css file import dose not work when build production files

[BboyAwey]

Version

3.12.1

Reproduction link

https://github.com/BboyAwey/vue-cli-css-file-lose-while-building-production-files

Environment info

System:
    OS: macOS 10.15
    CPU: (8) x64 Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
  Binaries:
    Node: 8.12.0 - /usr/local/bin/node
    Yarn: 1.9.4 - /usr/local/bin/yarn
    npm: 5.1.0 - /usr/local/bin/npm
  Browsers:
    Chrome: 78.0.3904.97
    Firefox: 70.0.1
    Safari: 13.0.2
  npmPackages:
    @vue/babel-helper-vue-jsx-merge-props:  1.0.0
    @vue/babel-plugin-transform-vue-jsx:  1.1.2
    @vue/babel-preset-app:  3.12.1
    @vue/babel-preset-jsx:  1.1.2
    @vue/babel-sugar-functional-vue:  1.1.2
    @vue/babel-sugar-inject-h:  1.1.2
    @vue/babel-sugar-v-model:  1.1.2
    @vue/babel-sugar-v-on:  1.1.2
    @vue/cli-overlay:  3.12.1
    @vue/cli-plugin-babel: ^3.12.0 => 3.12.1
    @vue/cli-plugin-eslint: ^3.12.0 => 3.12.1
    @vue/cli-plugin-unit-jest: ^3.12.0 => 3.12.1
    @vue/cli-service: ^3.12.0 => 3.12.1
    @vue/cli-shared-utils:  3.12.1
    @vue/component-compiler-utils:  3.0.2
    @vue/eslint-config-standard: ^4.0.0 => 4.0.0
    @vue/preload-webpack-plugin:  1.1.1
    @vue/test-utils: 1.0.0-beta.29 => 1.0.0-beta.29
    @vue/web-component-wrapper:  1.2.0
    eslint-plugin-vue: ^5.0.0 => 5.2.3 (4.7.1)
    jest-serializer-vue:  2.0.2
    vue: ^2.6.10 => 2.6.10
    vue-eslint-parser:  5.0.0 (2.0.3)
    vue-hot-reload-api:  2.3.4
    vue-jest:  3.0.5
    vue-loader:  15.7.2
    vue-router: ^3.0.3 => 3.1.3
    vue-style-loader:  4.1.2
    vue-template-compiler: ^2.6.10 => 2.6.10
    vue-template-es2015-compiler:  1.9.1
    vuex: ^3.0.1 => 3.1.2
  npmGlobalPackages:
    @vue/cli: Not Found

Steps to reproduce

1.run yarn to install all deps
2.run yarn serve to start dev server
I imported a css file from admin-ui/dist/style.css in main.js, and It works fine. Then we build:
3.run yarn build to build production files
and check the dist dir, the code from that css file is missing!!
Then I create another directory in node_modules and put a css file in it and import it in main.js, then build again, it works fine!!
Magic!!!

What is expected?

The code from css file admin-ui/dist/style.css should work and show themselves in production files.

What is actually happening?

The code from css file admin-ui/dist/style.css works fine when in dev mode but disappear in production files

---

What a magic moment!!

[haoqunjiang]

https://vue-loader.vuejs.org/guide/#manual-setup
Scroll to the bottom.

If you are developing a library or in a monorepo, please be aware that CSS imports are side effects. Make sure to remove "sideEffects": false in the package.json, otherwise CSS chunks will be dropped by webpack in production builds.

[haoqunjiang]

I know it's not the best place to put such a warning… Maybe we need to put it in the Vue CLI documentation.

[BboyAwey]

Oh that's a little bit awkward. I Appreciate your support,Thanks a lot!