yarn check reports errors after install succeeds

[haines]

Do you want to request a feature or report a bug?
Report a bug 🐛

What is the current behavior?
yarn check reports errors after a fresh and seemingly correct install.

If the current behavior is a bug, please provide the steps to reproduce.
Distilled from webpack/webpack#5262:

$ yarn init --yes
$ yarn add [email protected] [email protected]
$ yarn check
yarn check v0.27.5
error "ajv" is wrong version: expected "4.11.8", got "5.2.2"
error "webpack#ajv" not installed
warning "chokidar#fsevents#node-pre-gyp@^0.6.36" could be deduped from "0.6.36" to "[email protected]"
error Found 2 errors.

What is the expected behavior?
yarn check does not report errors.

It appears the problem is that [email protected] depends on ajv@^5.1.5 and transitively on ajv@^4.9.1, while [email protected] transitively depends on ajv@^4.7.0.

$ yarn why ajv
info Has been hoisted to "ajv"
info Reasons this module exists
   - "eslint#table" depends on it
   - "webpack#watchpack#chokidar#fsevents#node-pre-gyp#request#har-validator" depends on it

The install appears correct, though; yarn.lock contains two resolved versions for ajv:

ajv@^4.7.0, ajv@^4.9.1:
  version "4.11.8"

ajv@^5.1.5:
  version "5.2.2"

[email protected] is installed in node_modules:

$ jq .version node_modules/ajv/package.json
"5.2.2"

[email protected] is installed in node_modules/har-validator/node_modules and node_modules/table/node_modules:

$ jq .version node_modules/har-validator/node_modules/ajv/package.json
"4.11.8"
$ jq .version node_modules/table/node_modules/ajv/package.json
"4.11.8"

The transitive aspect appears to be key,

$ yarn add [email protected] [email protected]

does not cause this error; interestingly, it installs [email protected] to node_modules and [email protected] to node_modules/webpack/node_modules.

Please mention your node.js, yarn and operating system version.
Node 6.10.3
Yarn 0.27.5
Mac OS X 10.12.5

[CrabDude]

Recommend closing this in favor of #3933, which contains a generalized reproduction (not specific to webpack) and a lengthy explanation of the rationale for treating peerDependencies as npm does.

[haines]

Closing in favour of #3933