🔎 Search Terms

hasinstance, instanceof, 2359, TS2359

🕗 Version & Regression Information

• I was unable to test this on prior versions because this is recent functionality introduced in Support custom 'Symbol.hasInstance' methods when narrowing 'instanceof' #55052

⏯ Playground Link

https://www.typescriptlang.org/play?declaration=false&target=9&module=7&ts=5.4.0-dev.20231124#code/C4TwDgpgBAEghgZwJIDsHDigxhA8gMygF4oBvAKCigG0BlEAWwCMB7AGwDoALRVdTHAF0AFAA8AXAFcUAaxQsA7igCU4qK3YRM5AL7lyAegNQWM8vmlZgASxYooDEHwzY8+MVNnylAGihZxeGQ0FxwCZQoqACcIYEko+1EoaxCBCBZCLF19I3Uo0wgUc0sbOwcnVNcCVGAIKIQIK1sUD2k5RRQ-AKDnNIIoADITJgArRuAIyigYuISoJJT+Vwz-bMNjJnyZQuLsUvtHXqr8AHFCuussAB4AYSgIUVqUABMEWF5KsPwAPlavDq64huk2isXiiWSn3SmV0QA

💻 Code

type HasInstanceOf = {
  [Symbol.hasInstance](x:unknown): boolean
}

// ok
function myInstanceOf(x:unknown, c:HasInstanceOf){
  return x instanceof c
}

// broken
function myInstanceOfIntersection(x:unknown, c:HasInstanceOf & object){
  return x instanceof c
}

// broken
function myInstanceOfGeneric<C extends HasInstanceOf>(x:unknown, c:C){
  return x instanceof c
}

🙁 Actual behavior

The second and third examples give error TS2359

The right-hand side of an 'instanceof' expression must be either of type 'any', a class, function, or other type assignable to the 'Function' interface type, or an object type with a 'Symbol.hasInstance' method.

In all cases, the right-hand side is an object type with a Symbol.hasInstance method.

🙂 Expected behavior

I expect all three examples to typecheck.

Additional information about the issue

No response