler {
    handle(message: Message): void;
}

function saySize(message: Message | Message[], handler: MessageHandler) {
    if (message instanceof Array) {
        message.forEach(m => handler.handle(m));
    }
    else {
        handler.handle(message); // Argument of type 'Message | Message[]' is not assignable to parameter of type 'Message'.
    }
}

[danquirk]

@battmanz see #1719

[mhegazy]

@battmanz this is tracked by #1719