fix(types): improve ref typing, close #759

test-dts/ref.test-d.ts

@@ -0,0 +1,16 @@
+import { expectType } from 'tsd'
+import { Ref, ref } from './index'
+import { isRef } from '@vue/reactivity'
+
+function foo(arg: number | Ref<number>) {
+  // ref coercing
+  const coerced = ref(arg)
+  expectType<Ref<number>>(coerced)
+
+  // isRef as type guard
+  if (isRef(arg)) {
+    expectType<Ref<number>>(arg)
+  }
+}
+
+foo(1)