class completions include implemented interface's optional attribute

[deepkolos]

Suggestion

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

let a: A;

a.isA
a.isB

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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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[RyanCavanaugh]

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