fix(compiler): lang-41 side defects — ?T-typed nil try, WO-E305 on moves out of a field

- emit.ml: a `try … catch (e) nil` is `?T` (ty_of_expr) and the nil arm takes that destination, so a `?Int` nil is WO_NIL_SCALAR and an Int body's legitimate 0 no longer reads as nil (it used to fall back to the zero word via the body type / enclosing return type)
- owner.ml: `transfer` on a projection (`d.tags`, `x[i]`) of an owned value reports WO-E305 instead of returning false silently — the silent path compiled `Out { tags: d.tags }` to an alias that both records dropped (the "json.decode as T corruption": not json's, a double free language 44's poison now aborts on); heap scalars exempt (store sites copy)
- error catalog: WO-E305 row; owner.ml module doc updated
- corpus: run/try-nil-int-zero, compile-fail/no-partial-move, run/decode-record-crosses-return (Text copied, record moved whole — the archived `.. ""` workaround is unnecessary)
- verified: oop-e2e 126/0, tests/regress/lang-41 compile, --emit sweep over the non-porch examples, web-app gate 56/0 (porch in project mode) — no legitimate program trips WO-E305
- story 41: both side defects marked fixed; board prose updated

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
(cherry picked from commit 2d54710e693fafee4b8d6561cc9cba7b95415d89)

compiler/src/emit.ml

@@ -1177,6 +1177,12 @@ let query_elem_scalar (p : pctx) (q : Ast.query) ~(src : string) : string =
     | None -> "Int")
   | _ -> "Int"
 
+(* `try … catch (e) nil`: the catch arm's value is the literal nil *)
+let try_handler_is_nil (handler : Ast.stmt list) : bool =
+  match List.rev handler with
+  | { Ast.s_kind = Ast.ExprStmt { Ast.kind = Ast.NilLit; _ }; _ } :: _ -> true
+  | _ -> false
+
 let rec ty_of_expr (p : pctx) (f : fstate) (e : Ast.expr) : Ast.field_ty option =
   match e.kind with
   | IntLit _ -> Some (Scalar "Int")
@@ -1192,8 +1198,14 @@ let rec ty_of_expr (p : pctx) (f : fstate) (e : Ast.expr) : Ast.field_ty option
   | NilLit -> None
   | As (_, ty) -> Some (Nullable ty)
   (* haxe-parity Task 5: a `try` yields its try arm's type — types.ml has
-     already required the catch arm to agree. *)
-  | Try t -> ty_of_expr p f t.body
+     already required the catch arm to agree. A `catch (e) nil` arm makes it
+     `?T`, so a `?scalar`'s nil is the sentinel and an Int body's 0 stays 0
+     (lang-41 side defect). *)
+  | Try t -> (
+    match ty_of_expr p f t.body with
+    | Some (Nullable _) as n -> n
+    | Some bt when try_handler_is_nil t.handler -> Some (Nullable bt)
+    | other -> other)
   | Ident n -> (
     match List.assoc_opt n f.f_env with
     | Some (_, t) -> Some t
@@ -2665,7 +2677,13 @@ and emit_try (p : pctx) (f : fstate) (v : views) ~(dst : int) ?expected (e : Ast
       f.f_cur_line <- last.Ast.s_pos.line;
       (match expected with
       | Some t -> emit_expr p f v ~dst ~expected:t ve
-      | None -> emit_expr p f v ~dst ve);
+      | None -> (
+        (* a `nil` arm takes the try's own type as its destination: `?Int`
+           selects the scalar sentinel, so an `Int` body's legitimate 0 is
+           never read as nil (lang-41 side defect; see ty_of_expr's Try) *)
+        match (if is_nil_lit ve then ty_of_expr p f e else None) with
+        | Some t -> emit_expr p f v ~dst ~expected:t ve
+        | None -> emit_expr p f v ~dst ve));
       (* iteration 24 fix (the catch half of the arm-copy rule): a bare
          `e.msg` arm aliases the Error record's field, and the record is
          dropped at CATCH scope end below — ASan-confirmed use-after-free

compiler/src/owner.ml

@@ -69,7 +69,8 @@
    a place where this pass is wrong-by-accident:
 
      - No partial moves. A move site must name a whole local (`x`), never
-       a projection (`x.f`, `x[0]`); see the `let` case above.
+       a projection (`x.f`, `x[0]`); see the `let` case above. A projection
+       at a transfer site is WO-E305, not a silent alias (transfer).
      - Alias provability is syntactic *after canonicalization*: a place
        written through a borrow binding is first rewritten to the storage
        that borrow names (see canon), then two places overlap only if they
@@ -131,6 +132,15 @@ let conflicting_borrow_code = Diag.ownership_prefix ^ "03"
    escape. Related: where the borrow was created. *)
 let borrow_escape_code = Diag.ownership_prefix ^ "04"
 
