I want to get type checking working for custom elements in JSX. The following examples should work.

gistfile1.tsx

type HTMLElementPrototype = HTMLElement extends { prototype: infer Prototype }
  ? Prototype
  : never;

declare namespace h {
  namespace JSX {
    interface Element {}
    type LibraryManagedAttributes<E, _> = E extends {
      prototype: infer Prototype;
    }
      ? Pick<Prototype, Extract<keyof HTMLElementPrototype, keyof Prototype>>
      : _;
  }
}

class CustomElement extends HTMLElement {
  // Prop 1 should be required.
  prop1: string;

  // This should not be required.
  prop2?: string;
}

<CustomElement title="This should work because HTMLElement." />;