Don't allow math operations on different branded numeric types

🔎 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 });

Comienza con el ejemplo de Playground proporcionado y compáralo con el ejemplo de mismatched-operator enlazado para comprender el comportamiento actual de comprobación. El trabajo estará terminado cuando las operaciones entre intersecciones numéricas branded distintas produzcan los errores de tipo esperados sin introducir regresiones en los errores de mismatched-operator existentes.