+(* WO-E305 — an owned value is moved out of a field or element (`x.f`,
+   `x[i]`) while its record/container still owns it: stored into a record,
+   pushed into a container, passed to a `take` parameter, or returned.
+   Milestone 1 has no partial moves, and silently allowing the store put one
+   owned value under two owners — a double free at the second drop (the
+   lang-41 side defect). Heap scalars are exempt: every store site copies
+   them (stores_by_copy). Primary site: the move. Related: the owner. *)
+let partial_move_code = Diag.ownership_prefix ^ "05"
+
 (* ============================================================
    Ownership classes and places
    ============================================================ *)
@@ -1112,7 +1122,22 @@ let transfer (ctx : ctx) (p : place) ~(what : string) : bool =
         | Moved _ -> false (* already reported at the read *)
         | Borrowed _ -> false (* unreachable: is_borrow_root covered it *)
         | Live ->
-          if p.projs <> [] then false (* no partial moves in milestone 1 *)
+          if p.projs <> [] then begin
+            (* no partial moves in milestone 1 — and no silent alias either:
+               the record/container still owns this place, so the transfer
+               would give one owned value two owners (WO-E305) *)
+            if not (stores_by_copy ctx p) then
+              report ctx ~code:partial_move_code ~pos:p.ppos
+                ~message:
+                  (Printf.sprintf
+                     "`%s` %s — it is part of `%s`, and an owned value cannot be moved out of a \
+                      field or element (no partial moves): move `%s` whole, or build a fresh \
+                      container from its elements"
+                     (place_text p) what l.l_name l.l_name)
+                ~rel:l.l_pos
+                ~label:(Printf.sprintf "`%s` owns it" l.l_name);
+            false
+          end
           else begin
             check_against_borrows ctx ~node:p.pnode ~pos:p.ppos p AMove;
             l.l_state <- Moved p.ppos;

docs/plan/oop-vm/01-error-catalog.md

@@ -139,6 +139,7 @@ renders indented beneath it.
 | WO-E302 | a place is moved while a live borrow of it, or of an overlapping place, still exists. | `cannot move \`bag.items\` while \`r\` is borrowed` |
 | WO-E303 | two exclusive (`mut`) accesses of the same place, or two accesses the analysis can *prove* overlap, conflict in one region (e.g. two `mut` element accesses through the same provable index, or the same place borrowed and then mutated). Cases the analysis can't prove either way become a residual site for the VM to guard at runtime, not this diagnostic. | `cannot borrow \`bag.items[i]\` as \`mut\` twice in the same call` |
 | WO-E304 | a borrow is returned or stored somewhere that outlives the scope it borrowed from. `@gc`-typed values are exempt (freely aliased by design). | `borrow of \`x\` returned — borrows cannot outlive their scope` |
+| WO-E305 | an owned value is moved out of a field or element (`x.f`, `x[i]`) — stored into a record, pushed into a container, passed to a `take` parameter, or returned — while its record/container still owns it (no partial moves; before this diagnostic the store silently aliased, a double free at the second drop). Heap scalars (`Text`, `Bytes`) are exempt: those sites copy. Move the whole owner, or build a fresh container from its elements. | `\`d.tags\` cannot be stored in \`Out.tags\` — it is part of \`d\`, and an owned value cannot be moved out of a field or element (no partial moves): move \`d\` whole, or build a fresh container from its elements` |
 
 ## WO-E4xx — emitter (plan 3 Task 1, `compiler/src/emit.ml`)
 

docs/stories/00-status.md

@@ -659,7 +659,12 @@ localises it: every failure belonged to the path with 5x the allocation inside
 sequential insert+delete volume on one key (N=4/5 crashed, N=1-3 clean over 12+
 trials). Two smaller runtime defects came with it — `try/catch` cannot tell a
 literal `Int 0` reply from a trap, and a `json.decode` value is corrupted when
-embedded in a struct crossing a function-return boundary.
+embedded in a struct crossing a function-return boundary. **Both fixed
+2026-09-09**: a nil-armed `try` is `?T` (its nil is the scalar sentinel, so 0
+stays 0), and the "corruption" was a silent alias — an owned container field
+stored into another record with both records dropping it — now **WO-E305** at
+compile time (corpus: `try-nil-int-zero`, `no-partial-move`,
+`decode-record-crosses-return`).
 
