fix(types): fix function prop type inference

close vuejs#9357

Author: kjleitz

types/options.d.ts

@@ -150,12 +150,12 @@ export type PropType<T> = Prop<T> | Prop<T>[];
 
 export type PropValidator<T> = PropOptions<T> | PropType<T>;
 
-export interface PropOptions<T=any> {
-  type?: PropType<T>;
+export type PropOptions<T=any> = (<TypeAsAccessed extends T, TypeAsDefined extends PropType<T>>() => {
+  type?: TypeAsDefined;
   required?: boolean;
-  default?: T | null | undefined | (() => T | null | undefined);
-  validator?(value: T): boolean;
-}
+  default?: TypeAsDefined extends PropType<(...args: any[]) => any> ? TypeAsAccessed | null | undefined : TypeAsAccessed | null | undefined | (() => TypeAsAccessed | null | undefined);
+  validator?(value: TypeAsAccessed): boolean;
+}) extends () => infer Opts ? Opts : never;
 
 export type RecordPropsDefinition<T> = {
   [K in keyof T]: PropValidator<T[K]>

types/test/vue-test.ts

@@ -1,5 +1,5 @@
 import Vue, { VNode } from "../index";
-import { ComponentOptions } from "../options";
+import { ComponentOptions, PropType } from "../options";
 
 class Test extends Vue {
   a: number = 0;
@@ -154,6 +154,45 @@ const FunctionalScopedSlotsComponent = Vue.extend({
   }
 });
 
+declare function assertBoolean<A>(value: string extends A ? never : (A extends boolean ? A : never)): true;
+
+declare const val1: boolean;
+// declare const val2: boolean | (() => boolean);
+// declare const val3: any;
+
+assertBoolean(val1);    //=> compiles (good)
+// assertBoolean(val2); //=> does not compile (good)
+// assertBoolean(val3); //=> does not compile (good)
+
+const ComponentWithFunctionProps = Vue.extend({
+  props: {
+    functionProp: {
+      type: Function,
+      default: () => true,
+    },
+    functionPropWithBooleanReturnType: {
+      type: Function as PropType<() => boolean>,
+      default: () => true,
+    },
+    booleanProp: {
+      type: Boolean,
+      default: true,
+    },
+    booleanPropWithFunctionDefault: {
+      type: Boolean,
+      default: () => true,
+    },
+  },
+  methods: {
+    test(): void {
+      this.functionProp(); // callable (good)
+      assertBoolean(this.functionPropWithBooleanReturnType())
+      assertBoolean(this.booleanProp);
+      assertBoolean(this.booleanPropWithFunctionDefault);
+    },
+  },
+});
+
 const Parent = Vue.extend({
   data() {
     return { greeting: 'Hello' }