Suggestion: Allow ES6 export syntax in namespaces

TypeScript Version: typescript@4.0.0-dev.20200801

Search Terms: namespace, import, export, re-export, module

Code

import { Point } from "./Point";

export type Line = {
  readonly p1: Point;
  readonly p2: Point;
  readonly points?: readonly [Point, ...(readonly Point[])];
};

export namespace Line {
  export { Point } from "~/Point"; // TS says "Export declarations are not permitted in a namespace"
  // `export { Point };` <-- Should also work.
}

type LinePoint = Line.Point;
// ^ Somehow this actually works, i.e. `LinePoint` equals `Point`

Expected behavior:
Exporting from a namespace should mirror ES6 module exports, i.e. export { X }, export { X } from "...", and export * as NS from "..."

The lack of syntax for re-exports makes it very hard to use namespace/type/function merging abilities to represent nice APIs. Here's an example of what a module could look like if this was allowed...

// User.tsx

import { Admin } from "./Admin";
import { Member } from "./Member";

export type User = Admin | Member;
export type Props = { readonly user: User; };

export function User({ user }: Props) {
  return Admin.isAdmin(user) ? <Admin admin={user} /> : <Member member={user} />;
}

export namespace User {
  export { Admin, Member }; // Nice.
  export const isAuthenticated = (user: User): boolean => true;
}

// Home.tsx

import { User } from "./User";

export function Home({ user }: User.Props) {
  return <User.Admin admin={useAdmin()} />; // An example of what this enables.
}

Actual behavior:
TypeScript says "Export declarations are not permitted in a namespace," even though it actually respects the export declaration outside of the namespace.

Playground Link:
https://www.typescriptlang.org/play/#code/HYQwtgpgzgDiDGEAEBBJBvAUEnSIA8YB7AJwBcl4jgoKQkBeJAIhGaRCkutoG5MAvpkyhIsBMgBCGbLgLFyGVEgH8hmKjQr5GSSQDoU+kLyA

Related Issues:
https://github.com/microsoft/TypeScript/issues/20273
This issue showcases some of the syntax gymnastics needed to get around this issue.

https://github.com/microsoft/TypeScript/issues/4529
https://github.com/microsoft/TypeScript/issues/4529#issuecomment-140932811
More examples of verbose workarounds.

https://github.com/microsoft/TypeScript/issues/38041#issuecomment-616807133
@rbuckton Provides a nice example of what this could look like if enabled.

https://github.com/microsoft/TypeScript/issues/38041#issuecomment-662142857
My comment with another motivating example after the issue was closed.

Commencez par examiner l’exemple Playground lié ainsi que les issues associées 20273, 4529 et 38041 afin de comprendre les formes d’export de namespace proposées et les workarounds existants. La tâche serait considérée comme terminée si export { X }, export { X } from "..." et export * as NS from "..." sont pris en charge dans les namespaces sans le diagnostic actuel.