type inference error

[emeiziying]

🔎 Search Terms

Type 'U' cannot be used to index type '{ name: string; capacity: number; } | { name: string; amount: number; }'.

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.3#code/JYOwLgpgTgZghgYwgAgMpjpZBvAsAKGWQHcB7KAa2gGcAuHZEOAWwnurClAHMBuZBHAAOiYGACe9EAFdmAI2jIAvgG0AurwJFicKBAAWpadQh0GTVu049+LI+CmyFUZes34lBAhKEp0mCABpCHFkAF5kKnFSGDQMSHcfFAA1OAAbaSCQgB4AFWQIAA9IEAATajiA4PEAPnDIkJjKyBVctRUZeWgNL3wEUhAOZA4A+n8sCLxCEnIqKDMVLSJzFjZkAHIAKVJ9EHWAGgFhUQl6AEZlfaWibEZV+nXc0mYDo5EEMUlkC6Ur6bU-tpdAYjCYFtcVpYNgB1UikUqvOzSBzIAAMlwhtwsa3W6AGEERzHsYHo6N+SwBBCU7gIMGRCDAwAGyAAYnC8gVihAyhVxllxIcAKr1VIZfl5Go1AAUI0g1XouUOADd0pl5chBQBKHBLfqDMDINLAIYRWUQFRm6o9aZGjgAOmYwilUrEEGY2rCdSmywEA2opDSEDtaVI3BdkGYKhVYqtmvcRCUcapBCAA

💻 Code

interface State {
  workers: { name: string; capacity: number }[];
  warehouses: { name: string; amount: number }[];
}

type StateKey = keyof State;
type ValueKey<T extends StateKey> = keyof State[T][number];

const state: State = {
  workers: [
    { name: 'John', capacity: 1 },
    { name: 'Tom', capacity: 1 },
  ],
  warehouses: [
    { name: 'Wood', amount: 0 },
    { name: 'Stone', amount: 0 },
  ],
};

function Foo<T extends StateKey, U = ValueKey<T>>(stateKey: T, valueKey: U) {
  const list = state[stateKey];
  list.map((item) => {
    console.log(item[valueKey]);
  });
}

🙁 Actual behavior

Type 'U' cannot be used to index type '{ name: string; capacity: number; } | { name: string; amount: number; }'.

🙂 Expected behavior

no error report

Additional information about the issue

No response

[jcalz]

• The issue template has not quite been filled out properly; could you fix it, please?

• Why do you have U = ValueKey<T> instead of U extends ValueKey<T>? Of course you can't use U to index anything if U is unconstrained. If you change to extends you still have the error so I presume this is a typo.

• Where is the bug, exactly? If you're expecting item to be of type State[T][number] instead of the union? Is that the "inference error" you're talking about? That doesn't sound like a bug; the inferred type isn't incorrect. And you could just annotate the type if you don't like the inferred one:

function Foo<
  T extends StateKey,
  U extends keyof State[T][number]
>(stateKey: T, valueKey: U) {
  const list = state[stateKey];
  list.map((item: State[T][number]) => {
    console.log(item[valueKey]);
  });
}

Playground

[emeiziying]

• The issue template has not quite been filled out properly; could you fix it, please?

• Why do you have U = ValueKey<T> instead of U extends ValueKey<T>? Of course you can't use U to index anything if U is unconstrained. If you change to extends you still have the error so I presume this is a typo.

• Where is the bug, exactly? If you're expecting item to be of type State[T][number] instead of the union? Is that the "inference error" you're talking about? That doesn't sound like a bug; the inferred type isn't incorrect. And you could just annotate the type if you don't like the inferred one:

function Foo<
  T extends StateKey,
  U extends keyof State[T][number]
>(stateKey: T, valueKey: U) {
  const list = state[stateKey];
  list.map((item: State[T][number]) => {
    console.log(item[valueKey]);
  });
}

Playground

Sorry, my ts skills are still in the novice stage. And why should I add type State[T][number] to annotate item? The type of list is State[T], So shouldn’t the type of item be State[T][number]?

[typescript-bot]

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