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)

The issue proposes tuple spreading, tuple concatenation, and tuple-to-intersection merging in TypeScript's type system, with examples for Array.concat and Object.assign. No implementation files, tests, or entry points are named; determine the relevant type-system area first, then establish tests showing the proposed tuple behavior and examples work.