const A = "A";
const B = "B";

interface AAction {
    type: typeof A;
    payload: {
        a: string;
        b: string;
    }
}

interface BAction {
    type: typeof B;
    payload: {
        c: string;
        d: string;
    }
}
interface ActionMap {
    [A]: AAction;
    [B]: BAction;
}

declare function test<K extends keyof ActionMap>(key: K, payload: ActionMap[K]["payload"]): void;
test("A", { /* completion here */ })