fix(types): retain union type narrowing with defaults applied (#12108)

close #12106

packages-private/dts-test/setupHelpers.test-d.ts

@@ -240,6 +240,23 @@ describe('withDefaults w/ defineProp type is different from the defaults type',
   res1.value
 })
 
+describe('withDefaults w/ defineProp discriminate union type', () => {
+  const props = withDefaults(
+    defineProps<
+      { type: 'button'; buttonType?: 'submit' } | { type: 'link'; href: string }
+    >(),
+    {
+      type: 'button',
+    },
+  )
+  if (props.type === 'button') {
+    expectType<'submit' | undefined>(props.buttonType)
+  }
+  if (props.type === 'link') {
+    expectType<string>(props.href)
+  }
+})
+
 describe('defineProps w/ runtime declaration', () => {
   // runtime declaration
   const props = defineProps({

packages/runtime-core/src/apiSetupHelpers.ts

@@ -331,7 +331,8 @@ type PropsWithDefaults<
   T,
   Defaults extends InferDefaults<T>,
   BKeys extends keyof T,
-> = Readonly<MappedOmit<T, keyof Defaults>> & {
+> = T extends unknown
+  ? Readonly<MappedOmit<T, keyof Defaults>> & {
       readonly [K in keyof Defaults as K extends keyof T
         ? K
         : never]-?: K extends keyof T
@@ -346,6 +347,7 @@ type PropsWithDefaults<
           : boolean
         : boolean
     }
+  : never
 
 /**
  * Vue `<script setup>` compiler macro for providing props default values when