[ASDAlexander77]

Phase 3 of -mm=own (spec §7): containers and unions. Only OwnershipInferencePass.cpp changes; gc, rc and none are untouched. Plan: docs/superpowers/plans/2026-09-29-own-phase-3.md. Results: spec §14.

What changes

• Unions and optionals are views. A cast between class, union and optional types, CreateUnionInstance, GetValueFromUnionOp, OptionalValue and Value hand back the block their operand holds. let u: C | null = new C(i), let u: C | string = c and let u: C | undefined now compile as one owner. Before, they were rejected as "takes a second reference".
• A read out of a field or an element is a borrow. This closes a soundness hole: const c = h.c; h.c = new C(); c.x used to compile into a read of freed memory under own (rc has the same bug). A read may not be kept ('c' borrows 'h.c' and cannot be stored, returned or captured). No use of it, or of what it produces, may be reachable from:
  • an overwrite of a place on its way to the root;
  • a pop, shift, splice or length =;
  • an end of the root (release, assignment, move);
  • a call that may reach the root.
• A let declared from a read borrows it (let c = h.c, let e = arr[j]): no reference taken, none given back.
• Borrows are followed through ?? merges and through locals that own nothing, each checked against where that value is made again.

Verification

• Release suite 3196/3196.
• Debug own tests 301/303. The two failures, test-jit-rc-owned-async and test-jit-none-owned-async, fail on main too (the ~AsyncToken debug-heap assert). ownership-verifier-4 was excluded; it also fails on main (the linkname assert).
• Memory over 1M iterations: own is flat like rc (4.7–4.8 MB); none goes up to 582 MB.
• Teeth:
  • with a move's releases kept, the union positives fail with heap corruption;
  • with a let borrower's releases kept, the let positives fail the same way;
  • the new negatives compiled before this change, and five of them printed 999 (freed memory) under rc and own.
• Corpus under own: 252 → 272 of 564 compile. 3 stopped, all ${this.color} … ${this.area()}, per the ruling on calls below.

Rulings

• any boxing moves to phase 4. Unboxing is a generated ___unbox<T> that returns +1, so treating it as a borrow needs phase 4's borrowed-from-argument result. Found along the way: rc never frees the box of const a: any = new C(i) (66 MB against gc's 6 MB).
• Any call drops a borrow of a parameter's, this's or a global's field. The callee may reach the owner through a global. This is sound but restrictive until phase 4's callee facts.
• Overwrites are matched by field position and type. This can only add errors, never miss one.

Final review

A fresh reviewer found two unsound cases and one false error, all fixed with tests that failed first:

• a borrow stored into a non-owning local and read on a later iteration after the overwrite;
• a read through cond ? a : b where a is then assigned;
• cond ? h.c : h.d rejected after any earlier call.

Deferred minors: 'this value' naming for temporaries and merges; an extra error for nested reads; u.c ??= new C() is rejected. Also found, not own: const a2 = arr; arr.push(...) in a loop crashes the compiler under every model.

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