promisify.ts

type Promisified<T extends Function> =
    T extends (cb: (err: any, data: infer TData) => any) => void ? () => Promise<TData> :
    T extends (arg0: infer TArg0, cb: (err: any, data: infer TData) => any) => void ? (arg0: TArg0) => Promise<TData> :
    T extends (arg0: infer TArg0, arg1: infer TArg1, cb: (err: any, data: infer TData) => any) => void ? (arg0: TArg0, arg1: TArg1) => Promise<TData> :
    /* etc */
    never;

function w0args(cb: (err: Error, data: string) => void) {}
function w1args(arg0: number, cb: (err: Error, data: string) => void) {}
function w2args(arg0: number, arg2: boolean, cb: (err: Error, data: string) => void) {}

type w0argsPromisified = Promisified<typeof w0args>;
type w1argsPromisified = Promisified<typeof w1args>;
type w2argsPromisified = Promisified<typeof w2args>;

type Source = {
    w0args: typeof w0args;
    w1args: typeof w1args;
    w2args: typeof w2args;
};

type PromisifiedSource = {
    [TKey in keyof Source]: Promisified<Source[TKey]>;
};

const x: PromisifiedSource = null as any;
const r0: Promise<string> = x.w0args();
const r1: Promise<string> = x.w1args(42);
const r2: Promise<string> = x.w2args(42, true);