allow function component with children (#3676)

* allow function component with children

* test for 3611

* better element type

* remove namespace

src/index.d.ts

@@ -87,7 +87,7 @@ export type ComponentProps<
 	: never;
 
 export interface FunctionComponent<P = {}> {
-	(props: RenderableProps<P>, context?: any): VNode<any> | null;
+	(props: RenderableProps<P>, context?: any): ComponentChild;
 	displayName?: string;
 	defaultProps?: Partial<P>;
 }

src/jsx.d.ts

@@ -4,6 +4,9 @@
 import {
 	ClassAttributes,
 	Component,
+	ComponentChild,
+	ComponentType,
+	FunctionComponent,
 	PreactDOMAttributes,
 	VNode
 } from './index';
@@ -26,9 +29,14 @@ export namespace JSXInternal {
 		key?: any;
 	}
 
-	export interface Element extends VNode<any> {}
-
-	export interface ElementClass extends Component<any, any> {}
+	export type ElementType<P = any> =
+		| {
+				[K in keyof IntrinsicElements]: P extends IntrinsicElements[K]
+					? K
+					: never;
+		  }[keyof IntrinsicElements]
+		| ComponentType<P>;
+	export type ElementClass = Component<any, any> | FunctionComponent<any>;
 
 	export interface ElementAttributesProperty {
 		props: any;

test/ts/preact.tsx

@@ -102,6 +102,22 @@ const UseOfComponentWithChildren = () => {
 	);
 };
 
+const DummyChildren: FunctionalComponent = ({ children }) => {
+	return children;
+};
+
+function ReturnChildren(props: { children: preact.ComponentChildren }) {
+	return props.children;
+}
+
+function TestUndefinedChildren() {
+	return (
+		<ReturnChildren>
+			<ReturnChildren>Hello</ReturnChildren>
+		</ReturnChildren>
+	);
+}
+
 // using ref and or jsx
 class ComponentUsingRef extends Component<any, any> {
 	private array: string[];