type inference error about typescript HOT 3 CLOSED

• 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]);
  });
}

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• 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]);
  });
}

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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]?

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