[BUG] Locally-installed package should satisfy peer dependency

[btmills]

Current Behavior:

npm install in the eslint/eslint repository fails when it tries to resolve a transitive peer dependency on eslint@>=5.0.0 and finds eslint@undefined.

We want to dogfood eslint in development, so we specify it in our devDependencies as "eslint": "file:.". We also depend on a third-party plugin, eslint-plugin-eslint-plugin, which lists eslint@>=5.0.0 in its peerDependencies.

Running npm install reports ERESOLVE unable to resolve dependency tree having found eslint@undefined, which fails the eslint@>=5.0.0 requirement. (Full output and eresolve-report.txt included in the repro steps.)

eslint@7.13.0`s package.json snippet

```json
{
	"name": "eslint",
	"version": "7.13.0",
	"devDependencies": {
		"eslint": "file:.",
		"eslint-plugin-eslint-plugin": "^2.2.1"
	}
}
```

eslint-plugin-eslint-plugin@2.3.0`s package.json snippet

```json
{
	"name": "eslint-plugin-eslint-plugin",
	"version": "2.3.0",
	"peerDependencies": {
		"eslint": ">=5.0.0"
	}
}
```

Expected Behavior:

Ideally, instead of finding eslint@undefined, eslint-plugin-eslint-plugin's eslint@>=5.0.0 peer dependency is satisifed by eslint@7.13.0, which is already "installed" from file:.. Install continues on as normal. (It won't succeed yet because I'm still working on upgrading another dependency to a newer version that lists eslint@^7.0.0 as a compatible peer dependency, but eslint-plugin-eslint-plugin at least should be happy.)

If that's not an option, our current solution so that we can test Node 15 in CI is to install with the --legacy-peer-deps flag. I also found that replacing file: dependencies with workspaces installs successfuly. At some point in the future when we can require at least npm v7, we could switch from --legacy-peer-deps to workspaces.

Steps To Reproduce:

$ node --version
v15.2.1
$ npm --version
7.0.10
$ git clone https://github.com/eslint/eslint.git
$ cd eslint
# Latest version as of writing in case a future commit no longer repros
$ git checkout v7.13.0
$ npm install
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: eslint@7.13.0
npm ERR! Found: eslint@undefined
npm ERR! node_modules/eslint
npm ERR!   dev eslint@"file:." from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer eslint@">=5.0.0" from eslint-plugin-eslint-plugin@2.3.0
npm ERR! node_modules/eslint-plugin-eslint-plugin
npm ERR!   dev eslint-plugin-eslint-plugin@"^2.2.1" from the root project
npm ERR!
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
npm ERR!
npm ERR! See ~/.npm/eresolve-report.txt for a full report.

npm ERR! A complete log of this run can be found in:
npm ERR!     ~/.npm/_logs/2020-11-19T05_00_04_565Z-debug.log

~/.npm/eresolve-report.txt

# npm resolution error report

2020-11-19T05:00:04.563Z

While resolving: eslint@7.13.0
Found: eslint@undefined
node_modules/eslint
  dev eslint@"file:." from the root project

Could not resolve dependency:
peer eslint@">=5.0.0" from eslint-plugin-eslint-plugin@2.3.0
node_modules/eslint-plugin-eslint-plugin
  dev eslint-plugin-eslint-plugin@"^2.2.1" from the root project

Fix the upstream dependency conflict, or retry
this command with --force, or --legacy-peer-deps
to accept an incorrect (and potentially broken) dependency resolution.

Raw JSON explanation object:

{
  "code": "ERESOLVE",
  "current": {
    "name": "eslint",
    "errors": [
      {}
    ],
    "package": {},
    "whileInstalling": {
      "name": "eslint",
      "version": "7.13.0",
      "path": "~/code/eslint/eslint"
    },
    "location": "node_modules/eslint",
    "dependents": [
      {
        "type": "dev",
        "name": "eslint",
        "spec": "file:.",
        "from": {
          "location": "~/code/eslint/eslint"
        }
      }
    ]
  },
  "edge": {
    "type": "peer",
    "name": "eslint",
    "spec": ">=5.0.0",
    "error": "INVALID",
    "from": {
      "name": "eslint-plugin-eslint-plugin",
      "version": "2.3.0",
      "whileInstalling": {
        "name": "eslint",
        "version": "7.13.0",
        "path": "~/code/eslint/eslint"
      },
      "location": "node_modules/eslint-plugin-eslint-plugin",
      "dependents": [
        {
          "type": "dev",
          "name": "eslint-plugin-eslint-plugin",
          "spec": "^2.2.1",
          "from": {
            "location": "~/code/eslint/eslint"
          }
        }
      ]
    }
  },
  "peerConflict": null,
  "strictPeerDeps": false,
  "force": false
}

