fix(webpack): normalize devtool for terser compatibility (#6566)

packages/webpack/src/config/base.js

@@ -439,11 +439,16 @@ export default class WebpackBaseConfig {
     if (typeof extend === 'function') {
       const extendedConfig = extend.call(
         this.builder, config, { loaders: this.loaders, ...this.nuxtEnv }
-      )
-      // Only overwrite config when something is returned for backwards compatibility
-      if (extendedConfig !== undefined) {
-        return extendedConfig
+      ) || config
+
+      const pragma = /@|#/
+      const { devtool } = extendedConfig
+      if (typeof devtool === 'string' && pragma.test(devtool)) {
+        extendedConfig.devtool = devtool.replace(pragma, '')
+        consola.warn(`devtool has been normalized to ${extendedConfig.devtool} as webpack documented value`)
       }
+
+      return extendedConfig
     }
     return config
   }

test/fixtures/with-config/nuxt.config.js

@@ -86,7 +86,7 @@ export default {
     transpile: 'vue-test',
     extend (config, options) {
       return Object.assign({}, config, {
-        devtool: '#source-map'
+        devtool: 'source-map'
       })
     }
   },