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.

Inizia con gli esempi TypeScript motivanti e il Playground collegato, confrontando le diagnosi per le funzioni assegnate immediatamente con i tipi di ritorno annotati esplicitamente. Il lavoro è completato quando una mancata corrispondenza del tipo di ritorno nella prima forma viene segnalata nell'istruzione return che la causa, in modo coerente con il secondo esempio, senza modificare il JavaScript emesso.