Declaring Symbol variable conflicts with namespaceToStringTag

[yyx990803]

• Rollup Version: 2.39.0
• Operating System (or Browser): MacOS
• Node Version (if applicable):
• Link to reproduction (IMPORTANT, read below): https://github.com/yyx990803/rollup-tostringtag-symbol-conflict

The bug occurs when combining the following:

1. -f cjs
2. --namespaceToStringTag
3. Code: var Symbol = ...

Expected Behavior

Generated code should work.

Actual Behavior

The generated code:

'use strict';

Object.defineProperty(exports, '__esModule', { value: true });
exports[Symbol.toStringTag] = 'Module';

var Symbol = 1;

exports.Symbol = Symbol;

Due to the var hoisting behavior, Symbol is undefined then exports[Symbol.toStringTag] = 'Module'; is executed.

This my seem like a very niche case but the actual code was found in lodash-es. i.e. if the user tries to bundle lodash-es with the above options they will run into the same error.

Possible Fixes

Make Symbol a reserved variable name (i.e. always consider it already declared) so that var Symbol in user code is rewritten to var Symbol$1 or something similar.

[lukastaegert]

I see, that is not good. Will need to check if this conflicts in any way with how Polyfills handle Symbol.

[lukastaegert]

Of course there are no issues with Polyfills as they all appear to first identify the global object and operate on that. Fix at #3971. This also avoids the same problem for Object and Promise, which Rollup also relies on for various things.