Is it possible to type output of child states?

[WIStudent]

The documentation describes how to type the root output of a machine here, but it is also possible to set output on final states of a child state. Take a look at the following example:

import { setup } from "xstate";

interface ChildOutput {
  result: "success" | "failure"
}

setup({}).createMachine({
  initial: "parentInitial",
  states: {
    parentInitial: {
      on: {
        NEXT: "child"
      }
    },
    child: {
      initial: "childInitial",
      states: {
        childInitial: {
          on: {
            SUCCESS: "childSuccess",
            FAILURE: "childFailure"
          }
        },
        childSuccess: {
          type: "final",
          output: (): ChildOutput => ({
            result: "success"
          })
        },
        childFailure: {
          type: "final",
          output: (): ChildOutput => ({
            result: "failure"
          })
        }
      },
      onDone: [
        {
          target: "parentA",
          guard: ({event}) => event.output.result === "success"  // Results in ts error because output is unknown
        },
        {
          target: "parentB",
          guard: ({event}) => (event.output as ChildOutput).result === "failure" // My current workaround
        },
        {
          target: "parentC"
        }
      ]
    },
    parentA: {},
    parentB: {},
    parentC: {}
  }
});

Link to playground

The child state has 2 final states, childSuccess and childFailure, that set the result field in the output to success or failure. The parent uses guards in onDone to decide the next state based on the result field in the output. Unfortunately the type of the event that the guard receives is always DoneStateEvent<unknown>. Is there a way to configure the type of the child state output so that the event the guard receives is correctly typed? Or is my only option here to cast inside the guard?

[crishoj]

Also curious to know this.

Typing output at the topmost level is possible with setup:

setup({
  types: {
    output: {} as ChildOutput
  }
}).createMachine({
  // ...
})

But I am not aware of a way to specify types of "inline" child actors.