Enable Typescript to understand JSX default props

package.json

@@ -119,7 +119,7 @@
     "rollup-plugin-node-resolve": "^3.0.0",
     "sinon": "^4.4.2",
     "sinon-chai": "^3.0.0",
-    "typescript": "^2.9.0-rc",
+    "typescript": "^3.0.1",
     "uglify-js": "^2.7.5",
     "webpack": "^4.3.0"
   },

src/preact.d.ts

@@ -124,6 +124,15 @@ declare namespace preact {
 	};
 }
 
+type Defaultize<Props, Defaults> =
+	// Distribute over unions
+	Props extends any
+		? 	// Make any properties included in Default optional
+			& Partial<Pick<Props, Extract<keyof Props, keyof Defaults>>>
+			// Include the remaining properties from Props
+			& Pick<Props, Exclude<keyof Props, keyof Defaults>>
+		: never;
+
 declare global {
 	namespace JSX {
 		interface Element extends preact.VNode<any> {
@@ -140,6 +149,11 @@ declare global {
 			children: any;
 		}
 
+		type LibraryManagedAttributes<Component, Props> =
+			Component extends { defaultProps: infer Defaults }
+				? Defaultize<Props, Defaults>
+				: Props;
+
 		interface SVGAttributes extends HTMLAttributes {
 			accentHeight?: number | string;
 			accumulate?: "none" | "sum";

test/ts/preact.tsx

@@ -125,3 +125,68 @@ class ComponentWithLifecycle extends Component<DummyProps, DummyState> {
 		console.log("componentDidUpdate", previousProps, previousState, previousContext);
 	}
 }
+
+// Default props: JSX.LibraryManagedAttributes
+
+class DefaultProps extends Component<{text: string, bool: boolean}> {
+	static defaultProps = {
+		text: "hello"
+	};
+
+	render() {
+		return <div>{this.props.text}</div>;
+	}
+}
+
+const d1 = <DefaultProps bool={false} text="foo" />;
+const d2 = <DefaultProps bool={false} />;
+
+class DefaultPropsWithUnion extends Component<
+	{ default: boolean } & (
+		| {
+				type: "string";
+				str: string;
+		  }
+		| {
+				type: "number";
+				num: number;
+		  })
+> {
+	static defaultProps = {
+		default: true
+	};
+
+	render() {
+		return <div />;
+	}
+}
+
+const d3 = <DefaultPropsWithUnion type="string" str={"foo"} />;
+const d4 = <DefaultPropsWithUnion type="number" num={0xf00} />;
+const d5 = <DefaultPropsWithUnion type="string" str={"foo"} default={false} />;
+const d6 = <DefaultPropsWithUnion type="number" num={0xf00} default={false} />;
+
+class DefaultUnion extends Component<
+	| {
+			type: "number";
+			num: number;
+	  }
+	| {
+			type: "string";
+			str: string;
+	  }
+> {
+	static defaultProps = {
+		type: "number",
+		num: 1
+	};
+
+	render() {
+		return <div />;
+	}
+}
+
+const d7 = <DefaultUnion />;
+const d8 = <DefaultUnion num={1} />;
+const d9 = <DefaultUnion type="number" />;
+const d10 = <DefaultUnion type="string" str="foo" />;