berry: [Bug] yarn add --peer after --dev doesn't add peer dependency

• I’d be willing to implement a fix

Describe the bug

I have a library that depends on rxjs as a peer dependency, but it also has tests inside it.

If I add rxjs as a normal dependency, tests run properly. If instead I run:

yarn add --peer --dev rxjs

My tests will complain about: Cannot find module 'rxjs' from '...'

Note that I’m also using typescript and ts-jest.

Ideally, the dependency should be available to both peer and dev dependencies.

To Reproduce

const {promises: {readFile}} = require(`fs`);

await packageJsonAndInstall({
  devDependencies: {},
});

await yarn(`add`, `--dev`, `rxjs`);
await yarn(`add`, `--peer`, `rxjs`);

const pkgJson = JSON.parse(await readFile(`package.json`, `utf8`));
expect(pkgJson).toHaveProperty(`peerDependencies`);
expect(pkgJson).toHaveProperty(`devDependencies`);
expect(pkgJson.peerDependencies).toHaveProperty(`rxjs`);
expect(pkgJson.devDependencies).toHaveProperty(`rxjs`);

Additional context

This is also a bug in yarn v1: https://github.com/yarnpkg/yarn/issues/5287