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

Bug.

What is the current behavior?

By way of example, I have:

• util.js: ES exports two methods: red and blue via two scenarios: one-file (all code within one file) and re-export (methods re-exported from external files)
• app1.js: Imports and uses only red
• app2.js: Imports and uses only blue

Expected: If I build webpack bundles for both apps, app1 will have red and blue will be eliminated. Conversely, app2 will have blue and red will be eliminated:

Actual: Depends on config

1. Array of Configs: If there is an array of configurations with a single entry point each for app1.js and app2.js the correct unused methods are dropped.
2. Multiple Entry Points: If there is a single configuration object with multiple entry points for both app1.js and app2.js, then neither blue nor red are eliminated in either bundle. I would consider this a bug. Perhaps there is some known thing for "multiple entry points get any esnext exports used by any other entry point", but that seems weird.

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

We made a repository with passing and failing mocha tests to show the situation: https://github.com/FormidableLabs/webpack-tree-shaking-multiple-entry-points.

Reproduction:

$ git clone https://github.com/FormidableLabs/webpack-tree-shaking-multiple-entry-points.git
$ cd webpack-tree-shaking-multiple-entry-points
$ yarn install
$ yarn run build
$ yarn run test

The test output is:

  tree shaking in webpack4
    array
      one-file
        ✓ app1 should have red, not blue
        ✓ app2 should have blue, not red
      re-export
        ✓ app1 should have red, not blue
        ✓ app2 should have blue, not red
    multiple-entries
      one-file
        1) app1 should have red, not blue
        2) app2 should have blue, not red
      re-export
        3) app1 should have red, not blue
        4) app2 should have blue, not red


  4 passing (22ms)
  4 failing

The failing ones show that the ES methods that should be dropped aren't.

What is the expected behavior?

See above section on current behavior.

If this is a feature request, what is motivation or use case for changing the behavior?

Nope, it's a bug.

Please mention other relevant information such as the browser version, Node.js version, webpack version and Operating System.

$ node --version
v8.4.0

$  $ yarn list webpack
yarn list v1.3.2
└─ webpack@4.0.0-beta.1

$ uname -a
Darwin small.lan 16.7.0 Darwin Kernel Version 16.7.0: Thu Jan 11 22:59:40 PST 2018; root:xnu-3789.73.8~1/RELEASE_X86_64 x86_64