diff --git a/packages/compiler/src/backend/llvm/dyn.ts b/packages/compiler/src/backend/llvm/dyn.ts
index 125203bf3..9ba8c3c87 100644
--- a/packages/compiler/src/backend/llvm/dyn.ts
+++ b/packages/compiler/src/backend/llvm/dyn.ts
@@ -66,6 +66,25 @@ import { recordMayKeepSource, recordSourceFieldOrder } from "./record-sources.js
 import type { WalkerHost } from "./walkers.js";
 import type { LlStreamTypedRefAdapter } from "./expr-context.js";
 
+/** typeKey with `?` after every `(T | undefined)[]` (optionalElements),
+ * at any depth of arrays, Maps, Sets and promises: checked conversions to
+ * `number[][]` and `(number | undefined)[][]` need separate helpers, though
+ * the two types share a typeKey. */
+function checkedConversionKey(t: IrType): string {
+  switch (t.kind) {
+    case "array":
+      return `array<${checkedConversionKey(t.elem)}>${t.optionalElements ? "?" : ""}`;
+    case "map":
+      return `map<${checkedConversionKey(t.key)},${checkedConversionKey(t.value)}>`;
+    case "set":
+      return `set<${checkedConversionKey(t.elem)}>`;
+    case "promise":
+      return `promise<${checkedConversionKey(t.inner)}>`;
+    default:
+      return typeKey(t);
+  }
+}
+
 /** Whether to-dyn of a record shape keeps the record itself: an empty shape
  * and an event-listener record (a `handleEvent` function field) box their
  * identity. Every other shape, tuples included, copies its fields into a
@@ -759,6 +778,13 @@ export class LlDyn {
   /* ── dyn node plumbing ─────────────────────────────────────────────── */
 
   /** Loads a dyn node's kind tag (after the size_t rc word). */
+  /** i1: the checked-dynamic value `d` is undefined. */
+  isUndefined(B: BlockBuilder, d: string): string {
+    const is = B.tmp();
+    B.line(`${is} = icmp eq i32 ${this.kindOf(B, d)}, ${DYN_KIND.UNDEF}`);
+    return is;
+  }
+
   private kindOf(B: BlockBuilder, d: string): string {
     const p = B.tmp();
     const k = B.tmp();
@@ -1145,7 +1171,7 @@ export class LlDyn {
 
   /** `sc_dm_<n>(ptr d) -> i1` — does this dyn fit T? Never throws. */
   dynMatchHelper(t: IrType, preserveRefs = false): string {
-    const key = typeKey(t);
+    const key = checkedConversionKey(t);
     const matcherKey = preserveRefs ? `identity:${key}` : key;
     const existing = this.dynMatchers.get(matcherKey);
     if (existing) return existing;
@@ -1460,7 +1486,7 @@ export class LlDyn {
         B.startBlock(l0);
         const n = this.lenOf(B, "%d");
         const items = this.itemsOf(B, "%d");
-        this.i64Loop(B, "dm.el", n, (i, brNext) => {
+        this.i64Loop(B, "dm.el", n, (i, brNext, next) => {
           this.host.declare(`declare zeroext i1 @scr_dyn_arr_has_index(ptr, ${this.S})`);
           const present = B.tmp();
           B.line(`${present} = call zeroext i1 @scr_dyn_arr_has_index(ptr %d, ${this.S} ${i})`);
@@ -1471,6 +1497,15 @@ export class LlDyn {
           brNext();
           B.startBlock(valueLabel);
           const e = this.itemAt(B, items, i);
+          if (t.optionalElements) {
+            // A `(T | undefined)[]` stored as T elements takes undefined
+            // elements as well.
+            const undef = B.tmp();
+            const checkLabel = B.newLabel("dm.check");
+            B.line(`${undef} = icmp eq i32 ${this.kindOf(B, e)}, ${DYN_KIND.UNDEF}`);
+            B.condBr(undef, next, checkLabel);
+            B.startBlock(checkLabel);
+          }
           const ok = B.tmp();
           B.line(`${ok} = call zeroext i1 @${m}(ptr ${e})`);
           const lOk = B.newLabel("dm.eo");
@@ -1946,7 +1981,8 @@ export class LlDyn {
    * BUILD the typed value (+1), or throw the catchable path-annotated
    * TypeError and return a dummy with the pending flag set. */
   dynCheckHelper(t: IrType, preserveRefs = false): string {
-    const key = typeKey(t);
+    // A `(T | undefined)[]` stored as T elements also takes undefined ones.
