typescript helper inferring type from union type by key

helper.ts

type Helper<T, K extends string | number> = Extract<T, Record<K, unknown>>;

test.ts

type A = {
    a: string
    qwe: string;
}

type B = {
    b: string;
    asd: string;
}

type C = {
    c: string;
    zxc: string;
}

type Res = Helper<A | B | C, "a">;

//  type Res = {
//      a: string;
//      qwe: string;
//  }