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

🔎 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) and hence context and state parameters of entry are not inferred.

🙂 Expected behavior

The first createMachine invocation should infer the type parameter correctly without having to pass in a non-context-sensitive property leading to inference of context and state parameters of entry

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

Start by reproducing the inference difference in the linked TypeScript Playground using the two createMachine examples. Read the generic declaration and IdentityObject type in the issue, then trace the compiler's type-parameter inference for the context-sensitive entry property. Done means the version without the extra _ property infers context as { foo: number } and state as "a".