+    const key = checkedConversionKey(t);
     const builderKey = preserveRefs ? `identity:${key}` : key;
     const existing = this.dynBuilders.get(builderKey);
     if (existing) return existing;
@@ -2753,6 +2789,21 @@ export class LlDyn {
           B.line(`${ip} = getelementptr inbounds %ScrDynPath, ptr ${pathSlot}, i64 0, i32 2`);
           B.line(`store ${host.sizeType} ${i}, ptr ${ip}`);
           const e = this.itemAt(B, items, i);
+          if (t.optionalElements) {
+            // An undefined element is a present undefined element.
+            const undef = B.tmp();
+            const storeUndefined = B.newLabel("dca.undef");
+            const checkLabel = B.newLabel("dca.check");
+            B.line(`${undef} = icmp eq i32 ${this.kindOf(B, e)}, ${DYN_KIND.UNDEF}`);
+            B.condBr(undef, storeUndefined, checkLabel);
+            B.startBlock(storeUndefined);
+            host.declare(`declare void @scr_arr_set_undefined(ptr, double)`);
+            const at = B.tmp();
+            B.line(`${at} = ${sizeToDouble(this.S)} ${this.S} ${i} to double`);
+            B.line(`call void @scr_arr_set_undefined(ptr ${a}, double ${at})`);
+            B.br(doneLabel);
+            B.startBlock(checkLabel);
+          }
           const v = B.tmp();
           B.line(`${v} = call ${this.valTy(elem)} @${c}(ptr ${e}, ptr ${pathSlot})`);
           this.pendingBail(
diff --git a/packages/compiler/src/backend/llvm/expr-stream-bridges.ts b/packages/compiler/src/backend/llvm/expr-stream-bridges.ts
index fa27869d0..57172068f 100644
--- a/packages/compiler/src/backend/llvm/expr-stream-bridges.ts
+++ b/packages/compiler/src/backend/llvm/expr-stream-bridges.ts
@@ -792,13 +792,16 @@ export function streamTypedRefMaterializeAdapter(
   t: IrType,
 ): LlStreamTypedRefAdapter {
   const key = typeKey(t);
-  const existing = host.liveDynRefAdapters.get(key);
+  // A `(T | undefined)[]` stored as T elements shares T[]'s capsule type,
+  // but its adapters also store undefined elements.
+  const adapterKey = t.kind === "array" && t.optionalElements ? `${key}?` : key;
+  const existing = host.liveDynRefAdapters.get(adapterKey);
   if (existing) return existing;
   // The converter depends only on the static type. Intern the complete
   // graph across roots and union arms so recursive types emit once.
   const snapshot = `sc_ldr_${host.liveDynRefAdapters.size}_materialize`;
   const adapter: LlStreamTypedRefAdapter = { snapshot, commit: "null" };
-  host.liveDynRefAdapters.set(key, adapter);
+  host.liveDynRefAdapters.set(adapterKey, adapter);
   host.journal?.allocated(
     snapshot.replace(/_materialize$/, ""),
     "streamTypedRefMaterializeAdapter",
@@ -1312,6 +1315,18 @@ function liveArrayOps(
   G.terminate(`ret ptr ${boxed}`);
 
   const S = new BlockBuilder();
+  if (t.optionalElements) {
+    // A `(T | undefined)[]` stores undefined as a present undefined.
