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'!).

issue にあるインデックス式と加算の例から始め、次にインデックス引数と算術オペランドの型チェック規則を追ってください。Symbol.toPrimitive と Number/String ラッパーの型強制を一貫して考慮しつつ、無効な操作を許可しない状態になれば完了です。