core-js's use by babel not resolving in workspace modules

[daveisfera]

• I'd be willing to implement a fix

Describe the bug

I'm trying to use yarn 2 to build an application with webpack and I get this error when building with both webpack 4 and pnp-webpack-plugin and webpack 5:

(node:415) UnhandledPromiseRejectionWarning: Error: Error on webpack build: ../lib/core/lib/nudash.js
Module not found: Error: Something that got detected as your top-level application (because it doesn't seem to belong to any package) tried to access a package that is not declared in your dependencies

Required package: core-js (via "core-js/modules/web.dom-collections.iterator")
Required by: /usr/src/app/lib/core/lib/nudash.js

It appears that the use of core-js that's being done by babel isn't resolving in the workspace modules.

To Reproduce

Use a module in a workspace that is processed by babel where core-js is used

Environment if relevant (please complete the following information):

• OS: Alpine 3.11
• Node version: 12.14.1
• Yarn version: 2.0.0-rc.28

[arcanis]

As @sokra rightly explained Babel injects core-js in your built files, but you still need to add it explicitly to your owns. Otherwise you would rely on hoisting, which is unsound (there's no telling which version of core-js you'd get depending on the packages that are part of the dependency tree).

[daveisfera]

Then why does this work with standard modules? And why can't modules in a workspace work the same way?

[arcanis]

Because hoisting made it accidentally work. Yarn 2 makes things stricter and let you know when you're using unsound pattern that will eventually cause problems in your deployment - instead of just letting you think that things are alright until the crash.

[daveisfera]

Sorry, I understand the difference between yarn 1 and 2 and what's going on there (I actually really like the explicit resolution that's happening with yarn 2).

The injection of core-js by babel into a "standard module" that's listed in package.json and pulled from npm appears to work just fine, so my question is why doesn't it behave the same way and work just like a "standard module" with my "workspace module" that is also listed in package.json?