function toHTML<T, K extends keyof T>(items: T[], key: T[K] extends T[] ? K : never) {
    if (!items.length) return '';
    // i[key] here is an error :(
    const innerHTML = items.map(i => `<li>${i}${toHTML(i[key], key)}</li>`).join('\n');
    return `<ul>\n${innerHTML}\n</ul>`;
}

class Item {
    constructor(public name: string, public nodes: Item[]) { }
    toString() { return this.name; }
}

const tree = [
    new Item("Root", [
        new Item("Node 1", [new Item("Subnode 1", [])]),
        new Item("Node 2", [])
    ])
];

// This works correctly! Anything but 'nodes' (eg. 'name') is an error.
document.write(toHTML(tree, 'nodes'));