getCjsExportFromNamespace messes up default exports

[Veetaha]

• Rollup Plugin Name: @rollup/plugin-commonjs
• Rollup Plugin Version: 13.0.0
• Rollup Version: 2.18.1
• Operating System (or Browser): Vscode extension host (I guess it is bare nodejs process)
• Node Version: 12.8.1

How Do We Reproduce?

Link to REPL

Use the following rollup.config.js:

import resolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import nodeBuiltins from 'builtin-modules';

export default {
    input: 'out/src/main.js',
    plugins: [
        resolve({
            preferBuiltins: true
        }),
        commonjs()
    ],
    external: [...nodeBuiltins, 'vscode'],
    output: {
        file: './out/src/main.js',
        format: 'cjs'
    }
};

Add "node-fetch": "^2.6.0" to package.json
Write the following code (in reality it is generated by tsc as the first step before invoking rollup):

const fetch = require("node-fetch");
console.log(typeof fetch.default);

When run in node REPL this works fine:

Output

$ node
Welcome to Node.js v14.4.0.
Type ".help" for more information.
> const fetch = require("node-fetch");
undefined
> console.log(typeof fetch.default)
function

Expected Behavior

When packaged with rollup it must work the same as in node REPL (it must not crash in our production!)

Actual Behavior

It outputs undefined.
The reason is that the bundle contains the following:

function getCjsExportFromNamespace (n) {
	return n && n['default'] || n;  // <--- this is the culprit
}

var lib = /*#__PURE__*/Object.freeze({
	__proto__: null,
	'default': fetch,
	Headers: Headers,
	Request: Request,
	Response: Response,
	FetchError: FetchError
});

var fetch = getCjsExportFromNamespace(lib);

console.log(typeof fetch.default);

cc @lukastaegert
related: rust-lang/rust-analyzer#5257 (comment)

[lukastaegert]

Looks like this one should be added to #481

[shellscape]

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Please add a reproduction and we'll be happy to triage further.

[Veetaha]

@shellscape here is the reproduction: https://repl.it/join/vhpbczoh-veetaha
try running
node ./output/bundle.js and node ./input.js to see the difference