+    const undef = host.dyn.isUndefined(S, "%v");
+    const storeUndefined = S.newLabel("live.array.undefined");
+    const other = S.newLabel("live.array.value");
+    S.condBr(undef, storeUndefined, other);
+    S.startBlock(storeUndefined);
+    host.declare(`declare void @scr_arr_set_undefined(ptr, double)`);
+    S.line(`call void @scr_arr_set_undefined(ptr %p, double %i)`);
+    S.terminate(`ret i1 true`);
+    S.startBlock(other);
+  }
   const fits = S.tmp();
   S.line(`${fits} = call zeroext i1 @${host.dyn.dynMatchHelper(elem)}(ptr %v)`);
   const store = S.newLabel("live.array.store");
diff --git a/packages/compiler/src/frontend/lowering/array-values.ts b/packages/compiler/src/frontend/lowering/array-values.ts
index f191ad942..6e4a45f86 100644
--- a/packages/compiler/src/frontend/lowering/array-values.ts
+++ b/packages/compiler/src/frontend/lowering/array-values.ts
@@ -269,6 +269,140 @@ function readsMutableLocal(lowerer: Lowerer, e: IrExpr): boolean {
   });
 }
 
+/** A number, string or boolean element type. Arrays of it hold a present
+ * undefined in the element's state, so `(T | undefined)[]` stores T
+ * elements (the type mapper) and a T[] seen as one stays the same array. */
+export function primitiveElement(elem: IrType): boolean {
+  return elem.kind === "f64" || elem.kind === "string" || elem.kind === "bool";
+}
+
+/** `value` is the element type with undefined, stored into an array of a
+ * primitive element type: a present undefined or the element. */
+export function optionalElementValue(lowerer: Lowerer, value: IrExpr, elem: IrType): boolean {
+  if (!primitiveElement(elem)) return false;
+  if (value.type.kind === "undefinedT") return true;
+  return (
+    value.type.kind === "union" &&
+    lowerer.armTag(value.type.unionId, UNDEFINED_T) >= 0 &&
+    typeEquals(lowerer.stripUndefinedArm(value.type), elem)
+  );
+}
+
+/** A value of a primitive element type that holds a slot until the value
+ * stored there replaces it (insertedValues), or stands for a slot that is
+ * not read. */
+export function elementPlaceholder(elem: IrType, loc: SrcLoc): IrExpr {
+  if (elem.kind === "f64") return { kind: "numLit", value: 0, type: F64, loc };
+  if (elem.kind === "bool") return { kind: "boolLit", value: false, type: BOOL, loc };
+  return { kind: "strLit", value: "", type: STRING, loc };
+}
+
+/** Values inserted into an array (push, unshift, splice, a literal), in
+ * source order. A value that can be undefined (optionalElementValue) is
+ * kept in a temporary: the insertion takes a placeholder for it, and
+ * `fill` then stores the value at the placeholder's index. */
+export function insertedValues(
+  lowerer: Lowerer,
+  values: readonly IrExpr[],
+  elem: IrType,
+  coerce: (value: IrExpr, index: number) => IrExpr,
+  loc: SrcLoc,
+): {
+  stmts: IrStmt[];
+  args: IrExpr[];
+  optional: boolean;
+  fill: (arr: IrExpr, first: IrExpr) => IrStmt[];
+} {
+  if (!values.some((value) => optionalElementValue(lowerer, value, elem)))
+    return { stmts: [], args: values.map(coerce), optional: false, fill: () => [] };
+  const stmts: IrStmt[] = [];
+  const args: IrExpr[] = [];
+  const held: { offset: number; value: IrExpr }[] = [];
+  values.forEach((value, offset) => {
+    // Every value is evaluated in order before the insertion.
