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.

まず、リンクされている Playground の例と、関連する issue 20273、4529、38041 を確認し、提案されている namespace の export 形式と既存の回避策を理解してください。export { X }、export { X } from "..."、export * as NS from "..." を namespace 内で現在の診断なしにサポートできれば完了です。