microsoft / microsoft/TypeScript
Object with all context-sensitive properties requires at least one non-context-sensitive property for inference to work
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Description
### 🔎 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
```ts
// 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:
void,
_?: null
}
}
}>
(definition: IdentityObject) =>
D
type Identity =
T extends any
? ( T extends (...a: never) => unknown ? T :
T extends object ? IdentityObject :
T
)
: never
type IdentityObject =
{ [K in keyof T]: Identity }
```
### 🙁 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
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
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".
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Active
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100