Bug Report

Eager mapped types result in the indexed access of an union created from all elements of a tuple resulting in the widest type in the tuple.

🔎 Search Terms

eager mapped types, array intersection filtering

🕗 Version & Regression Information

This bug exists in every version of TypeScript available on the playground with support for the necessary language features through nightly. In 3.8.3, both assignments to test fail.

• This changed between versions 3.8.3 and 3.9.7

⏯ Playground Link

Playground link with relevant code

💻 Code

interface ListElement {
    condition: boolean;
    id: string;
}

const list = [
    { condition: true, id: "yes" },
    { condition: false, id: "no" },
] as const;

type CheckCondition<
    List extends readonly ListElement[],
    ID extends List[number]["id"],
    > = ({ id: ID } & List[number])["condition"]

type Mapped = {
    [ID in "yes"]: CheckCondition<typeof list, ID>
}["yes"]

type NotMapped = CheckCondition<typeof list, "yes">

declare const mapped: Mapped
declare const notMapped: NotMapped

let test: true = true
test = mapped // error, should pass
test = notMapped

🙁 Actual behavior

The type constructed using a mapped type loses specificity, becoming (typeof list)[number]["condition"], which in this case is true | false == boolean

🙂 Expected behavior

The mapped type should have no effect on the resulting type.

This is the result of a question on the TS discord.