The type system now keeps its nullability promise: a `?T` value cannot be
used, stored, or dereferenced as a plain `T` without narrowing. The canonical
evidence probe (return b.v where v: ?Int, fn -> Int) that compiled clean for
months now fails with WO-E211.
- WO-E211 (un-narrowed use): arithmetic and </<=/>/>= operands, and/or
operands (?Bool), interpolation segments, for-iterables, and returns whose
declared type is not nullable.
- WO-E212 (boundary): nil or ?T stored into a non-nullable slot — annotated
let, assignment to a confidently-typed local (cenv, never the placeholder
env — a placeholder target must stay silent) or a resolvable class field.
- WO-E213 (deref): field/index access through a possibly-nil base.
- Narrowing (locals only — a field place can be re-assigned between check
and use, so chains bind to a local first): `if x != nil { }` narrows the
branch; a DIVERGING then-branch (`if x == nil { return }`) narrows after
the if; `x != nil and x.n > 3` narrows and/or right operands
(short-circuit); `while x != nil` narrows the body. The narrow is
un-applied when an else-less then-env leaks out un-diverged (the existing
env-leak convention must not leak the narrow).
- No false positives by construction: env/cenv types are declared or
confidently inferred; the placeholder fallbacks are plain scalars, never
?T. The whole golden suite passed untouched (540/0).
- Samples updated to the bind-then-narrow idiom (log-watcher config decode +
supervisor lock/next_fire, gc-cycle ring print) — 22 genuine unnarrowed-nil
sites; employee needed zero changes. All acceptances green.
- Corpus: compile-fail/{nullable-unnarrowed-use,nullable-nil-into-plain,
nullable-deref-unchecked} + run/nullable-narrowing (all four forms) — 83/0.
- Catalog: E211/E212/E213 move from "Reserved, not yet emitted" to the main
table; nullable-types-implementation.md status flipped to ENFORCED
(historical record kept); plan 8 Task 6 ticked (boxed scalar cells
superseded by WO_NIL_SCALAR); board updated.
Verified: woc-test 540/0 + test_diag 14/0; oop-e2e 83/0; oop-accept ALL MET;
log-watcher 7/0; employee 8/0; gc-cycle ring prints + reclaims.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Design-first deliverable for iteration 7b (no runtime/compiler code yet).
docs/examples/gc-cycle explains, by example, how pointers flow through the heap
and the collector's mark-sweep logic:
- types.wo: Node (self-referential ?Node -> inferred `gc`/traced) vs Segment
(acyclic -> `owned`, deterministically dropped)
- main.wo: ring_demo builds a->b->c->a and abandons it; owned_demo shows the
drop path with no collector
- README.md: the model (ownership frees the 99%, tracing only the cyclic/
aliased residue, inference decides), the 16-byte header rewrite (retire
rc+borrow -> 8-byte sweep-list link, colors in flag bits), where a traced
pointer lives (root via pc gc-mask / GCREF field / container), and the
tri-color incremental algorithm with the Yuasa deletion barrier. Two mermaid
step diagrams (heap+roots, collector cycle) + the owned contrast.
Grounded in the approved spec (2026-08-11-inferred-gc-mark-sweep-design.md) and
the real runtime structures (obj.h/wob.h: wo_hdr, WO_K_GCREF, arena, wo_obj_size).
Run status: honest — the sample does NOT build today; woc reports WO-E301
(use-after-move at the ring-closing store), which is exactly the aliasing that
"traced classes alias freely" unblocks under 7b. README records this.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
