Problem returning union of promises in `then` callback

TypeScript Version: 3.2.0-dev.20181004

Code

function f(b: boolean, pn: Promise<number>, ps: Promise<string>) {
 return Promise.resolve().then(() => b ? pn : ps);
}

Expected behavior:

No error.

Actual behavior:

Pyramid of doom error message:

src/a.ts:2:35 - error TS2345: Argument of type '() => Promise<string> | Promise<number>' is not assignable to parameter of type '(value: void) => string | PromiseLike<string>'.
  Type 'Promise<string> | Promise<number>' is not assignable to type 'string | PromiseLike<string>'.
    Type 'Promise<number>' is not assignable to type 'string | PromiseLike<string>'.
      Type 'Promise<number>' is not assignable to type 'PromiseLike<string>'.
        Types of property 'then' are incompatible.
          Type '<TResult1 = number, TResult2 = never>(onfulfilled?: ((value: number) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => Promise<...>' is not assignable to type '<TResult1 = string, TResult2 = never>(onfulfilled?: ((value: string) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => PromiseLike<...>'.
            Types of parameters 'onfulfilled' and 'onfulfilled' are incompatible.
              Types of parameters 'value' and 'value' are incompatible.
                Type 'number' is not assignable to type 'string'.

2     return Promise.resolve().then(() => b ? pn : ps);

Related Issues: #20531

Start with the reproducer in src/a.ts at line 2 and compare the behavior with related issue #20531. Investigate how the Promise.resolve().then callback is typed for a union of promises; done means the shown code compiles without an error while retaining the expected promise result types.