Recursive storage

Classes and user enums may refer to themselves or one another, including through Option, Result, tuples, and transparent aliases. Every storage cycle must pass through a heap boundary: Box[T], a list, a set, or a dictionary. A cycle without one would require an infinite-sized value and is rejected.

class Node[T]:
    value: T
    next: Option[Box[Node[T]]]

Finite concrete generic instances can recur, including mutual instances with different arguments. Alias-only cycles remain invalid. Endless type expansion, such as a Node[T] containing a Node[list[T]], is rejected when specialized; the diagnostic names the first three instances and points at the type application that grows. Finite nesting deeper than 64 levels reports that limit. These limits concern type descriptions rather than the number of nodes in a runtime chain or tree.

Recursive values and aggregates containing them move. Automatic copy, equality (including membership), and formatting reject types whose stored values can recur, even when a particular value is shallow. Callable signatures do not constitute stored recursive values. Consuming matching transfers payloads; borrowed matching preserves the owner. See ownership.

Moving a field through a Box, stored references, multiple mutable owners of one node, and source ownership cycles are unsupported. A terminating variant, Nothing, or an empty collection supplies a construction base case. A list can own all nodes while integer IDs represent parent links, shared targets, or cycles: such graph edges do not carry ownership or create loans. Index stability, stale IDs, and graph consistency remain the application’s responsibility. See representation choices. Shared[T] nodes may form immutable trees and DAGs, but not cycles: a Shared[T] whose content can reach it again through a Mutex or channel is rejected, so back-references use these IDs.

Automatic destruction of recursive owners uses bounded stack space and allocates nothing. Explicit recursion in user methods and destructor hooks still uses the native stack. Box construction, content conversion, projections, and borrowing are described under facilities.

The recursive representation proposal records the storage and destruction rationale. The owned-structure assessment compares linked lists, trees, and checked-index arenas using these facilities.