fix: infer rootOptions for late invoked generators

close #1820

packages/@vue/cli/lib/Generator.js

@@ -3,6 +3,7 @@ const debug = require('debug')
 const GeneratorAPI = require('./GeneratorAPI')
 const sortObject = require('./util/sortObject')
 const writeFileTree = require('./util/writeFileTree')
+const inferRootOptions = require('./util/inferRootOptions')
 const normalizeFilePaths = require('./util/normalizeFilePaths')
 const injectImportsAndOptions = require('./util/injectImportsAndOptions')
 const { toShortPluginId, matchesPluginId } = require('@vue/cli-shared-utils')
@@ -86,10 +87,12 @@ module.exports = class Generator {
     this.exitLogs = []
 
     const cliService = plugins.find(p => p.id === '@vue/cli-service')
-    const rootOptions = cliService && cliService.options
+    const rootOptions = cliService
+      ? cliService.options
+      : inferRootOptions(pkg)
     // apply generators from plugins
     plugins.forEach(({ id, apply, options }) => {
-      const api = new GeneratorAPI(id, this, options, rootOptions || {})
+      const api = new GeneratorAPI(id, this, options, rootOptions)
       apply(api, options, rootOptions)
     })
   }

packages/@vue/cli/lib/util/inferRootOptions.js

@@ -0,0 +1,31 @@
+// Infer rootOptions for individual generators being invoked
+// in an existing project.
+
+module.exports = function inferRootOptions (pkg) {
+  const rootOptions = {}
+  const deps = Object.assign({}, pkg.dependencies, pkg.devDependencies)
+
+  // projectName
+  rootOptions.projectName = pkg.name
+
+  // router
+  if ('vue-router' in deps) {
+    rootOptions.router = true
+  }
+
+  // vuex
+  if ('vuex' in deps) {
+    rootOptions.vuex = true
+  }
+
+  // cssPreprocessors
+  if ('sass-loader' in deps) {
+    rootOptions.cssPreprocessor = 'sass'
+  } else if ('less-loader' in deps) {
+    rootOptions.cssPreprocessor = 'less'
+  } else if ('stylus-loader' in deps) {
+    rootOptions.cssPreprocessor = 'stylus'
+  }
+
+  return rootOptions
+}