Array.map typechecking issues, doesn't typecheck what the .map callback returns #31425

type Token = {
    value: string
}

const things = ['one', 'two'];

// using .map generic — doesn't fail typechecking
function getTokens1() {
    return things.map<Token>(thing => ({
        value: thing,
        // should say `somethingElse` does not exist on type `Token`
        somethingElse: true
    }));
}

// using return type — doesn't fail typechecking
function getTokens2(): Token[] {
    const tokens = things.map(thing => ({
        value: thing,
        // should say `somethingElse` does not exist on type `Token`
        somethingElse: true
    }));

    // type of tokens is reported as `{ value: string; somethingElse: boolean; }[]`
    // so how could it be compatible with `Token[]`?
    return tokens;
}


// following use cases as working proofs and potential workarounds

// fails typechecking correctly
const token: Token = {
    value: 'one',
    somethingElse: true // `somethingElse` does not exist on type `Token`
};

// fails typechecking correctly — potential workaround
function getTokens3() {
    // note the `Token` inline return type
    return things.map((thing): Token => ({
        value: thing,
        somethingElse: true // `somethingElse` does not exist on type `Token`
    }));
}