Refine type checking for class accessor when using decorator

Suggestion

🔍 Search Terms

decorator, class accessor, auto-accessor, types, null checks.

✅ 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

If a class accessor decorator will always add an initial value for the decorated accessor, TypeScript should infer that the accessor is always non-null, even if no initial value in its original declaration.

📃 Motivating Example

See playground.

The decorator dec will always initialize the accessor with a non-null value, but TS reports an error on @dec accessor data: Wrapper<string>;. It says "Property 'data' has no initializer and is not definitely assigned in the constructor" when strictNullChecks is enabled.

function dec<This, Value>(
    target: ClassAccessorDecoratorTarget<This, Wrapper<Value>>,
    context: ClassAccessorDecoratorContext<This, Wrapper<Value>>,
): ClassAccessorDecoratorResult<This, Wrapper<Value>> {
    return {
        init(initialValue) {
            if (initialValue !== undefined) {
                return initialValue;
            }
            return {};
        }
    }
}

interface Wrapper<T = unknown> {
    value?: T;
}

class Test {
    @dec accessor data: Wrapper<string>;
}

Are there any workarounds, other than putting an ! after data, to make the type checking works?

💻 Use Cases

Start with the linked TypeScript Playground example and reproduce the diagnostic under strictNullChecks for the decorated accessor. Read the decorator and class-accessor behavior described in the issue, then determine the type-checking change needed so the accessor is treated as initialized; done means the example no longer requires a non-null assertion while preserving existing runtime behavior.