Fix sparse array semantic gaps

packages/compiler/src/backend/emission/emit-exprs.ts

@@ -45,12 +45,15 @@ function streamTypedRefCommitAdapter(
       `  size_t sc_target_len = sc_target->len;`,
       `  size_t sc_target_cap = sc_target->cap;`,
       `  uint64_t *sc_target_data = sc_target->data;`,
+      `  uint8_t *sc_target_present = sc_target->present;`,
       `  sc_target->len = sc_next->len;`,
       `  sc_target->cap = sc_next->cap;`,
       `  sc_target->data = sc_next->data;`,
+      `  sc_target->present = sc_next->present;`,
       `  sc_next->len = sc_target_len;`,
       `  sc_next->cap = sc_target_cap;`,
       `  sc_next->data = sc_target_data;`,
+      `  sc_next->present = sc_target_present;`,
       `  scr_arr_release(sc_next);`,
       `}`,
       ``,
@@ -200,6 +203,7 @@ function streamTypedRefAdapter(
     lines.push(
       `  ScrDyn *d = scr_dyn_new_arr();`,
       `  for (size_t sc_i = 0; sc_i < v->len; sc_i++) {`,
+      `    if (!scr_arr_has(v, (double)sc_i)) { scr_dyn_arr_push_hole(d); continue; }`,
     );
     if (elem.kind === "f64") {
       lines.push(

packages/compiler/src/backend/emission/emit-stmts.ts

@@ -360,8 +360,16 @@ export function emitStmt(E: CEmitter, s: IrStmt): void {
         const idx = E.emitExpr(s.index);
         const v = E.emitExpr(s.value);
         if (s.arr.type.kind !== "array") throw new Error("emitter bug: arraySet on non-array");
-        const acc = elemAccess(s.arr.type.elem);
+        const elem = s.arr.type.elem;
+        const acc = elemAccess(elem);
         if (acc === "ref") E.moveTemp(v);
+        if (elem.kind === "union") {
+          const tag = E.undefinedArmTag(elem);
+          if (tag >= 0) {
+            E.line(`scr_arr_set_ref_fill(${arr.name}, ${idx.name}, ${v.name}, ${E.unitInstanceRef(elem.unionId, tag)});${E.srcComment(s.loc)}`);
+            break;
+          }
+        }
         E.line(`scr_arr_set_${acc}(${arr.name}, ${idx.name}, ${v.name});${E.srcComment(s.loc)}`);
         break;
       }

packages/compiler/src/backend/emission/emit-walkers.ts

@@ -260,6 +260,7 @@ import { OVERFLOW_MEMBER } from "./emit-shapes.js";
       `  size_t n = (size_t)scr_arr_len(a);`,
       `  for (size_t i = 0; i < n; i++) {`,
       `    if (i) for (size_t j = 0; j < sep->len; j++) scr_jb_putc(&b, sep->data[j]);`,
+      `    if (!scr_arr_has(a, (double)i)) continue;`,
       `    ScrUnion *u = (ScrUnion *)scr_arr_get_ref(a, (double)i);`,
       ...(unitTags.length > 0
         ? [`    if (${unitTags.map((t) => `u->tag == ${t}`).join(" || ")}) { scr_union_release(u); continue; }`]
@@ -1197,12 +1198,15 @@ export function jsonWriteHelper(E: CEmitter, t: IrType): string {
             `      size_t sc_old_len = sc_cached->len;`,
             `      size_t sc_old_cap = sc_cached->cap;`,
             `      uint64_t *sc_old_data = sc_cached->data;`,
+            `      uint8_t *sc_old_present = sc_cached->present;`,
             `      sc_cached->len = sc_out->len;`,
             `      sc_cached->cap = sc_out->cap;`,
             `      sc_cached->data = sc_out->data;`,
+            `      sc_cached->present = sc_out->present;`,
             `      sc_out->len = sc_old_len;`,
             `      sc_out->cap = sc_old_cap;`,
             `      sc_out->data = sc_old_data;`,
+            `      sc_out->present = sc_old_present;`,
             `      scr_arr_release(sc_out);`,
             `      return sc_cached;`,
             `    }`,

packages/compiler/src/backend/llvm/dyn.ts

@@ -668,6 +668,7 @@ export class LlDyn {
                 { index: 1, type: "i64" as const, name: "length" },
                 { index: 2, type: "i64" as const, name: "capacity" },
                 { index: 7, type: "ptr" as const, name: "data" },
+                { index: 8, type: "ptr" as const, name: "present" },
               ];
           members.forEach((member) => {
             const cachedPtr = B.tmp();

packages/compiler/src/backend/llvm/emitter.ts

@@ -3169,9 +3169,18 @@ class LlEmitter {
         const idx = this.emitExpr(s.index);
         const v = this.emitExpr(s.value);
         if (s.arr.type.kind !== "array") throw new Error("llvm emitter bug: arraySet on non-array");
-        const acc = elemAccess(s.arr.type.elem);
+        const elem = s.arr.type.elem;
+        const acc = elemAccess(elem);
         if (acc === "ref") this.moveTemp(v);
         const argTy = acc === "f64" ? "double" : acc === "bool" ? "i1" : "ptr";
+        if (elem.kind === "union") {
+          const tag = this.undefinedArmTag(elem);
+          if (tag >= 0) {
+            this.declare(`declare void @scr_arr_set_ref_fill(ptr, double, ptr, ptr)`);
+            B.line(`call void @scr_arr_set_ref_fill(ptr ${arr.name}, double ${idx.name}, ptr ${v.name}, ptr ${this.unitInstanceRef(elem.unionId, tag)})`);
+            break;
+          }
+        }
         this.declare(`declare void @scr_arr_set_${acc}(ptr, double, ${argTy === "i1" ? "i1 zeroext" : argTy})`);
         B.line(`call void @scr_arr_set_${acc}(ptr ${arr.name}, double ${idx.name}, ${argTy} ${v.name})`);
         break;
@@ -10148,6 +10157,12 @@ class LlEmitter {
         `  %next_data = load ptr, ptr %next_data_ptr`,
         `  store ptr %next_data, ptr %target_data_ptr`,
         `  store ptr %target_data, ptr %next_data_ptr`,
+        `  %target_present_ptr = getelementptr inbounds %ScrArr, ptr %target, i64 0, i32 8`,
+        `  %next_present_ptr = getelementptr inbounds %ScrArr, ptr %next, i64 0, i32 8`,
+        `  %target_present = load ptr, ptr %target_present_ptr`,
+        `  %next_present = load ptr, ptr %next_present_ptr`,
+        `  store ptr %next_present, ptr %target_present_ptr`,
+        `  store ptr %target_present, ptr %next_present_ptr`,
         `  call void @scr_arr_release(ptr %next)`,
         `  br label %done`,
         `done:`,
@@ -10416,7 +10431,9 @@ class LlEmitter {
       const elem = t.elem;
       this.declare(`declare ptr @scr_dyn_new_arr()`);
       this.declare(`declare void @scr_dyn_arr_push(ptr, ptr)`);
+      this.declare(`declare void @scr_dyn_arr_push_hole(ptr)`);
       this.declare(`declare double @scr_arr_len(ptr)`);
+      this.declare(`declare zeroext i1 @scr_arr_has(ptr, double)`);
       const out = B.tmp();
       B.line(`${out} = call ptr @scr_dyn_new_arr()`);
       const len = B.tmp();
@@ -10426,6 +10443,9 @@ class LlEmitter {
       B.line(`store double ${f64Lit(0)}, ptr ${iSlot}`);
       const condition = B.newLabel("strm.c");
       const body = B.newLabel("strm.b");
+      const present = B.newLabel("strm.p");
+      const hole = B.newLabel("strm.h");
+      const next = B.newLabel("strm.n");
       const done = B.newLabel("strm.e");
       B.br(condition);
       B.startBlock(condition);
@@ -10435,6 +10455,13 @@ class LlEmitter {
       B.line(`${more} = fcmp olt double ${index}, ${len}`);
       B.condBr(more, body, done);
       B.startBlock(body);
+      const has = B.tmp();
+      B.line(`${has} = call zeroext i1 @scr_arr_has(ptr %p, double ${index})`);
+      B.condBr(has, present, hole);
+      B.startBlock(hole);
+      B.line(`call void @scr_dyn_arr_push_hole(ptr ${out})`);
+      B.br(next);
+      B.startBlock(present);
       let value: string;
       if (elem.kind === "f64" || elem.kind === "bool") {
         const valueTy = elem.kind === "f64" ? "double" : "i1";
@@ -10453,9 +10480,11 @@ class LlEmitter {
       if (isRefCounted(elem)) {
         B.line(`call void ${releaseSym(this, elem)}(ptr ${value})`);
       }
-      const next = B.tmp();
-      B.line(`${next} = fadd double ${index}, ${f64Lit(1)}`);
-      B.line(`store double ${next}, ptr ${iSlot}`);
+      B.br(next);
+      B.startBlock(next);
+      const nextIndex = B.tmp();
+      B.line(`${nextIndex} = fadd double ${index}, ${f64Lit(1)}`);
+      B.line(`store double ${nextIndex}, ptr ${iSlot}`);
       B.br(condition);
       B.startBlock(done);
       B.terminate(`ret ptr ${out}`);

packages/compiler/src/backend/llvm/walkers.ts

@@ -874,6 +874,7 @@ export class LlWalkers {
     host.declare(`declare void @scr_jb_init(ptr)`);
     host.declare(`declare ptr @scr_jb_finish(ptr)`);
     host.declare(`declare double @scr_arr_len(ptr)`);
+    host.declare(`declare zeroext i1 @scr_arr_has(ptr, double)`);
     host.declare(`declare ptr @scr_arr_get_ref(ptr, double)`);
     host.declare(`declare void @scr_union_release(ptr)`);
     host.declare(`declare void @scr_str_release(ptr)`);
@@ -900,13 +901,18 @@ export class LlWalkers {
     // `if (i) put sep bytes` — separators between elements only.
     const nz = B.tmp();
     const lsep = B.newLabel("uj.s");
-    const lel = B.newLabel("uj.v");
+    const lhas = B.newLabel("uj.h");
+    const lval = B.newLabel("uj.v");
     B.line(`${nz} = fcmp ogt double ${i}, ${f64Lit(0)}`);
-    B.condBr(nz, lsep, lel);
+    B.condBr(nz, lsep, lhas);
     B.startBlock(lsep);
     this.putScrStr(B, buf, "%sep");
-    B.br(lel);
-    B.startBlock(lel);
+    B.br(lhas);
+    B.startBlock(lhas);
+    const present = B.tmp();
+    B.line(`${present} = call i1 @scr_arr_has(ptr %a, double ${i})`);
+    B.condBr(present, lval, ln);
+    B.startBlock(lval);
     const u = B.tmp();
     B.line(`${u} = call ptr @scr_arr_get_ref(ptr %a, double ${i}) ; element (+1)`);
     if (unitTags.length > 0) {

