What should I do about "warning ... unmet peer dependency" when using yarn? - yarnpkg

Question

I used yarn to install almost 1000 js libraries. When it's done I run yarn install --check-files and it warns me that it left out just a few. I find this bizarre that it installs almost all and only warns about not installing a small handful (but I'll read up on why yarn's doing that later), for now, I need to figure out what to do about these unmet peer dependencies?
Example output
yarn install --check-files
yarn install v1.22.19
[1/4] 🔍 Resolving packages...
[2/4] 🚚 Fetching packages...
[3/4] 🔗 Linking dependencies...
warning " > babel-loader#8.3.0" has unmet peer dependency "#babel/core#^7.0.0".
warning " > babel-plugin-polyfill-corejs2#0.3.3" has unmet peer dependency "#babel/core#^7.0.0-0".
warning " > babel-plugin-polyfill-corejs3#0.6.0" has unmet peer dependency "#babel/core#^7.0.0-0".
warning " > babel-plugin-polyfill-regenerator#0.4.1" has unmet peer dependency "#babel/core#^7.0.0-0".
warning " > bootstrap#4.6.2" has unmet peer dependency "popper.js#^1.16.1".
[4/4] 🔨 Building fresh packages...
✨ Done in 1.52s.
My guess is simply install them with
yarn add #popper.js#^1.16.1
yarn add #babel/core#^7.0.0-0
I'm not totally sure though and want to get it right (also not sure if the first # and ^ should be there).
Note
Although I already mentioned it above and link to it, I reiterate, that this question differs from What does 'has unmet peer dependency' mean when installing a package with yarn? in that it asked about what a peer dependency is, whereas I (and 90 others) would like to know what must be done with yarn to ensure all (peer) dependencies are installed.

Answer 1

How to solve
Copy the name of the unmet peer dependency from the warning message, and add it using yarn like so:
yarn add '<unmet peer dependency>'
don't forget the ' and ' around the dependency, (and keep the # and ^ symbols if the unmet peer dependency has them).
Example
The error message in the question has
warning " > babel-plugin-polyfill-corejs2#0.3.3" has unmet peer dependency "#babel/core#^7.0.0-0".
so you'd run
yarn add '#babel/core#^7.0.0-0'
to add it.
Repeat this until all the warnings have gone when you run yarn install --check-files and the output just looks something like this (no warnings any more):
yarn install --check-files
yarn install v1.22.19
[1/4] 🔍 Resolving packages...
[2/4] 🚚 Fetching packages...
[3/4] 🔗 Linking dependencies...
[4/4] 🔨 Building fresh packages...
✨ Done in 1.54s.