fix(toRef): ref created from union typed prop can't be used in watch (#3048)

test-dts/ref.test-d.ts

@@ -9,7 +9,8 @@ import {
   proxyRefs,
   toRef,
   toRefs,
-  ToRefs
+  ToRefs,
+  watch
 } from './index'
 
 function plainType(arg: number | Ref<number>) {
@@ -165,6 +166,14 @@ const obj = {
 expectType<Ref<number>>(toRef(obj, 'a'))
 expectType<Ref<number>>(toRef(obj, 'b'))
 
+const objWithUnionProp: { a: string | number } = {
+  a: 1
+}
+
+watch(toRef(objWithUnionProp, 'a'), value => {
+  expectType<string | number>(value)
+})
+
 // toRefs
 const objRefs = toRefs(obj)
 expectType<{