packages/compiler/src/frontend/lowering/array-density.ts

@@ -0,0 +1,166 @@
+import * as ts from "../ts7/adapter.js";
+import type { IrType } from "../../ir/nodes.js";
+import type { Lowerer } from "./lowerer.js";
+
+/** A hole can be read as JavaScript undefined only when the element slot has
+ * an actual undefined value. */
+export function arrayElementCanRepresentUndefined(L: Lowerer, elem: IrType): boolean {
+  if (elem.kind === "jsval" || elem.kind === "dyn" || elem.kind === "undefinedT") return true;
+  return elem.kind === "union" &&
+    (L.unions.get(elem.unionId)?.arms.some((arm) => arm.kind === "undefinedT") ?? false);
+}
+
+export function requireArrayGetSafe(
+  L: Lowerer,
+  source: ts.Expression,
+  elem: IrType,
+  blame: ts.Node,
+  construct: string,
+): void {
+  if (arrayElementCanRepresentUndefined(L, elem) || !arrayExprIsProvablySparse(L, source)) return;
+  L.unsupported(
+    "SC1090",
+    blame,
+    `${construct} on a sparse '${L.fmt({ kind: "array", elem })}' value ` +
+      `(a hole must be observed as undefined, which '${L.fmt(elem)}' element slots cannot represent - ` +
+      "use an undefined-capable element type or a dense array)",
+  );
+}
+
+function stripWrappers(expr: ts.Expression): ts.Expression {
+  let value = expr;
+  while (
+    ts.isParenthesizedExpression(value) || ts.isNonNullExpression(value) ||
+    ts.isAsExpression(value) || ts.isTypeAssertion(value) || ts.isSatisfiesExpression(value)
+  ) {
+    value = value.expression;
+  }
+  return value;
+}
+
+function isAssignmentOperator(kind: ts.SyntaxKind): boolean {
+  return kind >= ts.SyntaxKind.FirstAssignment && kind <= ts.SyntaxKind.LastAssignment;
+}
+
+function isWriteTarget(node: ts.Expression): boolean {
+  let value = node;
+  let parent = value.parent;
+  while (
+    parent &&
+    (ts.isParenthesizedExpression(parent) || ts.isNonNullExpression(parent) ||
+      ts.isAsExpression(parent) || ts.isTypeAssertion(parent) || ts.isSatisfiesExpression(parent))
+  ) {
+    value = parent;
+    parent = parent.parent;
+  }
+  if (!parent) return false;
+  if (ts.isDeleteExpression(parent) && parent.expression === value) return true;
+  if (
+    (ts.isPrefixUnaryExpression(parent) || ts.isPostfixUnaryExpression(parent)) &&
+    parent.operand === value &&
+    (parent.operator === ts.SyntaxKind.PlusPlusToken || parent.operator === ts.SyntaxKind.MinusMinusToken)
+  ) {
+    return true;
+  }
+  return ts.isBinaryExpression(parent) && parent.left === value && isAssignmentOperator(parent.operatorToken.kind);
+}
+
+const READ_ONLY_ARRAY_METHODS = new Set([
+  "at", "concat", "entries", "every", "filter", "find", "findIndex", "findLast",
+  "findLastIndex", "flat", "flatMap", "forEach", "includes", "indexOf", "join", "keys",
+  "lastIndexOf", "map", "reduce", "reduceRight", "slice", "some", "toReversed", "toSorted",
+  "toSpliced", "values", "with",
+]);
+
+function symbolUsePreservesSparsity(L: Lowerer, node: ts.Identifier): boolean {
+  const parent = node.parent;
+  if (
+    (ts.isVariableDeclaration(parent) || ts.isParameter(parent) || ts.isBindingElement(parent) ||
+      ts.isFunctionDeclaration(parent)) && parent.name === node
+  ) {
+    return true;
+  }
+  if (ts.isPropertyAccessExpression(parent) && parent.expression === node) {
+    if (isWriteTarget(parent)) return false;
+    if (parent.name.text === "length") return true;
+    return ts.isCallExpression(parent.parent) && parent.parent.expression === parent &&
+      READ_ONLY_ARRAY_METHODS.has(parent.name.text);
+  }
+  if (ts.isElementAccessExpression(parent) && parent.expression === node) {
+    return !isWriteTarget(parent);
+  }
+  if ((ts.isForOfStatement(parent) || ts.isForInStatement(parent)) && parent.expression === node) return true;
+  if (ts.isSpreadElement(parent) && parent.expression === node) return true;
+  if (ts.isBinaryExpression(parent)) {
+    if (parent.right === node && parent.operatorToken.kind === ts.SyntaxKind.InKeyword) return true;
+    return !isAssignmentOperator(parent.operatorToken.kind) &&
+      (parent.operatorToken.kind === ts.SyntaxKind.EqualsEqualsToken ||
+        parent.operatorToken.kind === ts.SyntaxKind.ExclamationEqualsToken ||
+        parent.operatorToken.kind === ts.SyntaxKind.EqualsEqualsEqualsToken ||
+        parent.operatorToken.kind === ts.SyntaxKind.ExclamationEqualsEqualsToken);
+  }
+  return parent.kind === ts.SyntaxKind.TypeQuery;
+}
+
+function symbolUsesPreserveSparsity(
+  L: Lowerer,
+  symbol: ts.Symbol,
+  file: ts.SourceFile,
+  before: number,
+): boolean {
+  let safe = true;
+  const visit = (node: ts.Node): void => {
+    if (!safe) return;
+    if (
+      ts.isIdentifier(node) && node.pos < before &&
+      node.text === symbol.name && L.resolveValueSymbol(node) === symbol
+    ) {
+      safe = symbolUsePreservesSparsity(L, node);
+      if (!safe) return;
+    }
+    node.forEachChild(visit);
+  };
+  file.forEachChild(visit);
+  return safe;
+}
+
+function isSparseArrayInitializer(L: Lowerer, source: ts.Expression): boolean {
+  const value = stripWrappers(source);
+  if (ts.isArrayLiteralExpression(value)) return value.elements.some(ts.isOmittedExpression);
+  if (!ts.isCallExpression(value) && !ts.isNewExpression(value)) return false;
+  const callee = value.expression;
+  if (
+    !ts.isIdentifier(callee) || callee.text !== "Array" ||
+    !L.isStdlibSymbol(L.resolveValueSymbol(callee) ?? undefined)
+  ) {
+    return false;
+  }
+  const args = value.arguments ?? [];
+  if (args.length !== 1) return false;
+  const length = stripWrappers(args[0]!);
+  return ts.isNumericLiteral(length) && Number.isInteger(Number(length.text)) && Number(length.text) > 0;
+}
+
+/** Prove only the narrow case needed to fence unrepresentable hole reads.
+ * Unknown provenance deliberately keeps the historical lowering behavior. */
+function arrayExprIsProvablySparse(L: Lowerer, source: ts.Expression): boolean {
+  const value = stripWrappers(source);
+  if (isSparseArrayInitializer(L, value)) return true;
+  if (!ts.isIdentifier(value)) return false;
+  const symbol = L.resolveValueSymbol(value);
+  if (!symbol) return false;
+  const declarations = L.checker.declarationsOf(symbol);
+  const declaration = declarations.length === 1 ? declarations[0] : undefined;
+  if (!declaration || !ts.isVariableDeclaration(declaration) || !declaration.initializer) return false;
+  const list = declaration.parent;
+  const statement = list.parent;
+  if (
+    !ts.isVariableDeclarationList(list) || list.declarations.length !== 1 ||
+    !ts.isVariableStatement(statement) ||
+    (ts.getCombinedModifierFlags(statement) & ts.ModifierFlags.Export) !== 0 ||
+    declaration.end > value.pos || !isSparseArrayInitializer(L, declaration.initializer)
+  ) {
+    return false;
+  }
+  return symbolUsesPreserveSparsity(L, symbol, declaration.getSourceFile(), value.pos);
+}

packages/compiler/src/frontend/lowering/lower-assert.ts

@@ -1340,15 +1340,26 @@ function deepEqHelper(L: Lowerer, t: IrType, loc: SrcLoc): string {
     case "array": {
       const len = (arr: IrExpr): IrExpr => ({ kind: "arrIntrinsic", method: "length", receiver: arr, args: [], type: F64, loc });
       const at = (arr: IrExpr, i: IrExpr): IrExpr => ({ kind: "arrayGet", arr, index: i, type: t.elem, loc });
+      const has = (arr: IrExpr, i: IrExpr): IrExpr => ({ kind: "arrayHas", arr, index: i, type: BOOL, loc });
+      const i = (): IrExpr => ref("i.0", F64);
       locals.push({ id: "i.0", name: "i", type: F64, mutable: true });
       body = [
         bailIf(neq(len(a()), len(b()))),
         {
           kind: "for",
           init: { kind: "varDecl", localId: "i.0", init: num(0), loc },
-          cond: { kind: "bin", op: "<", left: ref("i.0", F64), right: len(a()), type: BOOL, loc },
-          update: { kind: "assign", localId: "i.0", value: { kind: "bin", op: "+", left: ref("i.0", F64), right: num(1), type: F64, loc }, loc },
-          body: [bailIf(not(deq(t.elem, at(a(), ref("i.0", F64)), at(b(), ref("i.0", F64)))))],
+          cond: { kind: "bin", op: "<", left: i(), right: len(a()), type: BOOL, loc },
+          update: { kind: "assign", localId: "i.0", value: { kind: "bin", op: "+", left: i(), right: num(1), type: F64, loc }, loc },
+          body: [
+            bailIf(neq(has(a(), i()), has(b(), i()))),
+            {
+              kind: "if",
+              cond: has(a(), i()),
+              then: [bailIf(not(deq(t.elem, at(a(), i()), at(b(), i()))))],
+              else_: null,
+              loc,
+            },
+          ],
           loc,
         },
         ret(boolLit(true)),

packages/compiler/src/frontend/lowering/lower-builtins.ts

@@ -32,6 +32,7 @@ import { CRYPTO_CIPHERS, CRYPTO_CONSTANTS, CRYPTO_CURVES, CRYPTO_HASHES } from "
 import { timerStyleCallback } from "./lower-calls.js";
 import { registerHttpClientFnBinding } from "./lower-server.js";
 import { BOOL, BYTES_U8, CHILD_T, CHILDSTREAM_T, DYN, F64, FSWATCHER_T, PROCSTREAM_T, IrExpr, IrFunction, IrLibFn, IrLocal, IrStmt, IrType, JSVAL, NULL_T, SEARCH_PARAMS_T, SPAWNRES_T, STRING, SrcLoc, UNDEFINED_T, VOID, arrayOf, canBoxFuncIntoDyn, canConvertToDyn, funcOf, isUnitType, typeEquals, typeKey } from "../../ir/nodes.js";
+import { requireArrayGetSafe } from "./array-density.js";
 
