fix(types): fix ToRefs type on union value types

fix #2687

test-dts/ref.test-d.ts

@@ -8,7 +8,8 @@ import {
   expectType,
   proxyRefs,
   toRef,
-  toRefs
+  toRefs,
+  ToRefs
 } from './index'
 
 function plainType(arg: number | Ref<number>) {
@@ -28,7 +29,6 @@ function plainType(arg: number | Ref<number>) {
   const nestedRef = ref({
     foo: ref(1)
   })
-  expectType<Ref<{ foo: number }>>(nestedRef)
   expectType<{ foo: number }>(nestedRef.value)
 
   // ref boolean
@@ -171,3 +171,26 @@ expectType<{
   a: Ref<number>
   b: Ref<number>
 }>(objRefs)
+
+// #2687
+interface AppData {
+  state: 'state1' | 'state2' | 'state3'
+}
+
+const data: ToRefs<AppData> = toRefs(
+  reactive({
+    state: 'state1'
+  })
+)
+
+switch (data.state.value) {
+  case 'state1':
+    data.state.value = 'state2'
+    break
+  case 'state2':
+    data.state.value = 'state3'
+    break
+  case 'state3':
+    data.state.value = 'state1'
+    break
+}