Bug Report

• I would like to work on a fix!

Current behavior

TypeError: Cannot redefine property: c

Namespace reexports are transpiled like this:

for(key ...){
  Object.defineProperty(exports, key, {
    enumerable: true,
    get: function () {
      return imported[key];
    }
  });
}

But the spec allows for reexporting a symbol twice in the same file if it is actually the same binding (see example code below).

This means that Object.defineProperty(exports, "c", ...) is called twice, causing the error.

One of the packages that do this is @fluentui/react (this came up in parcel-bundler/parcel#4399)

Input Code

• https://github.com/mischnic/parcel-issue-4399 (lib/ already contains the Babel output of src-valid)

export * from "./a.js";
export * from "./b.js";

// ---- everything below is just for context ---

// a.js
export { c } from './c.js'; // <-- 
export const a = "a";
// b.js
export { c } from './c.js'; // <-- 
export const b = "b";

// c.js
export function c() {
	return false;
}

Expected behavior

The first file should export a, b, c (and not throw).

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

babel --plugins @babel/plugin-transform-modules-commonjs --delete-dir-on-start src-valid -d lib

Environment

  System:
    OS: macOS 10.15.4
  Binaries:
    Node: 14.4.0 - /usr/local/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.4 - /usr/local/bin/npm
  npmPackages:
    @babel/cli: ^7.10.1 => 7.10.1
    @babel/core: ^7.10.2 => 7.10.2
    @babel/plugin-transform-modules-commonjs: ^7.10.1 => 7.10.1

Possible Solution
Not use Object.defineProperty? configurable: true ? But all of these have other sideffects

Additional context

Technically, f there are conflicting reexports (see src-invalid), there should be a SyntaxError (!).

But even Rollup doesn't handle this correctly (and Babel cannot work across multiple files anyway). rollup/rollup#3629