Suggestion

🔍 Search Terms

catch argument type
catch error type
useUnknownInCatchVariables

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

📃 Motivating Example

This would be the same kind of setting as useUnknownInCatchVariables, except simply replace "Unknown" with "Error".

try {
    someFuncThatErrors();
} catch (err) {
    console.log(err.message); // compiler happy because assumes that `err` is an instance of Error
}

💻 Use Cases

Yes, we all know that a maniac could decide to throw a string if they really wanted to--but if that's happening in your project you've got much bigger problems than type safety. TypeScript is about enforcing a contract, so why not allow users to say "all thrown objects will extend Error"? This will reduce annoying boilerplate/explicit casts in catch blocks when you just want to access general error properties, or pass err to a function that expects an Error.

Obviously this is related to other issues (for example, #20024), but I'm not talking about allowing type annotations in catch variables. Just automatically assuming type Error and nothing more.