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 propone di trattare T[keyof T] come covariante durante la misurazione della varianza e osserva che i mapped types non vengono sempre riconosciuti come covarianti. Poiché non sono identificati file sorgente, test o punti di ingresso del compilatore, inizia individuando la misurazione della varianza e la gestione dei mapped types nel TypeScript compiler. Il lavoro è completato quando il comportamento di covarianza previsto è specificato e coperto da test appropriati di verifica dei tipi.