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

Suggestion

See title. Will fill in more details later.

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✅ 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

L’issue propose de traiter T[keyof T] comme covariant lors de la mesure de la variance et note que les mapped types ne sont pas toujours reconnus comme covariants. Comme aucun fichier source, test ou point d’entrée du compilateur n’est identifié, commencez par localiser la mesure de la variance et la gestion des mapped types dans le TypeScript compiler. Le travail est considéré comme terminé lorsque le comportement de covariance attendu est spécifié et couvert par des tests appropriés de vérification des types.