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.

[chancancode]

Just wanted to point out that this generally works without errors with the “No implicit returns” flag:

class Foo {
  maybeReturn(): string | void {
    if (Math.random() > 0.5) {
      return 'hello';
    }
  }
}

Playground link

[rohitkhurmi095]

compilerOptions:{ “noImplicitReturns”: false }