Generic key does not match narrow type in nested objects

[alexfauquette]

🔎 Search Terms

discriminate union in nested objects

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about generic.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.3#code/C4TwDgpgBAggdgSwLYEMA2BnKBeKByAEwHsBzPKAH3wGMVg8BuAKCeqLg2ChUVUwC5YvdBgDaAXRxRRhUngA0NOnnHMmAenVQAIhABmCONBRQAKgGUobOAZJQie7mjTdhmKEgjAAFkQJYACnY0EG4oNCJSCAAnKEMoHwQsCAAPFCQwNAgAShZrTit2W0EAb2lTOLghZBFxUvDSEhjBABlGmIAeUwA+BigAXwGpEqYoMas6eoiSJujBALAUaPTBOABXJAAjGOycbqgAAx9oWi4IOigAEhLF5aRBgwxvCAwDxX75UfHiEin2uagCyWKygnGihhIu2w+yOzygP3CCAA1i8rjdgfcDgNPv01BotAAFaIQW7GBqzeybABWEGoXGARCgBmcUE2KAwEAI9iqx0KNgQJBYoEgUDaM06FVSwAgcH81T4GH2uElKWlsqweFOeC+YwA-IDbiD1lsdntQcBwXBBeMoPNDUhBGCIVD9k6rWphdAxbMsLgRjbRBV4vAaphxLrWv8ur0mLi8uwCk1gNLohhI+LU8M40weKGMAA6PREaIAURQ1G8AS6UClMrlIYV3QCub4glMLqg-vG+S4ADd0GsIKYiOYvFJrLZRC30OJ89NZn1NKKoz0dXyCv20IOAOJeFNSJMpsTTtCqKBL70xMSmcRrw9XqduSS4CcCx+h2fzmKLrTaIiouAiC4NZZSvYAeC5HwLkvVNA2fbBcBg6M7z3B9NWUZ8+UndD6E-f4fygABVUDU3A9UEm8aD-jEHCVBwBDlwzDpaO6WNsjUJcAFpuJ43i+P4gTBKE4SePxKBzGQBA0CWQoCGgKCuAAd2LJEsG2Wg1g5CieFUhJGRMDA0AFbwuFfOwkD8BA9FOBB2DEkSHMcpzBJYVgE1MooBQAJnqQNKnlWp01maMhj9NdTjtDFVg2bZog7WETguc4uGue0HiSZ5Xk+G0fkiu5HQtZ0zQS+FSERFEsFSjF+gOWM8RPAsi1LctK2rWtyIbEQmxPNsOy7MYeygTdB2HUcuBfTySC898+DPJcylyqB82WvoIqWlaBjg8L3KGgcIF3ZMYgPVDYJPOatBg69bxte9TqfcdJums6CL-VEQLk0iIO0rhLrg+jEJXVibpOmitUwszpto86iJIzgIKwBTGJ9GQwf+pHOhYtjmCAA

💻 Code

const config: { [T in Keys]: { logger: Logger<T>; } }  = { ... };

const loggers: { [T in Keys]?: Logger<T>; } = { ... };


items.forEach(<T extends Keys>(key: T) => {
	// TS does not understand that Loggers[T] (type of getters[key]) is the same as Logger<T> (type of config[key].logger)
    getters[key] = config[key].logger; 
});

🙁 Actual behavior

TS does not understand that Loggers[T] (type of getters[key]) is the same as Logger (type of config[key].logger)

🙂 Expected behavior

TS should understand that Loggers[T] (type of getters[key]) is the same as Logger (type of config[key].logger)

Additional information about the issue

No response

[RyanCavanaugh]

This is only safe by construction because you're using the key value in both positions, but from TypeScript's analysis perspective this is no different from

function func<T extends Keys>(key1: T, key2: T) => {
    // Not safe!!
    getters[key1] = config[key2].logger; 
}
func<"a" | "b">("a", "b");

TS doesn't currently have any notion of tracking that two utterances of the same type variable must refer to the same value

[alexfauquette]

Thanks for the explanation. But I don't understand why it's the .logger addition that adds the error.

I can reproduce the Not safe example without getting any error

const config: { [T in Keys]: Logger<T> } = { ... };
const getters: { [T in Keys]?: Logger<T> } = {};

function func<T extends Keys>(key1: T, key2: T) {
    // Not safe!!
    getters[key1] = config[key2]; 
    
}
func<"a" | "b">("a", "b");

[RyanCavanaugh]

Oops 😅. There is special carve out for exactly the form

x[k] = y[k]

where both k are T, but this is (intentionally) a special case that doesn't apply anywhere else

[alexfauquette]

Thanks for the explanation

I don't know if it's feasible, but having a more explicit error message when typescript detects a A[T] = B[T].x pattern because I've probably spent more than a day trying to let TS understand that the T in A[T] and B[T] are the same.

[typescript-bot]

This issue has been marked as "Design Limitation" and has seen no recent activity. It has been automatically closed for house-keeping purposes.