Environment:

• OS: macOS 10.15.7
• Node: 15.2.1
• npm: 7.0.10

[RomanHotsiy]

I get a similar warning on the latest npm version:

• npm --version
  7.3.0
• node --version
  v15.5.1

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR! 
npm ERR! Found: type-fest@0.8.1
npm ERR! node_modules/type-fest
npm ERR!   type-fest@"^0.8.1" from boxen@4.2.0
npm ERR!   node_modules/boxen
npm ERR!     boxen@"^4.1.0" from @storybook/core@5.3.21
npm ERR!     node_modules/@storybook/core
npm ERR!       @storybook/core@"5.3.21" from @storybook/react@5.3.21
npm ERR!       node_modules/@storybook/react
npm ERR!         dev @storybook/react@"^5.2.0" from the root project
npm ERR!     boxen@"^4.2.0" from gatsby-telemetry@1.8.0
npm ERR!     node_modules/gatsby-telemetry
npm ERR!       gatsby-telemetry@"^1.8.0" from gatsby@2.30.1
npm ERR!       node_modules/gatsby
npm ERR!         gatsby@"2.30.1" from the root project
npm ERR!         17 more (@redocly/gatsby-plugin-favicon, ...)
npm ERR!       2 more (gatsby-cli, gatsby-plugin-page-creator)
npm ERR!   type-fest@"^0.8.0" from hasha@5.2.2
npm ERR!   node_modules/hasha
npm ERR!     hasha@"^5.2.0" from gatsby@2.30.1
npm ERR!     node_modules/gatsby
npm ERR!       gatsby@"2.30.1" from the root project
npm ERR!       17 more (@redocly/gatsby-plugin-favicon, ...)
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peerOptional type-fest@"^0.13.1" from @pmmmwh/react-refresh-webpack-plugin@0.4.3
npm ERR! node_modules/@pmmmwh/react-refresh-webpack-plugin
npm ERR!   @pmmmwh/react-refresh-webpack-plugin@"^0.4.1" from gatsby@2.30.1
npm ERR!   node_modules/gatsby
npm ERR!     gatsby@"2.30.1" from the root project
npm ERR!     17 more (@redocly/gatsby-plugin-favicon, ...)
npm ERR! 
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.

[ljharb]

@RomanHotsiy try v7.4.0?

[RomanHotsiy]

@ljharb same.

[rhansen]

Shell script to reproduce this problem with the latest npm v7.x:

tmp=$(mktemp -d)
echo "tmp dir: $tmp"

mkdir "$tmp/npm"
cd "$tmp/npm"
npm i npm@^7.0.0
npm() { "$tmp"/npm/node_modules/.bin/npm "$@"; }

# Create a dummy ep_etherpad-lite package. Later, a real package
# will be installed that has a peer dependency on this package.
mkdir "$tmp/ep_etherpad-lite"
cd "$tmp/ep_etherpad-lite"
npm init -y
npm version major # Bump version to 2.0.0 (needed below).

# Create dummy main package that depends on the above
# ep_etherpad-lite package via 'file:'.
mkdir "$tmp/main"
cd "$tmp/main"
npm init -y
npm i ep_etherpad-lite@file:../ep_etherpad-lite

# Install ep_readonly_guest in main. ep_readonly_guest has a peer dependency on
# ep_etherpad-lite@>=1.8.7, which is why the version was bumped to 2.0.0 above.
# The installation of ep_readonly_guest fails because npm can't determine if the
# installed ep_etherpad-lite package satisfies ep_readonly_guests's peer
# dependency on ep_etherpad-lite.
cd "$tmp/main"
npm i ep_readonly_guest@1.0.28

rm -rf "$tmp"