 
 
@@ -1326,6 +1327,10 @@ function optionMember(p: ts.ObjectLiteralElementLike): { name: string; value: ts
       if (spread) {
         if (expr.arguments.length === 1) {
           // The whole-array form forwards the operand directly (no copy).
+          const sourceType = L.mapTypeOf(L.typeOf(spread.expression));
+          if (sourceType?.kind === "array") {
+            requireArrayGetSafe(L, spread.expression, sourceType.elem, spread, "array argument spread");
+          }
           const packed = L.lowerExprExpecting(spread.expression, arrayOf(STRING));
           return { kind: "libCall", fn: fn.fn, args: [packed], type: fn.result, loc };
         }
@@ -1333,6 +1338,13 @@ function optionMember(p: ts.ObjectLiteralElementLike): { name: string; value: ts
         // tmpdir.resolve: plain arguments and spread arrays pack into one
         // fresh string[] (arrayLit's spread positions copy element-wise,
         // JS-exact; a dyn spread source rides the validated extraction).
+        for (const arg of expr.arguments) {
+          if (!ts.isSpreadElement(arg)) continue;
+          const sourceType = L.mapTypeOf(L.typeOf(arg.expression));
+          if (sourceType?.kind === "array") {
+            requireArrayGetSafe(L, arg.expression, sourceType.elem, arg, "array argument spread");
+          }
+        }
         const elems = expr.arguments.map((a) =>
           ts.isSpreadElement(a)
             ? L.lowerExprExpecting(a.expression, arrayOf(STRING))
@@ -6415,6 +6427,7 @@ type TextCodecCtor = {
         return { kind: "libCall", fn: "string.fromCharCode", args: [packed], type: STRING, loc };
       }
       const packed = L.lowerExprExpecting(spread.expression, arrayOf(F64));
+      requireArrayGetSafe(L, spread.expression, F64, spread, "array argument spread");
       return { kind: "libCall", fn: "string.fromCharCode", args: [packed], type: STRING, loc };
     }
     const elems = call.arguments.map((a) => L.lowerExprExpecting(a, F64));

packages/compiler/src/frontend/lowering/lower-calls.ts

@@ -27,6 +27,7 @@ import { ambientNsRootOf, ambientUndefReadType, ambientUndefVarRootOf, ambientUn
 import { declSymbolOf } from "./lower-modules.js";
 import { expandoMemberRead } from "./lower-expando.js";
 import { npmStaticPackageOfPath } from "../npm-static.js";
+import { requireArrayGetSafe } from "./array-density.js";
 
 /** How a parameter participates in CALL-SITE COMPLETION (the frontend
  * completes every call to the one full signature, so the IR and backends
@@ -479,6 +480,10 @@ export interface GenericInstance {
               `spreading '${L.fmt(src.type)}' into a '${L.fmt(restType)}' rest parameter (only a same-element-type array spreads)`,
             );
           }
+          const sourceType = L.mapTypeOf(L.typeOf(a.expression));
+          if (sourceType?.kind === "array") {
+            requireArrayGetSafe(L, a.expression, sourceType.elem, a, "array argument spread");
+          }
           spreads.push(i);
           return src;
         }

packages/compiler/src/frontend/lowering/lower-containers.ts

@@ -12,6 +12,7 @@ import { isJsSourceFile, locOf } from "../program.js";
 import { DYN_DISPATCH_METHODS, islandPrimitiveExit } from "./lower-calls.js";
 import { typeKey } from "../types.js";
 import { dynUndefinedExpr, own, WidthLift } from "./lowerer.js";
+import { requireArrayGetSafe } from "./array-density.js";
 
 /** Lower an expression whose checker type is statically `undefined`/`void`.
  * Optional builtin arguments use this before their ordinary expected-type
@@ -190,6 +191,7 @@ function lowerOptionalDefaultArg(
       if (call.arguments.length !== 0) {
         L.noLowering(`.toReversed with ${call.arguments.length} arguments`, call);
       }
+      requireArrayGetSafe(L, access.expression, elem, call, "Array.toReversed");
       return {
         kind: "arrIntrinsic",
         method: "toReversed",
@@ -203,6 +205,7 @@ function lowerOptionalDefaultArg(
       if (call.arguments.length !== 2 || call.arguments.some(ts.isSpreadElement)) {
         L.noLowering(`.with with ${call.arguments.length} arguments`, call);
       }
+      requireArrayGetSafe(L, access.expression, elem, call, "Array.with");
       const receiver = L.lowerExpr(access.expression);
       const index = L.lowerExprExpecting(call.arguments[0]!, F64);
       const value = L.coerceInto(
@@ -223,6 +226,7 @@ function lowerOptionalDefaultArg(
       if (call.arguments.some(ts.isSpreadElement)) {
         L.unsupported("SC1090", call, "spread arguments to Array.toSpliced");
       }
+      requireArrayGetSafe(L, access.expression, elem, call, "Array.toSpliced");
       const receiver = L.lowerExpr(access.expression);
       const start = call.arguments[0]
         ? L.lowerExprExpecting(call.arguments[0], F64)
@@ -320,6 +324,9 @@ function lowerOptionalDefaultArg(
             `pushing a spread of '${L.fmt(src.type)}' onto a '${L.fmt(receiverIr)}' array (only a same-element-type array spreads)`,
           );
         }
+        if (L.mapTypeOf(L.typeOf(spreadArg.expression))?.kind === "array") {
+          requireArrayGetSafe(L, spreadArg.expression, elem, spreadArg, "array argument spread");
+        }
         return { kind: "arrIntrinsic", method: "pushSpread", receiver, args: [src], type: F64, loc };
       }
       // Pushed values flow into the element slot like an assignment would:
@@ -1015,6 +1022,9 @@ function lowerOptionalDefaultArg(
     const loc = locOf(call);
     const receiver = L.lowerExpr(access.expression);
     const arrT = arrayOf(elem);
+    if (method !== "some" && method !== "every") {
+      requireArrayGetSafe(L, access.expression, elem, call, `Array.${method}`);
+    }
     const argNode = call.arguments[0];
     if (!argNode) L.unsupported("SC1090", call, "this call form"); // tsc-guarded
     const { fnArg, arity } = hofCallbackArg(L, argNode, [elem], arrT);
@@ -1583,6 +1593,7 @@ function lowerOptionalDefaultArg(
     const loc = locOf(call);
     const method = access.name.text === "toSorted" ? "toSorted" : "sort";
     const copyFirst = method === "toSorted";
+    if (copyFirst) requireArrayGetSafe(L, access.expression, elem, call, "Array.toSorted");
     if (call.arguments.length === 0 && elem.kind === "string") {
       const receiver = L.lowerExpr(access.expression);
       const cmp = defaultStringCmpHelper(L, loc);
@@ -1691,14 +1702,16 @@ function lowerOptionalDefaultArg(
     return name;
   }
 
