Allow use of `infer` in the type parameter of a type declaration

Suggestion

For many type declarations, it shouldn't be necessary to use a conditional type which repeats a constraint already found in type parameter.

For example, instead of:

type ConstructorParameters<T extends abstract new (...args: any    ) => any> =
                           T extends abstract new (...args: infer P) => any ? P : never;

This would be better expressed as:

type ConstructorParameters<T extends abstract new (...args: infer P) => any> = P;

🔍 Search Terms

infer type parameter conditional type type declaration

✅ 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

See above

📃 Motivating Example

TypeScript's own built-in types have other examples where constraints are repeated in both a type parameter and conditional type. I've also seen cases where developers just don't want to repeat constraints, so they define:

type RecordValueType<T> =
    T extends Record<any, infer V> ? V : never;

And then incorrect usage results in a type of never, rather than an error.

💻 Use Cases

• Improved readability
• Less repetition when authoring type declarations
• Less frustration when consuming lazily-declared types

Inizia dagli esempi motivanti e dalla sintassi infer proposta nell’issue, quindi esamina come TypeScript gestisce attualmente i vincoli dei parametri di tipo e i tipi condizionali. Il lavoro è completato quando le dichiarazioni di tipo possono inferire un parametro di tipo direttamente dal relativo vincolo senza ripetere un tipo condizionale, mantenendo il comportamento indicato in termini di leggibilità e convalida.