Object with all context-sensitive properties requires at least one non-context-sensitive property for inference to work

[devanshj]

🔎 Search Terms

object with all context-sensitive properties, self referencial types, xstate

🕗 Version & Regression Information

Tested with TypeScript 7.0.2, the issue exists even in versions as old as 4.4.4

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.3#code/PTAEAcCcHsCMBsCmBbAXKAJtRBnAdgOQAuoAlngGaKSgAioAxtJJIg0fAJ4A0j0eRRAA8SAQzwZQOIqMGhRreXk4AoBq1mIAsqIYALcogAUAbxWgyeUkVKj46AESiH3c3wHCi6E6ArRo6ACMoAC+rhbSmjjebhaiMRaJoIgCkJzoRkweIryRggCUoAC8AHygZklJWYIisZUgoAB6APx1SXmIbYkNLV0hdf0W-SH5KioNONDwAK42-OiiGJKioNM4iEg4OKB4-AC01Z5763g41qQAbogQMODURJygAAYA+uh40-DwT6BqGoI6fSGUxucjnOyOZzhdw1LzlXz+IKhaEdaLlOrxdGVZKpdKgTL8WG5GQFYplCrYmGeLoWHqtSlSEmdBl0vrQpJvHafeADNzDUYqDBseAKa5ZaSMf7aXQGPCIVBuAA89E8KQw2wpFjBNghoAA1ohONAKHQANoOVEOAC67KpInQcAAVmwiLbUQlKqaAMqWfWG41mi1MnDWq0e7EpIhpZoZQ72wNxojW4madBewqlUAXaCkDC2jkxrlfPq8oYlOpGIUUbWkeagACSQoE1k4AHlYM72MqShny9jaGMHncG02bA9FQAVMpFNwT5IiNXbcSqJLNfGgOeqiTbIwAOn3mLlV0gvdWeD1uwA7nhQGu5wqGZuF9vQE6XbeR5GW+3O0RJ2UH0pCculGJJ3kQY9B04YdGy-B4fxdf9ijcHxTQAaV9A0jRNCcw0-ZtxwndCrTKfogAOfEUiAA

💻 Code

// problem: doesn't infer D correctly, context and state are any
createMachine({
  initial: "a",
  context: { foo: 1 },
  states: {
    a: {
      entry: (context, state) => {
        context
        // ^? any
        state
        // ^? any
      }
    }
  }
})

// solution: add a useless non-context-sensitive property `_: null` 
createMachine({
  initial: "a",
  context: { foo: 1 },
  states: {
    a: {
      entry: (context, state) => {
        context
        // ^? { foo: number }
        state
        // ^? "a"
      },
      _: null
    }
  }
})

declare const createMachine:
  <D extends {
    initial: keyof D["states"],
    context: object,
    states: {
      [S in keyof D["states"]]: {
        entry?: (context: D["context"], state: S) => void,
        _?: null
      }
    }
  }>
    (definition: IdentityObject<D>) =>
      D

type Identity<T> =
  T extends any
    ? ( T extends (...a: never) => unknown ? T :
        T extends object ? IdentityObject<T> :
        T
      )
    : never

type IdentityObject<T> =
  { [K in keyof T]: Identity<T[K]> }

🙁 Actual behavior

The first createMachine invocation doesn't infer the type parameter correctly (it requires at least one non-context-sensitive property)

🙂 Expected behavior

The first createMachine invocation should infer the type parameter correctly without having to pass in a non-context-sensitive property

Additional information about the issue

The premise is adding (or rather not adding) an extra useless property shouldn't make a difference to the inference