Allow nesting user-defined type guards within a return object

[HenryMorgan-cd2]

Suggestion

🔍 Search Terms

nested user-defined type guards

✅ 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

Currently, type guards can only return a boolean value (x is SomeType). I think it would be beneficial to allow this assertion to be nested within a more complex return type.

Playground link

📃 Motivating Example

function validate(data: IData): { error?: string, isValid: data is ValidData } {
    if (!data.name) {
        return { error: "Name is required", isValid: false }
    } else {
        return { isValid: true }
    }
}

💻 Use Cases

Currently, we have to either have to assert properties when we use them (data.name!) or we manually type-cast the object within the check:

if (isValid) {
  const validData = someData as IValidData
}

[MartinJohns]

Related: #46650