-/** The insertion-sort loop, from existing IR nodes. Present elements compact
-   * to the front first; CompareArrayElements then sinks undefined past every
-   * value without calling the comparator, so the order is values, then
-   * undefineds. The tail beyond the present count is deleted for the
-   * in-place sort (holes land last, SortIndexedProperties' SKIP-HOLES) and
-   * assigned the interned undefined arm for toSorted (`undefExpr`, dense
-   * per READ-THROUGH-HOLES); a toSorted element type with no undefined arm
-   * cannot represent a read-through hole and keeps the holes.
+/** The insertion-sort loop, from existing IR nodes. Both methods first take a
+   * shallow working snapshot. Present elements compact within that private
+   * array; CompareArrayElements then sinks undefined past every value without
+   * calling the comparator, so the order is values, then undefineds. sort
+   * commits the sorted prefix and hole tail to the receiver only after every
+   * comparator call succeeds. This preserves the original receiver during
+   * comparator observation and leaves it untouched when a comparator throws.
+   * toSorted returns the working array directly; its hole tail becomes present
+   * undefined (the call site fences scalar receivers unless they are proven
+   * dense).
    *
    *   n = a.length;
     *   for (i = 1; i < n; i++) {
@@ -1728,7 +1741,9 @@ function lowerOptionalDefaultArg(
     const i = ref("i.0", F64);
     const j = ref("j.0", F64);
     const m = ref("m.0", F64);
-    const at = (index: IrExpr): IrExpr => ({ kind: "arrayGet", arr: ref("a.0", arrT), index, type: elem, loc });
+    const working = (): IrExpr => ref("w.0", arrT);
+    const at = (index: IrExpr): IrExpr => ({ kind: "arrayGet", arr: working(), index, type: elem, loc });
+    const hasWorking: IrExpr = { kind: "arrayHas", arr: working(), index: i, type: BOOL, loc };
     const jPlus1: IrExpr = { kind: "bin", op: "+", left: j, right: num(1), type: F64, loc };
     const isUndefined = (value: IrExpr): IrExpr | null => {
       if (elem.kind === "union" && undefinedTag !== null) {
@@ -1806,7 +1821,7 @@ function lowerOptionalDefaultArg(
           kind: "if",
           cond: shouldShift,
           then: [
-            { kind: "arraySet", arr: ref("a.0", arrT), index: jPlus1, value: at(j), loc },
+            { kind: "arraySet", arr: working(), index: jPlus1, value: at(j), loc },
             { kind: "assign", localId: "j.0", value: { kind: "bin", op: "-", left: j, right: num(1), type: F64, loc }, loc },
           ],
           else_: [{ kind: "break", loc }],
@@ -1816,28 +1831,26 @@ function lowerOptionalDefaultArg(
       loc,
     };
     const body: IrStmt[] = [
-      ...(copyFirst
-        ? [{
-            kind: "assign" as const,
-            localId: "a.0",
-            value: {
-              kind: "arrIntrinsic" as const,
-              method: "slice" as const,
-              receiver: ref("a.0", arrT),
-              args: [],
-              type: arrT,
-              loc,
-            },
-            loc,
-          }]
-        : []),
       readLenStmt(arrT, loc),
+      {
+        kind: "varDecl",
+        localId: "w.0",
+        init: {
+          kind: "arrIntrinsic",
+          method: "slice",
+          receiver: ref("a.0", arrT),
+          args: [],
+          type: arrT,
+          loc,
+        },
+        loc,
+      },
       { kind: "varDecl", localId: "m.0", init: num(0), loc },
       countedForLoop(loc, [{
         kind: "if",
-        cond: hasElemExpr(arrT, loc),
+        cond: hasWorking,
         then: [
-          { kind: "arraySet", arr: ref("a.0", arrT), index: m, value: at(i), loc },
+          { kind: "arraySet", arr: working(), index: m, value: at(i), loc },
           { kind: "assign", localId: "m.0", value: { kind: "bin", op: "+", left: m, right: num(1), type: F64, loc }, loc },
         ],
         else_: null,
@@ -1850,8 +1863,8 @@ function lowerOptionalDefaultArg(
         update: { kind: "assign", localId: "i.0", value: { kind: "bin", op: "+", left: i, right: num(1), type: F64, loc }, loc },
         body: [
           copyFirst && undefExpr !== null
-            ? { kind: "arraySet", arr: ref("a.0", arrT), index: i, value: undefExpr, loc }
-            : { kind: "arrayDelete", arr: ref("a.0", arrT), index: i, loc },
+            ? { kind: "arraySet", arr: working(), index: i, value: undefExpr, loc }
+            : { kind: "arrayDelete", arr: working(), index: i, loc },
         ],
         loc,
       },
@@ -1866,14 +1879,44 @@ function lowerOptionalDefaultArg(
           loc,
         },
         body: [
-          { kind: "varDecl", localId: "v.0", init: getElemExpr(arrT, elem, loc), loc },
+          { kind: "varDecl", localId: "v.0", init: at(i), loc },
           { kind: "varDecl", localId: "j.0", init: { kind: "bin", op: "-", left: ref("i.0", F64), right: num(1), type: F64, loc }, loc },
           shiftLoop,
-          { kind: "arraySet", arr: ref("a.0", arrT), index: jPlus1, value: ref("v.0", elem), loc },
+          { kind: "arraySet", arr: working(), index: jPlus1, value: ref("v.0", elem), loc },
         ],
         loc,
       },
-      { kind: "return", value: ref("a.0", arrT), loc },
+      ...(copyFirst
+        ? []
+        : [
+            {
+              kind: "for" as const,
+              init: { kind: "varDecl" as const, localId: "i.0", init: num(0), loc },
+              cond: { kind: "bin" as const, op: "<" as const, left: i, right: m, type: BOOL, loc },
+              update: {
+                kind: "assign" as const,
+                localId: "i.0",
+                value: { kind: "bin" as const, op: "+" as const, left: i, right: num(1), type: F64, loc },
+                loc,
+              },
+              body: [{ kind: "arraySet" as const, arr: ref("a.0", arrT), index: i, value: at(i), loc }],
+              loc,
+            },
+            {
+              kind: "for" as const,
+              init: { kind: "varDecl" as const, localId: "i.0", init: m, loc },
+              cond: { kind: "bin" as const, op: "<" as const, left: i, right: ref("n.0", F64), type: BOOL, loc },
+              update: {
+                kind: "assign" as const,
+                localId: "i.0",
+                value: { kind: "bin" as const, op: "+" as const, left: i, right: num(1), type: F64, loc },
+                loc,
+              },
+              body: [{ kind: "arrayDelete" as const, arr: ref("a.0", arrT), index: i, loc }],
+              loc,
+            },
+          ]),
+      { kind: "return", value: copyFirst ? working() : ref("a.0", arrT), loc },
     ];
     return {
       name,
@@ -1885,6 +1928,7 @@ function lowerOptionalDefaultArg(
       locals: [
         { id: "a.0", name: "a", type: arrT, mutable: true },
         { id: "f.0", name: "f", type: fnT, mutable: true },
+        { id: "w.0", name: "w", type: arrT, mutable: false },
         { id: "n.0", name: "n", type: F64, mutable: false },
         { id: "i.0", name: "i", type: F64, mutable: true },
         { id: "m.0", name: "m", type: F64, mutable: true },
@@ -2392,6 +2436,13 @@ function lowerOptionalDefaultArg(
           else_: null,
           loc,
         },
+        {
+          kind: "if",
+          cond: { kind: "unary", op: "!", operand: { kind: "arrayHas", arr: ref("a.0", arrT), index: t, type: BOOL, loc }, type: BOOL, loc },
+          then: [{ kind: "return", value: miss, loc }],
+          else_: null,
+          loc,
+        },
         {
           kind: "varDecl",
           localId: "v.0",

packages/compiler/src/frontend/lowering/lower-exprs.ts

@@ -25,6 +25,7 @@ import { mixinFnOfCallee } from "./lower-mixins.js";
 import { isConstAssertionTypeNode, isGenericCallableMemberType, underConstAssertion, unitOnlyUnion } from "../types.js";
 import { lowerYield } from "./lower-generators.js";
 import { lowerStreamProperty, lowerStreamStateProperty, streamSidesOf } from "./lower-stream.js";
+import { requireArrayGetSafe } from "./array-density.js";
 
 /** An assignable `obj.field` target — a class field, a record field, or a
  * class ACCESSOR property (reads become getter calls, writes setter calls;
@@ -3680,6 +3681,10 @@ export function lowerOptionalChain(L: Lowerer, expr: ts.CallExpression | ts.Prop
             `spreading '${L.fmt(src.type)}' into a '${L.fmt(type)}' literal (only a same-element-type array spreads)`,
           );
         }
+        const sourceType = L.mapTypeOf(L.typeOf(el.expression));
+        if (sourceType?.kind === "array") {
+          requireArrayGetSafe(L, el.expression, sourceType.elem, el, "array spread");
+        }
         spreads.push(i);
         return src;
       }

packages/compiler/src/frontend/lowering/lower-island.ts

@@ -10,6 +10,7 @@ import { requiresDynamicApiDiag, requiresDynamicPackageDiag } from "../../diagno
 import { isCjsJsFile, isJsSourceFile, locOf, npmPackageNameOf } from "../program.js";
 import { foldedStringKeyOf, lowerDynObjectLiteral, pureReemittable } from "./lower-exprs.js";
 import { PoisonError, dynUndefinedExpr, newFnCtx, nodeThrowExpr, own } from "./lowerer.js";
+import { requireArrayGetSafe } from "./array-density.js";
 import {
   NODE24_FETCH_COMPAT_PROFILE,
   STATIC_HEADERS_CALLS,
@@ -2947,6 +2948,7 @@ export function lowerStaticReadableStreamReaderCall(
           `spreading '${L.fmt(src.type)}' into Math.${name} (only a number[] spreads)`,
         );
       }
+      requireArrayGetSafe(L, spread.expression, F64, spread, `array spread into Math.${name}`);
       return {
         kind: "libCall",
         fn: name === "max" ? "math.maxArr" : "math.minArr",

packages/compiler/src/frontend/lowering/lower-stmts.ts

@@ -27,6 +27,7 @@ import { UNSUPPORTED, checkerPanicDiag, isCheckerPanic, requiresDynamicDiag } fr
 import { isUnitOnlyTsType, unitOnlyUnion } from "../types.js";
 import { canonicalBuiltinModule, isRelativeSpecifier } from "../shared.js";
 import { probeNodeRequireRefusal } from "../npm.js";
+import { requireArrayGetSafe } from "./array-density.js";
 
 /** `const X = /* @__PURE__ *\/ makeX()` at a module's top level: every
  * declarator initialized by a call (or `new`) carrying the bundler PURE
@@ -997,6 +998,9 @@ export function lowerStmt(L: Lowerer, stmt: ts.Statement): IrStmt | IrStmt[] | n
     ) {
       L.badType(sourceNode, L.typeOf(sourceNode));
     }
+    if (source.type.kind === "array") {
+      requireArrayGetSafe(L, sourceNode, source.type.elem, decl.initializer!, "stored array value iteration");
+    }
     const loc = locOf(decl);
     const indexedSource = L.declareHiddenLocal("%numiterSource", source.type);
     const index = L.declareHiddenLocal("%numiterIndex", F64);
@@ -5898,6 +5902,15 @@ function isEsModuleStamp(expr: ts.Expression): boolean {
         if (recv?.kind === "array" && (proj === "keys" || proj === "entries")) {
           const container = L.lowerExpr(src.expression.expression);
           if (container.type.kind === "array") {
+            if (proj === "entries") {
+              requireArrayGetSafe(
+                L,
+                src.expression.expression,
+                container.type.elem,
+                stmt.expression,
+                "Array.entries iteration",
+              );
+            }
             return lowerForOfArrayIter(L, stmt, container as IrExpr & { type: IrType & { kind: "array" } }, proj);
           }
         }
@@ -6102,6 +6115,7 @@ function isEsModuleStamp(expr: ts.Expression): boolean {
         L.badType(stmt.expression, L.typeOf(stmt.expression));
       }
     }
+    requireArrayGetSafe(L, iterSrc, iterable.type.elem, stmt.expression, "array for-of iteration");
     if (!ts.isVariableDeclarationList(stmt.initializer)) {
       // `for (x of xs)` over a PRE-DECLARED writable binding: JS assigns
       // the existing binding once per pass — one shared binding across

packages/compiler/src/ir/dense-arrays.ts

@@ -88,7 +88,10 @@ function markFunction(fn: IrFunction): void {
         case "exprStmt": expr(s.expr); break;
         case "arraySet": {
           const id = directCandidate(s.arr);
-          if (id === null) expr(s.arr);
+          // A dynamic index can now grow past length and introduce holes.
+          // Without a range proof, later reads must inspect presence.
+          if (id !== null) invalidate(id);
+          else expr(s.arr);
           expr(s.index);
           expr(s.value);
           break;

packages/compiler/src/ir/nodes.ts

@@ -1155,10 +1155,10 @@ export type IrStmt =
       labels?: string[];
       loc: SrcLoc;
     }
-  /** Element write `a[i] = v` — statement-only, like `assign`. Valid indices
-   * are [0, length]; i == length appends (JS would create a hole past that —
-   * scriptc traps instead, see SEMANTICS.md). Ownership of a refcounted
-   * value MOVES into the array; the replaced element is released. */
+  /** Element write `a[i] = v` — statement-only, like `assign`. Valid array
+   * indices are [0, 2^32-2]; writes at or beyond length grow through i + 1
+   * and leave intermediate holes. Ownership of a refcounted value MOVES
+   * into the array; the replaced element is released. */
   | { kind: "arraySet"; arr: IrExpr; index: IrExpr; value: IrExpr; loc: SrcLoc }
   /** `delete a[i]`: clears presence without changing length. */
   | { kind: "arrayDelete"; arr: IrExpr; index: IrExpr; loc: SrcLoc }

