[BUG] Peer dependency warnings are never seen when using legacy-peer-deps

[sscaff1]

Is there an existing issue for this?

• I have searched the existing issues

This issue exists in the latest npm version

• I am using the latest npm

Current Behavior

When using legacy peer deps, npm no longer throws a warning for uninstalled peer dependencies.

Notice no warning for eslint-config-airbnb's peers.

Expected Behavior

You should see a warning. Notice that yarn throws a warning:

Steps To Reproduce

1. Go to codesandbox
2. Create a new node project
3. yarn add eslint-config-airbnb - note you get warnings
4. rm -rf node_modules
5. npm i eslint-config-airbnb --legacy-peer-deps=true - note you get no warnings
6. Look in the package-lock.json and note that none of the peers have been installed

Environment

• npm: latest
• Node.js: LTS
• OS Name: N/A
• System Model Name: N/A
• npm config: N/A

[ljharb]

Good catch. You should also, ofc, not use legacy peer deps :-)

[darcyclarke]

+1 @ljharb. --legacy-peer-deps was meant as a stopgap solution for the ecosystem to catch up/fix issues. I think it's time to rip off the bandaid. I imagine the npm CLI team should be queuing up deprecating the flag for v10 🙏🏻 I know that pnpm will be installing peer deps by default in v8 (ref. https://twitter.com/ZoltanKochan/status/1630956825012064256) so I think it's time to drop any config that avoids/hides these conflicts.

[valiant-code]

If you are discussing deprecating the flag then we should definitely fix this to throw warnings so that people can use those warnings to help inform them on how to resolve the conflicts and be aware how many issues are potentially building up by using the legacy flag.

[Susccy]

This just cost me hours of angry debugging because I didn't understand why my peer deps weren't installing and not even showing any warnings in the console. I eventually found that legacy-peer-deps has been silently enabled for months in the .npmrc of our project and everyone forgot about it because npm never tells you anywhere about it being enabled.

However, even with knowing that option was enabled I still didn't understand why npm install and npm ci wouldn't even show any warnings about missing peer deps. It just succeeded the install like everything was perfectly fine. But of course the code wouldn't run due to missing deps. So frustrating!