 **Corrections to our own record:** the earlier claim that `Pool` being traced
 forces a one-slot re-wrap was WRONG — WO-E222 fires on the class, not on

docs/stories/language-runtime-database/41-actor-arena-crash.md

@@ -378,7 +378,21 @@ fixed.
   dropping, run 5× under `WO_SHARDS=4` + ASan by `scripts/db-actor-accept.sh`
   (`just db-actor`). It lives in `tests/regress` rather than the corpus because
   the corpus runner pins `WO_SHARDS=1`.
-- **The two smaller runtime defects found alongside** (above): the
-  `try EXPR catch (e) nil` Int-0-vs-trap ambiguity and the `json.decode ... as T`
-  cross-return-boundary corruption. Both worked around in the archived code;
-  each deserves its own minimal fixture and fix, neither blocks this.
+- ~~**The two smaller runtime defects found alongside** (above)~~ ✅ **both
+  fixed 2026-09-09**, each with its corpus fixture:
+  - `try EXPR catch (e) nil` over an `Int` body spelled its nil as the zero
+    word (the try took the body's type, `Int`, so `nil_const_for` chose 0 and
+    `x == nil` compared against 0). The emitter now types a nil-armed try as
+    `?T` and the nil arm takes that destination, so a `?Int` nil is the
+    `WO_NIL_SCALAR` sentinel and a legitimate 0 stays 0 —
+    `tests/corpus/run/try-nil-int-zero`.
+  - the "`json.decode … as T` corruption" was not json's: storing a record's
+    owned **container** field into another record (`Out { tags: d.tags }`)
+    compiled to a silent alias — `transfer` returned false for any projection
+    ("no partial moves") without reporting, so `d` and `Out` both dropped the
+    multi (a double free, which language 44's poisoned header now aborts on).
+    A projection at a transfer site is **WO-E305** (heap scalars exempt: those
+    sites copy) — `tests/corpus/compile-fail/no-partial-move`; the sanctioned
+    shapes (Text copied, record moved whole) —
+    `tests/corpus/run/decode-record-crosses-return`. The archived `.. ""`
+    workaround is unnecessary.

tests/corpus/compile-fail/no-partial-move/fixture.code

@@ -0,0 +1 @@
+WO-E305

tests/corpus/compile-fail/no-partial-move/fixture.wo

@@ -0,0 +1,21 @@
+-- lang-41 side defect 2: storing `d.tags` into another record used to
+-- compile to a silent alias — `d` still owned the multi, so both records
+-- dropped it (a double free the poisoned header now aborts on). A
+-- projection at a transfer site is WO-E305; the Text field is exempt
+-- because SETF copies it.
+class D {
+  name: Text
+  tags: multi Text
+}
+class Out {
+  label: Text
+  tags: multi Text
+}
+fn build() -> Out {
+  let d = D { name: "alpha", tags: ["t1", "t2"] }
+  return Out { label: d.name, tags: d.tags }
+}
+fn main() {
+  let o = build()
+  print(o.label)
+}

tests/corpus/run/decode-record-crosses-return/fixture.out

@@ -0,0 +1,3 @@
+alpha
+7
+beta

tests/corpus/run/decode-record-crosses-return/fixture.wo

@@ -0,0 +1,32 @@
+-- lang-41 side defect 2, the sanctioned shapes: a Text read off a decoded
+-- record into another record's ctor is COPIED (SETF's rule), and the decoded
+-- record itself moves whole across a return. Neither needs the archived
+-- porch code's `.. ""` workaround; a container field must move with its
+-- record (see compile-fail/no-partial-move).
+use json
+class D {
+  name: Text
+  n: Int
+}
+class Out {
+  label: Text
+  n: Int
+}
+fn build(t: Text) -> Out {
+  let d = json.decode(t) as D
+  if d != nil { return Out { label: d.name, n: d.n } }
+  return Out { label: "none", n: 0 }
+}
+fn build2(t: Text) -> ?D {
+  let d = json.decode(t) as D
+  return d
+}
+fn main() {
+  let o = build("{\"name\":\"alpha\",\"n\":7}")
+  let junk = "z" .. "y" .. "x"
+  print(o.label)
+  print_int(o.n)
+  let d2 = build2("{\"name\":\"beta\",\"n\":8}")
+  let junk2 = "a" .. "s" .. "d"
+  if d2 != nil { print(d2.name) }
+}

tests/corpus/run/try-nil-int-zero/fixture.out

@@ -0,0 +1,3 @@
+0
+trap-is-nil
+zero-kept-in-int-fn

tests/corpus/run/try-nil-int-zero/fixture.wo

@@ -0,0 +1,18 @@
+-- lang-41 side defect 1: `try EXPR catch (e) nil` over an Int body used to
+-- spell its nil as the zero word, so a legitimate 0 reply read as a trap.
+-- The try is `?Int`: nil is the scalar sentinel, 0 stays 0, in a void fn
+-- and in one whose own return type is Int (the archived porch shape).
+fn zero() -> Int { return 0 }
+fn boom() -> Int { let d = 0; return 1 / d }
+fn probe() -> Int {
+  let z = try zero() catch (e) nil
+  if z == nil { print("ZERO-READ-AS-NIL-IN-INT-FN") } else { print("zero-kept-in-int-fn") }
+  return 1
+}
+fn main() {
+  let z = try zero() catch (e) nil
+  if z == nil { print("ZERO-READ-AS-NIL") } else { print_int(z) }
+  let t = try boom() catch (e) nil
+  if t == nil { print("trap-is-nil") } else { print("TRAP-READ-AS-VALUE") }
+  probe()
+}