Yarn doesn't install non-optional sub-dependencies when --ignore-optional is used and one or more other packages mark it as optional

[mnpenner]

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

Bug

If the current behavior is a bug, please provide the steps to reproduce.

Sorry, I tried to create an SSCCE but couldn't recreate this issue outside of my project.

First, here's my attempt. Create this package.json:

{
  "name": "yarntest",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "webpack": "^1.13.2"
  },
  "scripts": {
    "build": "webpack --progress"
  }
}

Run yarn.

It installs a ton of dependencies including one called graceful-fs. If you run yarn why graceful-fs it tells us:

This module exists because "webpack#enhanced-resolve" depends on it.

Which is great.

However, in my other project which also depends on webpack, graceful-fs is not installed!

However, when I run yarn why graceful-fs it tells me:

This module exists because "less" depends on it.

So... does that mean it thinks it installed graceful-fs? Because the directory node_modules/graceful-fs simply isn't there.

You will probably want to see the package.json for that project, so here it is.

I did manage to reproduce this error by copying those dependencies (and devDependencies) into my test project. I tried deleting node_modules and yarn.lock. Happens every time. graceful-fs is missing.

It might be some strange conflict between the less and webpack packages.

What is the current behavior?

Missing package(s).

What is the expected behavior?

All sub-dependencies are installed.

Please mention your node.js, yarn and operating system version.

Node: v6.3.1
Yarn: 0.16.0
OS: Win10 Anniversary

[mnpenner]

I verified that the issue is reproducible with just those two packages:

{
  "name": "yarntest",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "devDependencies": {
    "less": "^2.7.1",
    "webpack": "^1.12.14"
  },
  "scripts": {
    "build": "webpack --progress"
  }
}

Oh... and I think I know that the problem is.

less lists graceful-fs as an "optional" dependency but I've been using --ignore-optional due to bug #1217.

The problem is that graceful-fs is not optional for webpack, so it should have got installed anyway (and it does if I omit less).

[bestander]

Merging into #2666