Improve support for Symbol.toPrimitive

Presently, TS limits index types to ‘string’, ‘number’, ‘symbol’, or ‘any’. That’s fine, but consider the following:

class Foo {
    [Symbol.toPrimitive](): number {
        return 1;
    }
}

var x = ['a','b','c'];
var f = new Foo();
x[f];

We give the error an index expression argument must be of type ... on the array access.

Foo is for all intents and purposes a wrapper around the number 1 (close to new Number(1)), yet it cannot be used to index an array, since Foo itself is not of type number, string, symbol, or any. We could do extra type-checking to see if an object has a user-defined Symbol.toPrimitive and consider its return types when checking type-coercing operations (after all, if you’ve opted in to using Symbol.toPrimitve it’s probably part of your goal to gain this behavior), such as indexing or mathematics operators.

Building on this, we currently don’t support the coercion of Number or String classes to their respective primitive types (which are a specialization of the above issue but which applies to ES5 and ES3 output). Presently one can’t do this:

function add(a: string, b: string): number {
  var x = new Number(a);
  var y = new Number(b);
  return x + y;
}
add('10e4', '0x8');

Because since neither x nor y is the ‘number’ (or ‘string’) primitive type, the addition operation is not allowed. Though it’s still valid js (and without the usage of the Number class, the addition would result in ‘10e40x8’!).

[CMCDragonkai]

I thought I might try the hack where you intersect with a primitive. This works for alot of cases.

type Id = IdInternal & string;

class IdInternal {
  // psuedo code
  [Symbol.toPrimitive]() {
    return 'abc';
  }
}

const id = new IdInternal as Id;

const obj = {};
obj[id] = 3;

However I found that strings intersection results in conflicts with the main type. Especially in terms of index accessors:

// TS now thinks this is `never`
id[0]

So it would be nice to support some way of doing this. Although I noticed that Date has the same problem atm.

[bschlenk]

I think this would make writing isomorphic packages that use setTimeout easier (https://github.com/microsoft/TypeScript/issues/842). You’d be able to assign the Timeout type to a number.