Don't allow math operations on different branded numeric types

[JoshuaKGoldberg]

🔎 Search Terms

number numeric branded types plus minus add subtract

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Nominal types (tagged, branded)

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAwgrgJwRAdgYxAHgCoD4oC8UKcAtgEYQJQBkUA3lAPpNqLLogBcU2UAvgG4AUKEhQAoogD2hWO1QZMAIggzluEWOgBVAMoAROfCSKsyuAGcAJhpHCAZnHTAAltJRQ0AQxQAFRDQAC29LCABJYAhSAAowBFc0CJQpBGkeVOkAGigAd28AGwKIYHCUfQMeCoBKBmEoBqgAeiaoADlZbSgqNIQc8jhgKHC86TgC6ygQgDdoCAAPSDQoyY8IesbkYERPfKKSsoqofHjE5MyRfmEgA

💻 Code

type Currency<T> = number & { __currency: T };
type Euro = Currency<"euro">;
type USD = Currency<"usd">;

function canPurchaseItem(priceInEuro: Euro, walletInUSD: USD) {
    // No type error, but I would have expected one
    return walletInUSD > priceInEuro;
}

🙁 Actual behavior

Euro and USD -two different branded numeric types- can be compared with binary operators such as -.

🙂 Expected behavior

These are two different branded numeric types. I would have expected that binary operations between the two of them would be disallowed.

More technically, if a mathematical binary operation contains:

• one value that is an intersection of number and some object type
• another value that is not assignable to that type

...then I'd expect to receive a type error.

Additional information about the issue

Per #202, TypeScript doesn't actually formally include a built-in branded/nominal typing. But other mismatched operators are disallowed [playground of mismatched operators]:

// These are all type errors:
console.log([] + []);
console.log([] + {});
console.log([] + 1);
console.log({} + 1);
console.log({} + {});
console.log({ a: 1 } + { b: 2 });

[uhyo]

There is no heuristic that fits all use cases I think. For example, I might argue that Price * TaxRate should be allowed.

To serve this feature request, I think TypeScript needs to support the “true” nominal typing so that interaction between different types could be made configurable.

[jcalz]

Even with nominal typing you'd need something like #42218 for it to have any effect on mathematical operators.

[RGFTheCoder]

There is no heuristic that fits all use cases I think. For example, I might argue that Price * TaxRate should be allowed.

To serve this feature request, I think TypeScript needs to support the “true” nominal typing so that interaction between different types could be made configurable.

The matching types are only really required for addition or subtraction.

For example, 5m/s + 3m/s makes sense, but 5m/s + 2 hours has no useful outcome. Multiplication and division will generally create new units: 1$ / 2€ = 0.5 $/€