Tagged unions not inferred when returning objects implementing interfaces

[mmc41]

TypeScript Version: 2.0.0

interface Category 
{
    id: string;
    description: string;
}

interface TreeNodeBase 
{
  readonly children: TreeNode[];
  title() : string;
  id(): string;
}

interface CategoryTreeNode extends TreeNodeBase
{
  readonly kind: "category";
  readonly item: Category;
}

interface EmptyTreeNode extends TreeNodeBase
{
  readonly kind: "empty";
}

type TreeNode = EmptyTreeNode | CategoryTreeNode; // More variants in actual case.

function createCategoryNode(category : Category, children : TreeNode[]) : CategoryTreeNode
{
    return {
      kind: "category",
      item: category,
      children: children,
      title() { return (<Category>this.item).description; }, // Why is type of this.item not inferred ?
      id() { return (<Category>this.item).id; } // Why is type of this.item not inferred ?
    };
}

Expected behavior:

The factory method createCategoryNode should be able to return an object that is a CategoryTreeNode in a type safe manner without having to specify the type as done above when implementing the title and id functions.

Actual behavior:

Right now, unless the type of this.item is explicitly given, typescript will regard the type of item as any instead of Category. Thus, the type of this.item is not inferred correctly.

[Arnavion]

Nothing to do with union types. this itself is any in methods in object literals. See #10835

Edit: i.e. in your case you would need to write

    title(this: CategoryTreeNode) { return this.item.description; }
    id(this: CategoryTreeNode) { return this.item.id; }