TS 5.5 type guards overly strict in some cases

[benasher44]

🔎 Search Terms

• predicate
• guard
• type guard

🕗 Version & Regression Information

• This changed between versions 5.4.x and 5.5

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAEDKAOBTYBLMICiAnLB7L0iAHgC6IB2AJjNngQN4C+AUMwGYCu5wJKu50FBATI0mHPgAUiCVgBc0WvgCU0es2jQsiEhywCZdQeQgkw3RLjZwkqdEqwBuZi3ZcefAWVOTV6zcD8poSy0AC80OSIAO6Ksr7OmijWkkIiduJ00rLKfhqaWjp65InQLAFBuCCIAHQguADm2XQ1ALaIUGANiMrOLEA

💻 Code

class SpecialError extends Error {}

function isSpecialError(error: Error) {
  return error instanceof SpecialError;
}

function test() {
  const error = new Error();
  if (isSpecialError(error)) {
    return;
  }
  console.log(error.message);
}

🙁 Actual behavior

Error: "Property 'message' does not exist on type 'never'."

Even if error isn't SpecialError, it's still of type Error, which means it has a message property.

🙂 Expected behavior

This should compile. The error not being a SpecialError does not mean the error isn't still of type Error, which has a message property.

Additional information about the issue

In TS 5.4, this was allowed, but it's broken as of 5.5. My initial thought was that this had something to do with automatic type predicates, but adding an explicit type guard to the function doesn't help.

[benasher44]

Interestingly, someone on my team discovered that adding a random getter to SpecialError works as a workaround:

class SpecialError extends Error {
  get blah() {
    return "blah";
  }
}

With that in place, the error goes away 🤔

It has to be a property/method other than message though

[MartinJohns]

Interestingly, someone on my team discovered that adding a random getter to SpecialError works as a workaround:

That's because the types are structurally the same without an additional property.

This really didn't change, it was the same behavior in earlier TypeScript versions. Just in earlier versions your isSpecialError function is not inferred to be a type guard, it's just a function returning a boolean, while in 5.5 it now is inferred as a type guard. When you add a type annotation you get the same behavior in 5.4: function isSpecialError(error: Error): error is SpecialError. Respectively you get the old behavior in 5.5 by adding a return type annotation of boolean.

This works as intended. See also: https://github.com/microsoft/TypeScript/wiki/FAQ#what-is-structural-typing

[benasher44]

Ah! I see. The fact that it's a different class doesn't matter. Because the types are structurally the same, the types may as well be the same, in the eyes of TypeScript. Makes sense! Closing