Tuple iteration and merging

Edit: Remove indexing part
Edit 2: Replace tuple mapping syntax with normal mapped types

Search Terms

Concatentate and merge tuples' entries.

Suggestion

1. I would like to be able to extract and spread in positions other than the last part of a tuple.
2. I would like to merge a tuple into an intersection or concatenation of its members.

Use Cases

• Converting a tuple to its intersection, such as with Object.assign's assignee.
• Typing Array.prototype.concat correctly for tuples.

Syntax

The syntax comes in a few new forms:

• [...A, B] appends B to the tuple A. Similarly, [...infer A, any] extends B ? A : [] drops the last item of the tuple and [...any[], infer A] extends B ? A : never extracts the first.
• {... ...T} for an n-ary merge and [... ...T] an n-ary concatenation.

Examples

Here's the types for each method I mentioned above.

interface Array<T> {
    concat<U extends any[]>(...others: U): [...this, ... ...{[I in keyof U]: (
        U[I] extends any[] ? N : U[I] extends ArrayLike<infer R> ? R[] : [U[I]]
    )}];
}

interface ObjectConstructor {
    assign(target: {... ...typeof sources}, ...sources: object[]): typeof target;
}

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. new expression-level syntax)

この issue では、TypeScript の型システムにおけるタプルのスプレッド、タプルの連結、およびタプルから交差型へのマージが提案されており、Array.concat と Object.assign の例が示されています。実装ファイル、テスト、エントリポイントは指定されていません。まず関連する型システムの領域を特定し、その後、提案されたタプルの動作と例が機能することを示すテストを用意してください。