[v15.0.0-beta.2] Normalization of rules fails

[loilo]

Version

15.0.0-beta.2

Reproduction link

https://github.com/Loilo/repro-vue-loader-normalize

Steps to reproduce

Clone the repository and install npm dependencies:

git clone https://github.com/Loilo/repro-vue-loader-normalize.git
cd repro-vue-loader-normalize
npm install

Run webpack:

npx webpack

What is expected?

Webpack should compile.

What is actually happening?

Webpack fails with a very terse Cannot set property 'undefined-0' of undefined and no stack trace.

---

I've dug into the issue and can make some educated guesses about the details:

• The normalization of a rule fails.
  It seems to have to do with setting the ident in particular.

  A stack trace is available if the normalization step is invoked manually (node standalone.js in the reproduction repo):

  require('webpack/lib/RuleSet').normalizeRule({
    test: /\.vue$/,
    use: [{
      loader: 'vue-loader',
      options: {}
    }]
  })

• The error occurs if there's an options property on an object inside a use array:

  {
    test: /\.vue$/,
    use: [{
      loader: 'vue-loader',
      options: {}
    }]
  }

  As opposed to that, it does not occur if the loader and options properties are attached to the rule directly:

  {
    test: /\.vue$/,
    loader: 'vue-loader',
    options: {}
  }

• The problem happens on every loader, it doesn't need to be the vue-loader itself. (Although that's not unexpected since vue-loader iterates over all loaders.)

Ultimately, I'm not sure if this is an issue with vue-loader or rather with webpack itself.