Do you want to request a feature or report a bug?
bug

What is the current behavior?
If a module polyfills an API, e.g. fetch, the module code might check if window.fetch is already defined:

if (typeof fetch === 'undefined') {
  module.exports = // fetch polyfill
} else {
  module.exports = fetch;
}

This module can then be included by using e.g. const fetch = require('fetch-polyfill'); or, ideally, via ProvidePlugin:

module.exports = {
  ...
  plugins: [
    ...
    new webpack.ProvidePlugin({
      fetch: 'fetch-polyfill'
    })
  ]
}

However, this doesn't work - since fetch is used in the module itself (if (typeof fetch...), this "confuses" webpack and fetch becomes an empty object ({}). Importing the module as anything other than "fetch" works fine, e.g.

module.exports = {
  ...
  plugins: [
    ...
    new webpack.ProvidePlugin({
      myfetch: 'fetch-polyfill'
    })
  ]
}

...but this defeats the purpose of a polyfill.

webextension-polyfill relies on this "polyfill guard", which if how I found out about the issue.

If the current behavior is a bug, please provide the steps to reproduce.
https://gist.github.com/fstanis/450776f48682e6ff85b7c7ba366a5d1e

What is the expected behavior?
ProvidePlugin matching the behaviour of const <name> = require('<module>');