Report better errors on return expressions for immediately-assigned functions

Suggestion

🔍 Search Terms

return type label:"Domain: Error Messages"

✅ 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

Error reporting for mismatches between a functions return type and what is being returned by a particular return statement should be consistently reported a the location of the offending return.

📃 Motivating Example

TypeScript Playground

type Foo = (input: string) => number;

const foo: Foo = (input) => {
  switch (input) {
    case "1":
      return 1;
    case "2":
      return "2";
    default:
      return Infinity;
  }
}

The following error is reported on the declaration of foo.

const foo: Foo
Type '(input: string) => number | "2"' is not assignable to type 'Foo'.
  Type 'number | "2"' is not assignable to type 'number'.
    Type 'string' is not assignable to type 'number'.(2322)

const bar = (input: string): number => {
  switch (input) {
    case "1":
      return 1;
    case "2":
      return "2";
    default:
      return Infinity;
  }
}

The following error is reported on return "2":

Type 'string' is not assignable to type 'number'.(2322)

💻 Use Cases

In the second example it's much easier to see where the problem is. In larger functions for more `returns it can be even hard to pin down which one is causing the issue. These two examples are very similar and I would expect the same error to be reported for each.

動機となる TypeScript の例とリンクされた Playground から始め、すぐに代入される関数と、戻り値の型が明示的に注釈付けされた関数の診断を比較します。最初の形式での戻り値の型の不一致が、2 番目の例と一貫して、問題のある return 文で報告され、出力される JavaScript が変更されなければ完了です。