Surprising `Not all code paths return a value`

TypeScript Version: 2.5.2

Code

type AsyncFunction<T> = () => Promise<T>;

const f: AsyncFunction<number | void> = async () => {
  if (Math.random()) {
    return 1;
  }
};

Expected behavior:

No tsc errors.

Actual behavior:

const f: AsyncFunction<number | void> = async () => {
//                                      ~~~~~~~~~~~~~ [ts] Not all code paths return a value.

  if (Math.random()) {
    return 1;
  }
};

Notably, when using return-type annotation, this error does not occur:

async function a(): Promise<number | void> {
  if (Math.random()) {
    return 1;
  }
}

const b = async (): Promise<number | void> => {
  if (Math.random()) {
    return 1;
  }
};

In our real-world scenario, it would not be sufficient to look for this syntactic pattern. Rather, the fact that the async function is assigned to a union containing void should be the trigger that silences this error.

Reproduce the TypeScript 2.5.2 examples with tsc, comparing the async arrow assigned to AsyncFunction<number | void> with the explicitly annotated functions. Done means the union containing void suppresses the unexpected diagnostic while the compiler still reports genuinely incomplete return paths.