packages/compiler/test/ir.test.ts

@@ -208,11 +208,17 @@ test("dense-array pass bypasses hole checks only for untouched packed locals", (
         name: "holey", params: [], returnType: F64, locals: [{ id: "a.0", name: "a", type: arr, mutable: false }], loc,
         body: [{ kind: "varDecl", localId: "a.0", init: { kind: "arrayLit", elems: [{ kind: "numLit", value: 1, type: F64, loc }], type: arr, loc }, loc }, { kind: "arrayDelete", arr: { kind: "varRef", localId: "a.0", type: arr, loc }, index: { kind: "numLit", value: 0, type: F64, loc }, loc }, { kind: "return", value: get("a.0"), loc }],
       },
+      {
+        name: "written", params: [], returnType: F64, locals: [{ id: "a.0", name: "a", type: arr, mutable: false }], loc,
+        body: [{ kind: "varDecl", localId: "a.0", init: { kind: "arrayLit", elems: [{ kind: "numLit", value: 1, type: F64, loc }], type: arr, loc }, loc }, { kind: "arraySet", arr: { kind: "varRef", localId: "a.0", type: arr, loc }, index: { kind: "numLit", value: 4, type: F64, loc }, value: { kind: "numLit", value: 5, type: F64, loc }, loc }, { kind: "return", value: get("a.0"), loc }],
+      },
     ],
   };
   markDenseArrayReads(mod);
   const dense = mod.functions[0]!.body[1]!;
   const holey = mod.functions[1]!.body[2]!;
+  const written = mod.functions[2]!.body[2]!;
   expect(dense.kind === "return" && dense.value?.kind === "arrayGet" && dense.value.dense).toBe(true);
   expect(holey.kind === "return" && holey.value?.kind === "arrayGet" && holey.value.dense).toBeFalsy();
+  expect(written.kind === "return" && written.value?.kind === "arrayGet" && written.value.dense).toBeFalsy();
 });

packages/compiler/test/ts7/baselines/order-parity.json

@@ -5482,12 +5482,36 @@
    ],
    "diags": []
   },
+  "<repo>/tests/corpus/2672-http-request-response-callback.ts": {
+   "order": [
+    "<repo>/tests/corpus/2672-http-request-response-callback.ts"
+   ],
+   "diags": []
+  },
   "<repo>/tests/corpus/2673-sparse-array-js.js": {
    "order": [
     "<repo>/tests/corpus/2673-sparse-array-js.js"
    ],
    "diags": []
   },
+  "<repo>/tests/corpus/2684-sparse-live-array-payload.ts": {
+   "order": [
+    "<repo>/tests/corpus/2684-sparse-live-array-payload.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/2693-sparse-far-index-copy.ts": {
+   "order": [
+    "<repo>/tests/corpus/2693-sparse-far-index-copy.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/2695-sparse-frontend-review-fixes.ts": {
+   "order": [
+    "<repo>/tests/corpus/2695-sparse-frontend-review-fixes.ts"
+   ],
+   "diags": []
+  },
   "<repo>/tests/corpus/300-if-else.ts": {
    "order": [
     "<repo>/tests/corpus/300-if-else.ts"
@@ -7461,6 +7485,12 @@
    ],
    "diags": []
   },
+  "<repo>/tests/diagnostics/sparse-get-scalar-fences.ts": {
+   "order": [
+    "<repo>/tests/diagnostics/sparse-get-scalar-fences.ts"
+   ],
+   "diags": []
+  },
   "<repo>/tests/diagnostics/spi-off/main.ts": {
    "order": [
     "<repo>/tests/diagnostics/spi-off/main.ts"

packages/runtime/src/scr_array.c

@@ -16,9 +16,9 @@ static void scr_arr_oom(void) {
   scr_trap("scriptc: out of memory\n");
 }
 
-/* JS would return undefined for an OOB read and create holes for a far OOB
- * write; both are unrepresentable here (see SEMANTICS.md), so any invalid
- * index — negative, fractional, NaN, or past the allowed end — traps. */
+/* Reads outside length and values that are not array indices remain
+ * unrepresentable. Writes accept every JS array index (through 2^32-2),
+ * growing length and creating holes as needed. */
 static void scr_arr_trap_oob(double i, size_t len) {
   char buf[32];
   scr_f64_to_str(i, buf);
@@ -26,12 +26,10 @@ static void scr_arr_trap_oob(double i, size_t len) {
                buf, len);
 }
 
-/* Validate i as an element index. limit is a->len for reads, a->len + 1 for
- * writes (i == len appends). NaN fails the >= 0 test; fractional indices
- * fail the trunc test. */
-static size_t scr_arr_check_index(const ScrArr *a, double i, bool allow_append) {
-  size_t limit = a->len + (allow_append ? 1 : 0);
-  if (!(i >= 0) || i != trunc(i) || i >= (double)limit) {
+/* 2^32-1 is the maximum array length and therefore not an array index. */
+static size_t scr_arr_check_index(const ScrArr *a, double i, bool write) {
+  double limit = write ? 4294967295.0 : (double)a->len;
+  if (!(i >= 0) || i != trunc(i) || i >= limit) {
     scr_arr_trap_oob(i, a->len);
   }
   return (size_t)i;
@@ -95,6 +93,16 @@ static bool scr_arr_present_at(const ScrArr *a, size_t i) {
   return !a->present || (a->present[i >> 3] & (uint8_t)(1u << (i & 7))) != 0;
 }
 
+static void scr_arr_make_sparse(ScrArr *a) {
+  if (a->present) return;
+  size_t bytes = (a->cap + 7) / 8;
+  a->present = calloc(bytes ? bytes : 1, 1);
+  if (!a->present) scr_arr_oom();
+  for (size_t i = 0; i < a->len; i++) {
+    a->present[i >> 3] |= (uint8_t)(1u << (i & 7));
+  }
+}
+
 static void scr_arr_mark_present(ScrArr *a, size_t i, bool present) {
   if (!a->present) {
     if (present) return;
@@ -233,12 +241,7 @@ void scr_arr_set_sparse_len(ScrArr *a, double n, void *fill) {
   }
   if (len == a->len) return;
   scr_arr_grow(a, len);
-  if (!a->present) {
-    size_t bytes = (a->cap + 7) / 8;
-    a->present = calloc(bytes ? bytes : 1, 1);
-    if (!a->present) scr_arr_oom();
-    for (size_t i = 0; i < a->len; i++) scr_arr_mark_present(a, i, true);
-  }
+  scr_arr_make_sparse(a);
   uint64_t slot = scr_slot_from_ptr(fill);
   for (size_t i = a->len; i < len; i++) {
     a->data[i] = slot;
@@ -360,15 +363,35 @@ void *scr_arr_get_dense_ref(ScrArr *a, double i) {
   return p;
 }
 
-/* ── writes: i == len appends ──────────────────────────────────────────── */
+/* ── writes: far indices grow length and create holes ──────────────────── */
 
-static void scr_arr_set_slot(ScrArr *a, double i, uint64_t slot) {
+static void scr_arr_set_slot(ScrArr *a, double i, uint64_t slot,
+                             uint64_t hole) {
   size_t idx = scr_arr_check_index(a, i, true);
-  if (idx == a->len) {
-    scr_arr_grow(a, a->len + 1);
-    a->len++;
+  if (idx >= a->len) {
+    size_t old_len = a->len;
+    /* An already-sparse REF array can carry the undefined union singleton
+     * in its holes. Extend with the same inert backing when the caller did
+     * not supply the static element type's undefined arm. */
+    if (idx > old_len && hole == 0 && a->present && a->elem == SCR_ELEM_REF) {
+      for (size_t j = 0; j < old_len; j++) {
+        if (!scr_arr_present_at(a, j)) {
+          hole = a->data[j];
+          break;
+        }
+      }
+    }
+    scr_arr_grow(a, idx + 1);
+    if (idx > old_len) {
+      scr_arr_make_sparse(a);
+      for (size_t j = old_len; j < idx; j++) {
+        a->data[j] = hole;
+        scr_arr_mark_present(a, j, false);
+      }
+    }
     a->data[idx] = slot;
     scr_arr_mark_present(a, idx, true);
+    a->len = idx + 1;
     return;
   }
   /* Unlink-then-release: a release can trigger a cycle collection, which
@@ -382,15 +405,19 @@ static void scr_arr_set_slot(ScrArr *a, double i, uint64_t slot) {
 }
 
 void scr_arr_set_f64(ScrArr *a, double i, double v) {
-  scr_arr_set_slot(a, i, scr_slot_from_f64(v));
+  scr_arr_set_slot(a, i, scr_slot_from_f64(v), 0);
 }
 
 void scr_arr_set_bool(ScrArr *a, double i, bool v) {
-  scr_arr_set_slot(a, i, (uint64_t)(v ? 1 : 0));
+  scr_arr_set_slot(a, i, (uint64_t)(v ? 1 : 0), 0);
 }
 
 void scr_arr_set_ref(ScrArr *a, double i, void *v) {
-  scr_arr_set_slot(a, i, scr_slot_from_ptr(v));
+  scr_arr_set_slot(a, i, scr_slot_from_ptr(v), 0);
+}
+
+void scr_arr_set_ref_fill(ScrArr *a, double i, void *v, void *fill) {
+  scr_arr_set_slot(a, i, scr_slot_from_ptr(v), scr_slot_from_ptr(fill));
 }
 
 /* ── push / pop ────────────────────────────────────────────────────────── */

packages/runtime/src/scr_copying.c

@@ -44,29 +44,15 @@ static bool copying_arr_present_at(const ScrArr *a, size_t i) {
   return !a->present || (a->present[i >> 3] & (uint8_t)(1u << (i & 7))) != 0;
 }
 
