Flow reports implicitly returned undefined even when not possible

[Gozala]

I keep running into more complex version of this example code fairly often:

/* @flow */

const exmpl = (input:number):number => {
  while (true) {
    switch (input) {
      case 0:
        return input
      default:
        return 0
    }
  }
}

Which flow fails to type check with a following error:

3: const exmpl = (input:number):number => {
                                ^ number. This type is incompatible with an implicitly-returned undefined.

Most likely flow is failing to infer that while block will ever be executed. I am not sure how difficult would it be make flow type check this code, but either way it would really help if flow provided a hint on why it assumes implicitly returned undefined something, telling user that while block may never run would greatly improve usability.

[pechitook]

Any reference to how this can be solved? Thanks!

[jcready]

A workaround is to explicitly throw or return after the while loop.