Asserts on this from a member function does not narrow this

[JarekToro]

Bug Report

🔎 Search Terms

• assert type guard
• this type guard

possibly related to #43368

🕗 Version & Regression Information

Seems to effect all versions

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about
  • https://github.com/microsoft/TypeScript/wiki/FAQ#why-is-astring-assignable-to-anumber-for-interface-at--
  • https://github.com/microsoft/TypeScript/wiki/FAQ#why-doesnt-x-instanceof-foo-narrow-x-to-foo
  • https://github.com/microsoft/TypeScript/wiki/FAQ#why-doesnt-isfoox-narrow-x-to-foo-when-isfoo-is-a-type-guard

⏯ Playground Link

Playground link with relevant code

💻 Code

/* Setup */
class SuccessResult<T> {
  success: true = true;
  constructor(public value: T) {}
  throwOnFailure(): asserts this is SuccessResult<T> {}
  isSuccess(): this is SuccessResult<T> {
    return this.success;
  }
}

class FailureResult<T, E extends Error> {
  success: false = false;
  constructor(public error: E) {}
  throwOnFailure(): asserts this is SuccessResult<T> {
    throw this.error;
  }
  isSuccess(): this is SuccessResult<T> {
    return this.success;
  }
}

type OperationResult<T = never, E extends Error = Error> =
  | SuccessResult<T>
  | FailureResult<T, E>;

function workingAssertGuard<T>(result: OperationResult<T>): asserts result is SuccessResult<T> {
  if (!result.success){
    throw result.error
  }
}
/* End Setup */



const result: OperationResult<string> = {} as OperationResult<string>

/* This is the issue */
function assertThisType(){
  result.throwOnFailure()
  // type is unchanged
  result // OperationResult<string, Error>
}
/* End of issue */


/* The rest below works as intended */
function narrowOnProperty(){
  // Working narrowing from prop
  if (result.success){
    result.value // SuccessResult<string>
  } else {
    result.error // FailureResult<string, Error>
  }

}

function assertTypeGuard(){

  // Working narrowing from Assert Type Guard Function 
  workingAssertGuard(result)
  result.value // SuccessResult<string>

}

function narrowThisTypeGuard(){
 // Working narrowing from boolean Type Guard Member Function 
    if (result.isSuccess()) {
    result.value // SuccessResult<string>
  } else {
    result.error // FailureResult<string, Error>
  }
}

🙁 Actual behavior

Asserts on this from a member function does not narrow this

🙂 Expected behavior

Asserts on this from a member function does narrow this

[RyanCavanaugh]

For performance reasons, method-based control-flow-affecting calls need to be immediately (definition left intentionally vague) resolvable to something with an asserts return type to be considered part of the control flow graph. A union type doesn't currently fit that definition, but possibly we could change that.

[JarekToro]

Thanks @RyanCavanaugh for the explanation. Is there any docs/code that helps explain this? I'd like to look into it some more but wouldn't know where in the codebase to start.