-static void copying_arr_mark_hole(ScrArr *a, size_t i) {
-  if (!a->present) {
-    size_t bytes = (a->cap + 7) / 8;
-    a->present = malloc(bytes ? bytes : 1);
-    if (!a->present) scr_trap("scriptc: out of memory\n");
-    memset(a->present, 0xff, bytes);
-    if (a->cap & 7) a->present[bytes - 1] &= (uint8_t)((1u << (a->cap & 7)) - 1u);
-  }
-  a->present[i >> 3] &= (uint8_t)~(1u << (i & 7));
-}
-
 /* The ES2023 copying methods read every index (Get), so a source hole
- * densifies to its backing where that carries the value Get answers — the
- * interned undefined arm of a union element (nonzero). A zero backing
- * (f64/bool/plain ref) has no undefined to give: the slot stays a hole so
- * reads keep the deliberate trap instead of silently answering 0/NULL. */
+ * densifies to the undefined value carried in a REF hole's backing slot.
+ * Scalar and NULL-backed holes cannot encode undefined in their typed ABI;
+ * fail explicitly rather than fabricating 0/NULL or preserving the hole. */
 static void copying_arr_copy_slot(ScrArr *out, const ScrArr *src, size_t i) {
   uint64_t slot = src->data[i];
-  if (!copying_arr_present_at(src, i) && slot == 0) {
-    out->data[out->len] = 0;
-    copying_arr_mark_hole(out, out->len);
-    out->len++;
-    return;
+  if (!copying_arr_present_at(src, i) &&
+      (src->elem != SCR_ELEM_REF || slot == 0)) {
+    scr_trap("scriptc: TypeError: array hole cannot be represented as undefined by the element type\n");
   }
   out->data[out->len++] = copying_arr_retain_slot(src, slot);
 }
@@ -95,31 +81,25 @@ ScrArr *scr_arr_to_spliced(const ScrArr *a, double start,
   size_t out_len = a->len - ndelete + items->len;
   ScrArr *out = copying_arr_new_like(a, out_len);
   for (size_t i = 0; i < from; i++) copying_arr_copy_slot(out, a, i);
-  for (size_t i = 0; i < items->len; i++) {
-    out->data[out->len++] =
-        copying_arr_retain_slot(a, items->data[i]);
-  }
+  for (size_t i = 0; i < items->len; i++)
+    copying_arr_copy_slot(out, items, i);
   for (size_t i = from + ndelete; i < a->len; i++) {
     copying_arr_copy_slot(out, a, i);
   }
   return out;
 }
 
-/* `with` builds on slice (which preserves holes); densify the copy under
- * the same representability rule as copying_arr_copy_slot. */
-static void copying_arr_densify(ScrArr *out) {
-  if (!out->present) return;
-  bool all_present = true;
-  for (size_t i = 0; i < out->len; i++) {
-    uint8_t mask = (uint8_t)(1u << (i & 7));
-    if (out->present[i >> 3] & mask) continue;
-    if (out->data[i] != 0) out->present[i >> 3] |= mask;
-    else all_present = false;
-  }
-  if (all_present) {
-    free(out->present);
-    out->present = NULL;
+static ScrArr *copying_arr_with_slot(ScrArr *a, size_t index,
+                                     uint64_t replacement) {
+  ScrArr *out = copying_arr_new_like(a, a->len);
+  for (size_t i = 0; i < a->len; i++) {
+    if (i == index) {
+      out->data[out->len++] = copying_arr_retain_slot(a, replacement);
+    } else {
+      copying_arr_copy_slot(out, a, i);
+    }
   }
+  return out;
 }
 
 static bool copying_arr_with_index(const ScrArr *a, double index,
@@ -142,30 +122,21 @@ static bool copying_arr_with_index(const ScrArr *a, double index,
 ScrArr *scr_arr_with_f64(ScrArr *a, double index, double value) {
   size_t i;
   if (!copying_arr_with_index(a, index, &i)) return NULL;
-  ScrArr *out = scr_arr_slice(a, 0, INFINITY);
-  copying_arr_densify(out);
-  scr_arr_set_f64(out, (double)i, value);
-  return out;
+  uint64_t slot;
+  memcpy(&slot, &value, sizeof slot);
+  return copying_arr_with_slot(a, i, slot);
 }
 
 ScrArr *scr_arr_with_bool(ScrArr *a, double index, bool value) {
   size_t i;
   if (!copying_arr_with_index(a, index, &i)) return NULL;
-  ScrArr *out = scr_arr_slice(a, 0, INFINITY);
-  copying_arr_densify(out);
-  scr_arr_set_bool(out, (double)i, value);
-  return out;
+  return copying_arr_with_slot(a, i, value ? 1 : 0);
 }
 
 ScrArr *scr_arr_with_ref(ScrArr *a, double index, void *value) {
   size_t i;
   if (!copying_arr_with_index(a, index, &i)) return NULL;
-  ScrArr *out = scr_arr_slice(a, 0, INFINITY);
-  copying_arr_densify(out);
-  uint64_t retained =
-      copying_arr_retain_slot(a, copying_slot_from_ptr(value));
-  scr_arr_set_ref(out, (double)i, copying_slot_to_ptr(retained));
-  return out;
+  return copying_arr_with_slot(a, i, copying_slot_from_ptr(value));
 }
 
 ScrBytes *scr_bytes_to_reversed(const ScrBytes *b) {

packages/runtime/src/scr_json.c

@@ -2695,7 +2695,7 @@ void scr_sc_validate_options(const ScrDyn *options) {
   if (tr == NULL || tr->kind == SCR_DYN_UNDEF) return;
   if (tr->kind != SCR_DYN_ARR) {
     static const char msg[] =
-        "Failed to execute 'structuredClone': transfer in Options can not be converted to sequence.";
+        "Failed to execute 'structuredClone': transfer in Options cannot be converted to sequence.";
     scr_throw_error_msg_code(SCR_ERR_TYPE, msg, sizeof msg - 1, "ERR_INVALID_ARG_TYPE");
     return;
   }

packages/runtime/src/scr_runtime.h

@@ -719,10 +719,10 @@ ScrStr *scr_b64_missing_arg(void);
  * layout is untouched (interned-literal statics depend on it), so the
  * element kind lives here instead of in a shared object header.
  *
- * Index arguments are doubles (JS numbers). A valid index is a non-negative
- * integer; reads must be < len, writes may also be == len (append — JS
- * would create a hole past the end; scriptc traps instead, see
- * SEMANTICS.md). Anything else prints
+ * Index arguments are doubles (JS numbers). A valid array index is an
+ * integer in [0, 2^32-2]. Reads must also be < len; writes grow length to
+ * index + 1 and create holes between the old end and the written slot.
+ * Anything else prints
  *   scriptc: RangeError: array index <i> out of bounds (length <n>)
  * to stderr and abort()s. pop() on an empty array likewise traps (JS
  * returns undefined — unrepresentable here).
@@ -825,20 +825,26 @@ double scr_arr_get_dense_f64(ScrArr *a, double i);
 bool scr_arr_get_dense_bool(ScrArr *a, double i);
 void *scr_arr_get_dense_ref(ScrArr *a, double i);
 
-/* i == len appends; the _ref variant releases the old element (when
- * replacing) and takes ownership of the new one. */
+/* A write at or beyond len grows through i + 1, leaving intermediate slots
+ * as holes. The _ref variants release the old element (when replacing) and
+ * take ownership of the new one. _ref_fill stores fill as inert backing for
+ * new holes, allowing a statically known undefined union arm to answer Get. */
 void scr_arr_set_f64(ScrArr *a, double i, double v);
 void scr_arr_set_bool(ScrArr *a, double i, bool v);
 void scr_arr_set_ref(ScrArr *a, double i, void *v);
+void scr_arr_set_ref_fill(ScrArr *a, double i, void *v, void *fill);
 
 /* slice(start?, end?): a fresh +1 shallow copy of the index range —
  * ToIntegerOrInfinity indices, negatives from the end, clamping; ref
  * elements retain into the copy. Borrows a. */
 ScrArr *scr_arr_slice(ScrArr *a, double start, double end);
 
-/* ES2023 copying methods. All borrow their inputs and return fresh +1
- * shallow copies. with() raises Node's catchable RangeError for an invalid
- * relative index; the ref variant retains the borrowed replacement. */
+/* ES2023 copying methods. All borrow their inputs and return fresh packed +1
+ * shallow copies; source holes densify to present undefined values via a
+ * REF array's non-NULL hole backing. A hole whose typed element ABI cannot
+ * encode undefined traps rather than becoming 0/NULL. with() raises Node's
+ * catchable RangeError for an invalid relative index; the ref variant
+ * retains the borrowed replacement. */
 ScrArr *scr_arr_to_reversed(const ScrArr *a);
 ScrArr *scr_arr_to_spliced(const ScrArr *a, double start,
                            double delete_count, const ScrArr *items);

packages/runtime/src/scr_web.c

@@ -1212,7 +1212,7 @@ static const char web_prelude[] =
     "          if (typeof tr === 'string') throw 0;\n"
     "          list = [...tr];\n"
     "        } catch (e) {\n"
-    "          throw scErr(\"Failed to execute 'structuredClone': transfer in Options can not be converted to sequence.\");\n"
+    "          throw scErr(\"Failed to execute 'structuredClone': transfer in Options cannot be converted to sequence.\");\n"
     "        }\n"
     "        if (list.length > 0) {\n"
     "          throw new DOMException('Found invalid value in transferList.', 'DataCloneError');\n"

packages/runtime/test/array.test.ts

@@ -19,6 +19,7 @@ beforeAll(async () => {
     "-o", bin,
     join(testDir, "test_array.c"),
     join(testDir, "../src/scr_array.c"),
+    join(testDir, "../src/scr_copying.c"),
     join(testDir, "../src/scr_string.c"),
     join(testDir, "../src/scr_number.c"),
     join(testDir, "../src/scr_bytes.c"),
@@ -35,13 +36,12 @@ test("array runtime: push/pop/set/get, RC recursion, growth", async () => {
   expect(stderr.trim()).toMatch(/^(\d+)\/\1 cases passed$/);
 });
 
-// JS returns undefined for OOB reads and creates holes for far writes; both
-// are unrepresentable, so the runtime must trap (documented divergence).
 test.each([
-  ["--crash-get-oob", "array index 1 out of bounds (length 1)"],
-  ["--crash-get-frac", "array index 0.5 out of bounds (length 1)"],
-  ["--crash-set-oob", "array index 2 out of bounds (length 1)"],
-  ["--crash-pop-empty", "pop() on an empty array"],
+  ["--crash-get-oob", "RangeError: array index 1 out of bounds (length 1)"],
+  ["--crash-get-frac", "RangeError: array index 0.5 out of bounds (length 1)"],
+  ["--crash-set-max", "RangeError: array index 4294967295 out of bounds (length 1)"],
+  ["--crash-pop-empty", "RangeError: pop() on an empty array"],
+  ["--crash-copy-hole", "TypeError: array hole cannot be represented as undefined by the element type"],
 ])("trap aborts (%s)", async (mode, message) => {
   const err = await execFileAsync(bin, [mode]).then(
     () => {
@@ -50,5 +50,5 @@ test.each([
     (e: Error & { signal?: string; stderr?: string }) => e,
   );
   expect(err.signal).toBe("SIGABRT");
-  expect(err.stderr).toContain(`scriptc: RangeError: ${message}`);
+  expect(err.stderr).toContain(`scriptc: ${message}`);
 });

packages/runtime/test/test_array.c

@@ -4,8 +4,9 @@
  *   (no args)          run all assertions; prints "N/N cases passed"
  *   --crash-get-oob    read past the end        → RangeError + abort()
  *   --crash-get-frac   read a fractional index  → RangeError + abort()
- *   --crash-set-oob    write past len (holes)   → RangeError + abort()
+ *   --crash-set-max    write at 2^32-1          → RangeError + abort()
  *   --crash-pop-empty  pop an empty array       → RangeError + abort()
+ *   --crash-copy-hole  densify a scalar hole    → TypeError + abort()
  *
  * The RC-recursion cases (array of strings, array of arrays of strings)
  * assert live counts directly: releasing the outer array must release
@@ -436,6 +437,30 @@ static void test_sparse(void) {
   scr_arr_release(slice);
   scr_arr_release(a);
 
