I'm wondering how it's possible that I didn't notice it while testing yarn 2 on this repository 🤔

It's only required in the typescript types.

Oh right, I forgot about it. Related: #10315, #9924.

I don't think that we should add a whole dependency only used for TS types. I think that we have 3 options:

• Add @babel/types in optionalPeerDependencies. Users using TS can then add @babel/types to their dependencies.
• Publish a package only with the type definitions
• Copy the .d.ts file from @babel/types to @babel/parser when publishing.

cc @babel/babel

Is there an optionalPeerDependencies? I think you'd need to add it in peerDeps, and then mark it optional in peerDepsMeta (which would make it required for older npm users).

Is there an optionalPeerDependencies?

It was in my search history and I assumed that it existed 😅

which would make it required for older npm users

We are dropping Node.js <10.13 in Babel 8, and v10.13.0 has npm v6.12.0 which supports peerDepsMeta. However, I think that this is sub-optimal and I would prefer my third suggestion.