fix(types): ensure correct oldValue typing based on lazy option

close #719

test-dts/watch.test-d.ts

@@ -0,0 +1,54 @@
+import { ref, computed, watch } from './index'
+import { expectType } from 'tsd'
+
+const source = ref('foo')
+const source2 = computed(() => source.value)
+const source3 = () => 1
+
+// eager watcher's oldValue will be undefined on first run.
+watch(source, (value, oldValue) => {
+  expectType<string>(value)
+  expectType<string | undefined>(oldValue)
+})
+
+watch([source, source2, source3], (values, oldValues) => {
+  expectType<(string | number)[]>(values)
+  expectType<(string | number | undefined)[]>(oldValues)
+})
+
+// const array
+watch([source, source2, source3] as const, (values, oldValues) => {
+  expectType<Readonly<[string, string, number]>>(values)
+  expectType<
+    Readonly<[string | undefined, string | undefined, number | undefined]>
+  >(oldValues)
+})
+
+// lazy watcher will have consistent types for oldValue.
+watch(
+  source,
+  (value, oldValue) => {
+    expectType<string>(value)
+    expectType<string>(oldValue)
+  },
+  { lazy: true }
+)
+
+watch(
+  [source, source2, source3],
+  (values, oldValues) => {
+    expectType<(string | number)[]>(values)
+    expectType<(string | number)[]>(oldValues)
+  },
+  { lazy: true }
+)
+
+// const array
+watch(
+  [source, source2, source3] as const,
+  (values, oldValues) => {
+    expectType<Readonly<[string, string, number]>>(values)
+    expectType<Readonly<[string, string, number]>>(oldValues)
+  },
+  { lazy: true }
+)