+  ScrArr *far = scr_arr_new(SCR_ELEM_F64, 0);
+  scr_arr_push_f64(far, 1);
+  scr_arr_set_f64(far, 4, 5);
+  check_f64(scr_arr_len(far), 5, "far indexed write grows length");
+  check(scr_arr_has(far, 0) && !scr_arr_has(far, 1) &&
+            !scr_arr_has(far, 2) && !scr_arr_has(far, 3) &&
+            scr_arr_has(far, 4),
+        "far indexed write creates intermediate holes");
+  check_f64(scr_arr_get_f64(far, 0), 1, "far write preserves prefix");
+  check_f64(scr_arr_get_f64(far, 4), 5, "far write stores endpoint");
+  scr_arr_set_f64(far, 5, 6);
+  check_f64(scr_arr_len(far), 6, "write at sparse length appends");
+  check(!scr_arr_has(far, 3) && scr_arr_has(far, 5),
+        "sparse append preserves existing holes");
+  scr_arr_release(far);
+
+  ScrArr *far_refs = scr_arr_new(SCR_ELEM_STR, 0);
+  scr_arr_push_ref(far_refs, scr_str_new("a", 1));
+  scr_arr_set_ref(far_refs, 3, scr_str_new("d", 1));
+  check_f64(scr_arr_len(far_refs), 4, "far ref write grows length");
+  check(!scr_arr_has(far_refs, 1) && !scr_arr_has(far_refs, 2),
+        "far ref write leaves unowned holes");
+  scr_arr_release(far_refs);
+
   ScrArr *dst = scr_arr_new(SCR_ELEM_F64, 0);
   ScrArr *src = scr_arr_new(SCR_ELEM_F64, 1);
   scr_arr_push_f64(src, 1);
@@ -453,6 +478,76 @@ static void test_sparse(void) {
 #endif
 }
 
+static void test_copying_holes(void) {
+  long live0 = mock_live;
+  MockRec *undefined_fill = mock_new(-1);
+  ScrArr *src = scr_arr_new_ref(&mock_retain, &mock_release, NULL, 0);
+  scr_arr_push_ref(src, mock_new(1));
+  scr_arr_set_sparse_len(src, 4, undefined_fill);
+  scr_arr_set_ref(src, 2, mock_new(3));
+
+  ScrArr *reversed = scr_arr_to_reversed(src);
+  check(reversed->present == NULL, "toReversed densifies represented holes");
+  check(scr_arr_has(reversed, 0) && scr_arr_has(reversed, 1) &&
+            scr_arr_has(reversed, 2) && scr_arr_has(reversed, 3),
+        "toReversed result has every index");
+  MockRec *v = scr_arr_get_ref(reversed, 0);
+  check(v == undefined_fill, "toReversed reads a hole as undefined backing");
+  mock_release(v);
+  check(undefined_fill->rc == 3,
+        "toReversed retains each densified undefined backing");
+  scr_arr_release(reversed);
+  check(undefined_fill->rc == 1,
+        "toReversed releases densified undefined backings");
+
+  ScrArr *items = scr_arr_new_ref(&mock_retain, &mock_release, NULL, 0);
+  scr_arr_push_ref(items, mock_new(2));
+  ScrArr *spliced = scr_arr_to_spliced(src, 1, 1, items);
+  check(spliced->present == NULL, "toSpliced densifies kept holes");
+  check_f64(scr_arr_len(spliced), 4, "toSpliced keeps expected length");
+  v = scr_arr_get_ref(spliced, 3);
+  check(v == undefined_fill, "toSpliced reads a kept hole as undefined backing");
+  mock_release(v);
+  check(undefined_fill->rc == 2,
+        "toSpliced retains a kept undefined backing");
+  scr_arr_release(spliced);
+  check(undefined_fill->rc == 1,
+        "toSpliced releases a kept undefined backing");
+
+  MockRec *replacement = mock_new(9);
+  ScrArr *with = scr_arr_with_ref(src, 0, replacement);
+  check(with->present == NULL, "with densifies represented holes");
+  check(undefined_fill->rc == 3, "with retains densified undefined backings");
+  check(replacement->rc == 2, "with retains the borrowed replacement");
+  scr_arr_release(with);
+  check(undefined_fill->rc == 1, "with releases densified undefined backings");
+  check(replacement->rc == 1, "with releases its replacement reference");
+  mock_release(replacement);
+
+  ScrArr *plain = scr_arr_new(SCR_ELEM_F64, 0);
+  scr_arr_push_f64(plain, 1);
+  scr_arr_set_f64(plain, 2, 3);
+  ScrArr *no_items = scr_arr_new(SCR_ELEM_F64, 0);
+  ScrArr *deleted = scr_arr_to_spliced(plain, 1, 1, no_items);
+  check(deleted->present == NULL && scr_arr_len(deleted) == 2,
+        "toSpliced does not Get a deleted unrepresentable hole");
+  check_f64(scr_arr_get_f64(deleted, 0), 1, "toSpliced deleted-hole prefix");
+  check_f64(scr_arr_get_f64(deleted, 1), 3, "toSpliced deleted-hole suffix");
+  ScrArr *replaced = scr_arr_with_f64(plain, 1, 2);
+  check(replaced->present == NULL && scr_arr_has(replaced, 1),
+        "with does not Get its replaced hole");
+  check_f64(scr_arr_get_f64(replaced, 1), 2, "with stores over a scalar hole");
+  scr_arr_release(replaced);
+  scr_arr_release(deleted);
+  scr_arr_release(no_items);
+  scr_arr_release(plain);
+
+  scr_arr_release(items);
+  scr_arr_release(src);
+  mock_release(undefined_fill);
+  check(mock_live == live0, "copying methods balance ref elements");
+}
+
 int main(int argc, char **argv) {
   if (argc > 1) {
     ScrArr *a = scr_arr_new(SCR_ELEM_F64, 0);
@@ -461,11 +556,15 @@ int main(int argc, char **argv) {
       scr_arr_get_f64(a, 1); /* len is 1 */
     } else if (strcmp(argv[1], "--crash-get-frac") == 0) {
       scr_arr_get_f64(a, 0.5);
-    } else if (strcmp(argv[1], "--crash-set-oob") == 0) {
-      scr_arr_set_f64(a, 2, 9); /* would create a hole */
+    } else if (strcmp(argv[1], "--crash-set-max") == 0) {
+      scr_arr_set_f64(a, 4294967295.0, 9);
     } else if (strcmp(argv[1], "--crash-pop-empty") == 0) {
       scr_arr_pop_f64(a);
       scr_arr_pop_f64(a); /* now empty */
+    } else if (strcmp(argv[1], "--crash-copy-hole") == 0) {
+      scr_arr_set_sparse_len(a, 2, NULL);
+      ScrArr *copy = scr_arr_to_reversed(a);
+      scr_arr_release(copy);
     } else {
       fprintf(stderr, "unknown mode %s\n", argv[1]);
       return 2;
@@ -481,6 +580,7 @@ int main(int argc, char **argv) {
   test_index_of_includes();
   test_ref_elements();
   test_ref_cycle();
+  test_copying_holes();
   test_join();
   test_sparse();
 

tests/corpus/2283-structured-clone.cjs

@@ -8,7 +8,7 @@ const assert = require('assert');
 assert.throws(() => structuredClone(), { code: 'ERR_MISSING_ARGS' });
 assert.throws(() => structuredClone(undefined, ''), { code: 'ERR_INVALID_ARG_TYPE', message: "Failed to execute 'structuredClone': Options cannot be converted to a dictionary" });
 assert.throws(() => structuredClone(undefined, 1), { code: 'ERR_INVALID_ARG_TYPE' });
-assert.throws(() => structuredClone(undefined, { transfer: 1 }), { code: 'ERR_INVALID_ARG_TYPE', message: "Failed to execute 'structuredClone': transfer in Options can not be converted to sequence." });
+assert.throws(() => structuredClone(undefined, { transfer: 1 }), { code: 'ERR_INVALID_ARG_TYPE', message: /Failed to execute 'structuredClone': transfer in Options can ?not be converted to sequence\./ });
 assert.throws(() => structuredClone(undefined, { transfer: null }), { code: 'ERR_INVALID_ARG_TYPE' });
 assert.strictEqual(structuredClone(undefined), undefined);
 assert.strictEqual(structuredClone(undefined, null), undefined);

tests/corpus/2684-sparse-live-array-payload.ts

@@ -0,0 +1,42 @@
+// Live Web references must carry an array's hole bitmap through dynamic
+// materialization, mutation commits, and cached structural-cast refreshes.
+
+const materializedSource: number[] = new Array(4);
+materializedSource[1] = 11;
+materializedSource[3] = 33;
+const materialized: unknown = AbortSignal.abort(materializedSource).reason;
+if (Array.isArray(materialized)) {
+  console.log(
+    "materialized",
+    materialized.length,
+    Object.keys(materialized).join(","),
+    JSON.stringify(materialized),
+  );
+}
+
+const committedSource: number[] = new Array(3);
+committedSource[0] = 1;
+committedSource[2] = 3;
+const committed: unknown = AbortSignal.abort(committedSource).reason;
+if (Array.isArray(committed)) committed[1] = 2;
+console.log(
+  "committed",
+  committedSource.length,
+  Object.keys(committedSource).join(","),
+  JSON.stringify(committedSource),
+);
+
+const refreshedSource: number[] = new Array(4);
+refreshedSource[1] = 10;
+const refreshValue: unknown = AbortSignal.abort(refreshedSource).reason;
+const first = refreshValue as (number | string)[];
+delete refreshedSource[1];
+refreshedSource[2] = 20;
+const second = refreshValue as (number | string)[];
+console.log(
+  "refreshed",
+  first === second,
+  second.length,
+  Object.keys(second).join(","),
+  JSON.stringify(second),
+);

