Vue-cli: --modern flag fails when chaining "copy" plugin

[scottbedard]

Version

3.0.3

Reproduction link

https://github.com/scottbedard/vue-cli-bug

Node and OS info

Node 8.9.4, Yarn 1.6.0, Windows 10

Steps to reproduce

Run yarn build:modern

What is expected?

The build should complete with no errors, and static assets should be copied to ./wwwroot

What is actually happening?

The build fails with the following output

$ vue-cli-service build --modern

-  Building legacy bundle for production... ERROR  TypeError: Cannot read property '__expression' of undefined
TypeError: Cannot read property '__expression' of undefined
    at Object.toConfig (C:\git\vue-cli-bug\node_modules\webpack-chain\src\Plugin.js:55:38)
    at clean.Object.assign.plugins.plugins.values.map.plugin (C:\git\vue-cli-bug\node_modules\webpack-chain\src\Config.js:129:61)
    at Array.map (<anonymous>)
    at module.exports.toConfig (C:\git\vue-cli-bug\node_modules\webpack-chain\src\Config.js:129:40)
    at Service.resolveWebpackConfig (C:\git\vue-cli-bug\node_modules\@vue\cli-service\lib\Service.js:218:34)
    at PluginAPI.resolveWebpackConfig (C:\git\vue-cli-bug\node_modules\@vue\cli-service\lib\PluginAPI.js:115:25)
    at module.exports (C:\git\vue-cli-bug\node_modules\@vue\cli-service\lib\commands\build\resolveAppConfig.js:34:25)
    at build (C:\git\vue-cli-bug\node_modules\@vue\cli-service\lib\commands\build\index.js:124:50)
    at api.registerCommand (C:\git\vue-cli-bug\node_modules\@vue\cli-service\lib\commands\build\index.js:50:13)
    at Service.run (C:\git\vue-cli-bug\node_modules\@vue\cli-service\lib\Service.js:203:12)
    at Object.<anonymous> (C:\git\vue-cli-bug\node_modules\@vue\cli-service\bin\vue-cli-service.js:36:9)
    at Module._compile (module.js:643:30)
    at Object.Module._extensions..js (module.js:654:10)
    at Module.load (module.js:556:32)
    at tryModuleLoad (module.js:499:12)
    at Function.Module._load (module.js:491:3)
error Command failed with exit code 1.

---

Building works as expected when the --modern flag is omitted, or when config.plugin('copy') is not called. Configuring other plugins also seems to work as expected.

[GoodeUser]

Also failing on OSX Version 10.13.5

[LinusBorg]

in modern mode, the config runs twice, once for legacy, once for modern. We add the copy plugin only to one of those, so that files are only copied once.

solution:

config.plugins.has('copy') && config.plugin('copy').tap(function() {