Suggestion

interface Type {
  isA?: true;
  isB?: false;
}
class A implements Type {
  isA: true = true;
  a = 1;
}

let a: A;

a.isA
a.isB

🔍 Search Terms

List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily and help provide feedback.

class implements interface completions

✅ 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

💻 Use Cases

use case

interface Type {
  isA?: false;
  isB?: false;
}
type PickType<T extends keyof Type> = {
  [K in keyof Type]?: K extends T ? true : false | undefined;
};
class A implements PickType<'isA'> {
  isA: true = true;
  a = 1;
}
class B implements PickType<'isB'> {
  isB: true = true;
  b = 1;
}
type AorB = A | B;
let a: AorB | undefined;
if (a) {
  if (a.isA) a.a; // error Property 'isA' does not exist on type 'B'
  if (a.isB) a.b; // error Property 'isB' does not exist on type 'A'
}

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