Treat `T[keyof T]` as covariant (rather than invariant) during variance measurement

Suggestion

See title. Will fill in more details later.

🔍 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.

✅ 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

A lot of mapped types are covariant by construction, but we're not always able to determine that

📃 Motivating Example

💻 Use Cases

See incoming linked issues

この issue では、分散性の測定時に T[keyof T] を共変として扱うことを提案しており、mapped types が常に共変として認識されるわけではないことを指摘しています。ソースファイル、テスト、コンパイラのエントリポイントは特定されていないため、まず TypeScript compiler における分散性の測定と mapped types の処理箇所を見つけてください。意図した共変性の動作が仕様化され、適切な型チェックテストでカバーされていれば完了です。