Allow wrapped values to be used in place of primitives.

class NumberWrapper {
    constructor(private value: number) {}
    valueOf(): number { return this.value; }
}

var x = new NumberWrapper(1);

// The right-hand side of an arithmetic operation 
// must be of type 'any', 'number' or an enum type.
console.log(2 + x);

It would be nice if an arithmetic operation could allow using the wrapped number because it’s valueOf() method returns the expected primitive type.

This can be generalized to: if type T is expected, then a value that implements the following interface can be used.

interface Wrapped<T> {
    valueOf(): T;
}

[RyanCavanaugh]

It was an internal discussion before we went public.

Date is insane and I think why we backed off from doing this. Its valueOf method produces a number, but its [[DefaultValue]] internal method defaults to a string hint type (http://www.ecma-international.org/ecma-262/5.1/#sec-8.12.8). This results in very surprising behavior:

> var x = new Date();
undefined
> x.valueOf()
1426616370842
> x + x
"Tue Mar 17 2015 11:19:30 GMT-0700 (Pacific Daylight Time)Tue Mar 17 2015 11:19:30 GMT-0700 (Pacific Daylight Time)"
> x - x
0
> x + 0
"Tue Mar 17 2015 11:19:30 GMT-0700 (Pacific Daylight Time)0"
> x - 0
1426616370842
> x * x / x
1426616370842
> x * x + x
"2.0352342695543988e+24Tue Mar 17 2015 11:19:30 GMT-0700 (Pacific Daylight Time)"

😢

[HerbCaudill]

@RyanCavanaugh this strikes me as a bug. TypeScript shouldn’t choke on valid JavaScript. It seems weird that this issue is still open after 4 years, especially considering that the community has provided multiple use cases, proof-of-concept code, and a specification.