Overload gets lost in mapped type with conditional type

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms: mapped type overload

Code

interface Overloads {
  foo(a: string): void;
  foo(a: number, b: string): void;
}

/** Converts all properties of an object to Promises and all methods to return Promises */
type ProxiedObject<T> = {
  [P in keyof T]: T[P] extends (...args: infer Arguments) => infer R
    ? (...args: Arguments) => Promise<R>
    : Promise<T[P]>
};

declare let x: ProxiedObject<Overloads>;
x.foo("abc"); // Error: [ts] Expected 2 arguments, but got 1. [2554]
x.foo(123, "abc");

Expected behavior:
No error, overload should be maintained.

This makes it impossible to use this pattern with popular types that contain overloads, like Rx Observable pipe()/subscribe().
The ProxiedObject type is used in https://github.com/GoogleChromeLabs/comlink.

Actual behavior:
Overload gets lost, compile error when trying to call the first overload.

Playground Link: link

Start by reproducing the mapped-type overload behavior in the linked TypeScript Playground using the provided Overloads and ProxiedObject example. Trace the compiler's handling of conditional and mapped types, then add a regression test for both overload calls; done means neither call produces an error and overload behavior is preserved.