fix(compat): handle and warn config.optionMergeStrategies

packages/runtime-core/src/compat/compatConfig.ts

@@ -26,6 +26,7 @@ export const enum DeprecationTypes {
   CONFIG_PRODUCTION_TIP = 'CONFIG_PRODUCTION_TIP',
   CONFIG_IGNORED_ELEMENTS = 'CONFIG_IGNORED_ELEMENTS',
   CONFIG_WHITESPACE = 'CONFIG_WHITESPACE',
+  CONFIG_OPTION_MERGE_STRATS = 'CONFIG_OPTION_MERGE_STRATS',
 
   INSTANCE_SET = 'INSTANCE_SET',
   INSTANCE_DELETE = 'INSTANCE_DELETE',
@@ -174,6 +175,12 @@ export const deprecationData: Record<DeprecationTypes, DeprecationData> = {
       `\`config.compilerOptions.whitespace\`.`
   },
 
+  [DeprecationTypes.CONFIG_OPTION_MERGE_STRATS]: {
+    message:
+      `config.optionMergeStrategies no longer exposes internal strategies. ` +
+      `Use custom merge functions instead.`
+  },
+
   [DeprecationTypes.INSTANCE_SET]: {
     message:
       `vm.$set() has been removed as it is no longer needed in Vue 3. ` +

packages/runtime-core/src/compat/global.ts

@@ -41,7 +41,8 @@ import { Directive } from '../directives'
 import { nextTick } from '../scheduler'
 import { version } from '..'
 import {
-  installLegacyConfigProperties,
+  installLegacyConfigWarnings,
+  installLegacyOptionMergeStrats,
   LegacyConfig,
   legacyOptionMergeStrats
 } from './globalConfig'
@@ -327,6 +328,7 @@ export function installAppCompatProperties(
   render: RootRenderFunction
 ) {
   installFilterMethod(app, context)
+  installLegacyOptionMergeStrats(app.config)
 
   if (!singletonApp) {
     // this is the call of creating the singleton itself so the rest is
@@ -337,7 +339,7 @@ export function installAppCompatProperties(
   installCompatMount(app, context, render)
   installLegacyAPIs(app)
   applySingletonAppMutations(app)
-  if (__DEV__) installLegacyConfigProperties(app.config)
+  if (__DEV__) installLegacyConfigWarnings(app.config)
 }
 
 function installFilterMethod(app: App, context: AppContext) {

packages/runtime-core/src/compat/globalConfig.ts

@@ -1,7 +1,11 @@
 import { extend, isArray } from '@vue/shared'
 import { AppConfig } from '../apiCreateApp'
 import { mergeDataOption } from './data'
-import { DeprecationTypes, warnDeprecation } from './compatConfig'
+import {
+  DeprecationTypes,
+  softAssertCompatEnabled,
+  warnDeprecation
+} from './compatConfig'
 import { isCopyingConfig } from './global'
 
 // legacy config warnings
@@ -33,7 +37,7 @@ export type LegacyConfig = {
 }
 
 // dev only
-export function installLegacyConfigProperties(config: AppConfig) {
+export function installLegacyConfigWarnings(config: AppConfig) {
   const legacyConfigOptions: Record<string, DeprecationTypes> = {
     silent: DeprecationTypes.CONFIG_SILENT,
     devtools: DeprecationTypes.CONFIG_DEVTOOLS,
@@ -57,11 +61,27 @@ export function installLegacyConfigProperties(config: AppConfig) {
       }
     })
   })
+}
 
-  // Internal merge strats which are no longer needed in v3, but we need to
-  // expose them because some v2 plugins will reuse these internal strats to
-  // merge their custom options.
-  extend(config.optionMergeStrategies, legacyOptionMergeStrats)
+export function installLegacyOptionMergeStrats(config: AppConfig) {
+  config.optionMergeStrategies = new Proxy({} as any, {
+    get(target, key) {
+      if (key in target) {
+        return target[key]
+      }
+      if (
+        key in legacyOptionMergeStrats &&
+        softAssertCompatEnabled(
+          DeprecationTypes.CONFIG_OPTION_MERGE_STRATS,
+          null
+        )
+      ) {
+        return legacyOptionMergeStrats[
+          key as keyof typeof legacyOptionMergeStrats
+        ]
+      }
+    }
+  })
 }
 
 export const legacyOptionMergeStrats = {