tests/corpus/2693-sparse-far-index-copy.ts

@@ -0,0 +1,47 @@
+// Far indexed writes grow ordinary arrays and leave intermediate holes.
+const numbers = [1];
+numbers[4] = 5;
+console.log(
+  "numbers",
+  numbers.length,
+  0 in numbers,
+  1 in numbers,
+  3 in numbers,
+  4 in numbers,
+  numbers.join(","),
+);
+
+const words = ["a"];
+words[3] = "d";
+console.log("words", words.length, 1 in words, 2 in words, words.join("|"));
+
+// A far write on an initially empty optional REF array gives each new hole
+// the undefined arm as backing. Join skips the holes, while copying methods
+// Get and densify them as present undefined values.
+const optional: (number | undefined)[] = [];
+optional[2] = 7;
+console.log(
+  "optional",
+  optional.length,
+  0 in optional,
+  1 in optional,
+  2 in optional,
+  optional.join(","),
+);
+const reversed = optional.toReversed();
+console.log(
+  "copy",
+  reversed.length,
+  0 in reversed,
+  1 in reversed,
+  2 in reversed,
+  reversed[0],
+  reversed[1] === undefined,
+  reversed[2] === undefined,
+);
+
+// Join must not read a hole even when its REF representation has no
+// undefined arm and therefore leaves zero backing.
+const mixed: (number | string)[] = [];
+mixed[2] = "x";
+console.log("mixed", mixed.join("|"));

tests/corpus/2695-sparse-frontend-review-fixes.ts

@@ -0,0 +1,79 @@
+import assert from "node:assert";
+
+// Static deep equality compares indexed-property presence before values. Two
+// scalar holes never reach an unrepresentable arrayGet, while a hole remains
+// distinct from a present undefined in an undefined-capable array.
+const scalarHolesA: number[] = new Array(3);
+const scalarHolesB: number[] = new Array(3);
+assert.deepStrictEqual(scalarHolesA, scalarHolesB);
+scalarHolesB[1] = 0;
+assert.notDeepStrictEqual(scalarHolesA, scalarHolesB);
+
+const unionHole: (number | undefined)[] = new Array(1);
+const unionUndefined: (number | undefined)[] = new Array(1);
+unionUndefined[0] = undefined;
+assert.notDeepStrictEqual(unionHole, unionUndefined);
+console.log("deep-presence", Object.keys(scalarHolesA).length, 0 in unionHole, 0 in unionUndefined);
+
+// In-place sort works privately until all comparisons succeed. Comparator
+// observation sees the original receiver, and a throw commits no writes.
+const observed = [3, 1, 2];
+let receiverStayedOriginal = true;
+observed.sort((a, b) => {
+  receiverStayedOriginal = receiverStayedOriginal && observed.join(",") === "3,1,2";
+  return a - b;
+});
+console.log("sort-observe", receiverStayedOriginal, observed.join(","));
+
+const throwing = [4, 2, 3];
+let caught = false;
+try {
+  throwing.sort((_a, _b): number => {
+    throw new Error("stop sort");
+  });
+} catch (error) {
+  caught = error instanceof Error && error.message === "stop sort";
+}
+console.log("sort-throw", caught, throwing.join(","));
+
+const sparseSort: number[] = new Array(5);
+sparseSort[1] = 3;
+sparseSort[3] = 1;
+const sparseKeys = Object.keys(sparseSort).join(",");
+try {
+  sparseSort.sort((_a, _b): number => {
+    throw new Error("sparse stop");
+  });
+} catch {}
+console.log("sparse-throw", Object.keys(sparseSort).join(",") === sparseKeys, sparseSort.length);
+sparseSort.sort((a, b) => a - b);
+console.log("sparse-commit", sparseSort.join(","), Object.keys(sparseSort).join(","), sparseSort.length);
+
+// at has an undefined-capable result even when the element slot is scalar,
+// so a presence miss returns that result arm without touching the slot.
+const scalarAt: number[] = new Array(2);
+scalarAt[1] = 7;
+console.log("at-hole", scalarAt.at(0) === undefined, scalarAt.at(1), scalarAt.at(-2) === undefined);
+
+// Undefined-capable element storage supports Get-based callback visits to
+// holes directly.
+const searchable: (number | undefined)[] = new Array(3);
+searchable[2] = 5;
+let visits = "";
+const holeIndex = searchable.findIndex((value, index) => {
+  visits += `${index}${value === undefined ? "u" : "v"}`;
+  return value === undefined;
+});
+const lastHole = searchable.findLastIndex((value) => value === undefined);
+console.log("find-holes", holeIndex, lastHole, visits);
+
+// Packed provenance discharges scalar Get fences through a direct parameter,
+// for-of, spread, and toSorted.
+function firstEven(values: number[]): number {
+  return values.findIndex((value) => value % 2 === 0);
+}
+const packed = [3, 2, 1];
+let packedSum = 0;
+for (const value of packed) packedSum += value;
+const packedCopy = [...packed];
+console.log("packed", firstEven(packed), packedSum, packedCopy.join(","), packed.toSorted((a, b) => a - b).join(","));

tests/diagnostics/sparse-get-scalar-fences.ts

@@ -0,0 +1,44 @@
+const scalarHoles: number[] = new Array(3);
+// Direct Get-based methods cannot materialize undefined in number slots.
+scalarHoles.find((value) => value > 0);
+scalarHoles.findIndex((value) => value > 0);
+scalarHoles.findLast((value) => value > 0);
+scalarHoles.findLastIndex((value) => value > 0);
+// Iteration and spread have the same read-through-holes behavior.
+for (const value of scalarHoles) console.log(value);
+const copied = [...scalarHoles];
+console.log(copied.length);
+
+function takeAll(...values: number[]): number {
+  return values.length;
+}
+takeAll(...scalarHoles);
+Math.max(...scalarHoles);
+String.fromCharCode(...scalarHoles);
+// Copying sort reads through holes and produces present undefined entries.
+scalarHoles.toSorted((a, b) => a - b);
+// A later write cannot make this earlier Get safe.
+const writtenLater = new Array<number>(2);
+writtenLater.find((value) => value > 0);
+writtenLater[0] = 1;
+
+// Dense construction is safe even with scalar element storage.
+const denseLiteral = [3, 1, 2];
+denseLiteral.find((value) => value > 0);
+for (const value of denseLiteral) console.log(value);
+console.log([...denseLiteral].length, denseLiteral.toSorted((a, b) => a - b).length);
+
+const filled = new Array<number>(3);
+filled[0] = 0;
+filled[1] = 0;
+filled[2] = 0;
+filled.find((value) => value > 0);
+for (const value of filled) console.log(value);
+console.log([...filled].length, filled.toSorted((a, b) => a - b).length);
+
+// Parameters and other unknown provenance retain historical lowering.
+function readUnknown(values: number[]): void {
+  values.find((value) => value > 0);
+  for (const value of values) console.log(value);
+  console.log([...values].length, values.toSorted((a, b) => a - b).length);
+}

tests/harness/__snapshots__/sparse-get-scalar-fences.ts.txt

@@ -0,0 +1,76 @@
+sparse-get-scalar-fences.ts:3:1 - error SC1090: Array.find on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  2 | // Direct Get-based methods cannot materialize undefined in number slots.
+  3 | scalarHoles.find((value) => value > 0);
+    | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  4 | scalarHoles.findIndex((value) => value > 0);
+
+sparse-get-scalar-fences.ts:4:1 - error SC1090: Array.findIndex on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  3 | scalarHoles.find((value) => value > 0);
+  4 | scalarHoles.findIndex((value) => value > 0);
+    | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  5 | scalarHoles.findLast((value) => value > 0);
+
+sparse-get-scalar-fences.ts:5:1 - error SC1090: Array.findLast on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  4 | scalarHoles.findIndex((value) => value > 0);
+  5 | scalarHoles.findLast((value) => value > 0);
+    | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  6 | scalarHoles.findLastIndex((value) => value > 0);
+
+sparse-get-scalar-fences.ts:6:1 - error SC1090: Array.findLastIndex on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  5 | scalarHoles.findLast((value) => value > 0);
+  6 | scalarHoles.findLastIndex((value) => value > 0);
+    | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  7 | // Iteration and spread have the same read-through-holes behavior.
+
+sparse-get-scalar-fences.ts:8:21 - error SC1090: array for-of iteration on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  7 | // Iteration and spread have the same read-through-holes behavior.
+  8 | for (const value of scalarHoles) console.log(value);
+    |                     ^~~~~~~~~~~
+  9 | const copied = [...scalarHoles];
+
+sparse-get-scalar-fences.ts:9:17 - error SC1090: array spread on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+   8 | for (const value of scalarHoles) console.log(value);
+   9 | const copied = [...scalarHoles];
+     |                 ^~~~~~~~~~~~~~
+  10 | console.log(copied.length);
+
+sparse-get-scalar-fences.ts:15:9 - error SC1090: array argument spread on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  14 | }
+  15 | takeAll(...scalarHoles);
+     |         ^~~~~~~~~~~~~~
+  16 | Math.max(...scalarHoles);
+
+sparse-get-scalar-fences.ts:16:10 - error SC1090: array spread into Math.max on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  15 | takeAll(...scalarHoles);
+  16 | Math.max(...scalarHoles);
+     |          ^~~~~~~~~~~~~~
+  17 | String.fromCharCode(...scalarHoles);
+
+sparse-get-scalar-fences.ts:17:21 - error SC1090: array argument spread on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  16 | Math.max(...scalarHoles);
+  17 | String.fromCharCode(...scalarHoles);
+     |                     ^~~~~~~~~~~~~~
+  18 | // Copying sort reads through holes and produces present undefined entries.
+
+sparse-get-scalar-fences.ts:19:1 - error SC1090: Array.toSorted on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  18 | // Copying sort reads through holes and produces present undefined entries.
+  19 | scalarHoles.toSorted((a, b) => a - b);
+     | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  20 | // A later write cannot make this earlier Get safe.
+
+sparse-get-scalar-fences.ts:22:1 - error SC1090: Array.find on a sparse 'number[]' value (a hole must be observed as undefined, which 'number' element slots cannot represent - use an undefined-capable element type or a dense array) is not supported yet
+
+  21 | const writtenLater = new Array<number>(2);
+  22 | writtenLater.find((value) => value > 0);
+     | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  23 | writtenLater[0] = 1;

tests/harness/web-globals.test.ts

@@ -129,7 +129,7 @@ describe(`island web globals (scriptc-only${sanitize ? ", sanitized" : ""})`, ()
         "DataCloneError:t:true",
         'TypeError/ERR_MISSING_ARGS: The "The value argument must be specified" argument must be specified',
         "TypeError/ERR_INVALID_ARG_TYPE: Failed to execute 'structuredClone': Options cannot be converted to a dictionary",
-        "TypeError/ERR_INVALID_ARG_TYPE: Failed to execute 'structuredClone': transfer in Options can not be converted to sequence.",
+        "TypeError/ERR_INVALID_ARG_TYPE: Failed to execute 'structuredClone': transfer in Options cannot be converted to sequence.",
         "true",
         "DataCloneError/25: () => {} could not be cloned.",
       ].join(" | "),