struct AnyView

A view whose type is decided at run time.

For the trees Swift cannot spell. A split holding a split holding a pane is a type that contains itself, and an opaque some View cannot recurse, so the recursion is erased here and nowhere else.

It costs what erasure costs everywhere: when the underlying type changes between two rebuilds — a pane turning into a split — the old subtree is unmounted and a new one mounted, and whatever state lived on its nodes (a scroll offset, a text field's caret) starts over. While the type stays the same, reconcile goes straight through to the view's own and is as cheap as it would be without the wrapper.