+    if (value.type.kind === "undefinedT") {
+      if (value.kind !== "unitLit") stmts.push({ kind: "exprStmt", expr: value, loc });
+      args.push(elementPlaceholder(elem, loc));
+      held.push({ offset, value: { kind: "unitLit", unit: "undefined", type: UNDEFINED_T, loc } });
+      return;
+    }
+    const temp = lowerer.declareHiddenLocal("%inserted", value.type);
+    stmts.push({ kind: "varDecl", localId: temp.id, init: value, loc });
+    const stable = varRef(temp.id, temp.type, loc);
+    if (optionalElementValue(lowerer, value, elem)) {
+      args.push(elementPlaceholder(elem, loc));
+      held.push({ offset, value: stable });
+    } else {
+      args.push(coerce(stable, offset));
+    }
+  });
+  const fill = (arr: IrExpr, first: IrExpr): IrStmt[] =>
+    held.map(({ offset, value }) =>
+      arrayValueStore(
+        lowerer,
+        arr,
+        offset === 0
+          ? first
+          : {
+              kind: "bin",
+              op: "+",
+              left: first,
+              right: { kind: "numLit", value: offset, type: F64, loc },
+              type: F64,
+              loc,
+            },
+        value,
+        elem,
+        loc,
+      ),
+    );
+  return { stmts, args, optional: true, fill };
+}
+
+/** Statements appending `value` to `arr` inside a lifted helper. A value
+ * that can be undefined in a primitive array (optionalElementValue) is held
+ * in the helper's local `slot` (which the caller declares with the value's
+ * type) and stored as a present undefined or the element; any other value
+ * converts to the element type. */
+export function appendElement(
+  lowerer: Lowerer,
+  arr: IrExpr,
+  value: IrExpr,
+  elem: IrType,
+  slot: string,
+  loc: SrcLoc,
+): IrStmt[] {
+  if (optionalElementValue(lowerer, value, elem) && value.type.kind !== "undefinedT") {
+    const held = varRef(slot, value.type, loc);
+    const length: IrExpr = {
+      kind: "arrIntrinsic",
+      method: "length",
+      receiver: arr,
+      args: [],
+      type: F64,
+      loc,
+    };
+    return [
+      { kind: "varDecl", localId: slot, init: value, loc },
+      arrayValueStore(lowerer, arr, length, held, elem, loc),
+    ];
+  }
+  return [
+    {
+      kind: "exprStmt",
+      expr: {
+        kind: "arrIntrinsic",
+        method: "push",
+        receiver: arr,
+        args: [lowerer.coerceToExpected(value, elem)],
+        type: F64,
+        loc,
+      },
+      loc,
+    },
+  ];
+}
+
 /** Store a stabilized value without changing the array's element ABI. */
 export function arrayValueStore(
   lowerer: Lowerer,
@@ -279,6 +413,12 @@ export function arrayValueStore(
   loc: SrcLoc,
 ): IrStmt {
   if (typeEquals(value.type, elem)) return { kind: "arraySet", arr, index, value, loc };
+  // An undefined value of a `(T | undefined)[]` (primitiveElement).
+  if (value.type.kind === "undefinedT" && primitiveElement(elem)) {
+    const write: IrStmt = { kind: "arraySetUndefined", arr, index, loc };
+    if (value.kind === "unitLit") return write;
+    return { kind: "block", body: [{ kind: "exprStmt", expr: value, loc }, write], loc };
+  }
   if (value.type.kind === "union" && lowerer.armTag(value.type.unionId, UNDEFINED_T) >= 0) {
     // The tag test and the VALUE arm extraction both consume the value. Keep
     // an arbitrary expression (especially an array read or call) single
diff --git a/packages/compiler/src/frontend/lowering/builtins/promises.ts b/packages/compiler/src/frontend/lowering/builtins/promises.ts
index e9d4322ad..c3b2cd479 100644
--- a/packages/compiler/src/frontend/lowering/builtins/promises.ts
+++ b/packages/compiler/src/frontend/lowering/builtins/promises.ts
@@ -3,7 +3,8 @@ import { checkedPromiseAll, thenReader } from "../checked-promise-all.js";
 import { InternalCompilerError } from "../../../errors.js";
 import * as ts from "../../ts7/adapter.js";
 import type { Lowerer } from "../lowerer.js";
-import { varRef } from "../../../ir/build.js";
+import { countedFor, varRef } from "../../../ir/build.js";
+import { arrayValueStore, optionalElementValue, primitiveElement } from "../array-values.js";
 import { maybePromiseResolve, maybePromiseUnionOf } from "../maybe-promise.js";
 import { isJsSourceFile, locOf } from "../../program.js";
 import {
@@ -14,6 +15,7 @@ import {
   type IrFunction,
   type IrStmt,
   type IrType,
+  type SrcLoc,
   UNDEFINED_T,
   VOID,
   arrayOf,
@@ -435,6 +437,28 @@ export function lowerPromiseStaticCall(
       inner.kind === "void"
         ? { kind: "promise", inner: VOID }
         : lowerer.mapTypeOf(lowerer.typeOf(call));
+    // Promise<T | undefined>[] combines into a `(T | undefined)[]`, which
+    // stores T elements (primitiveElement): collect the values, then copy
+    // them in with their present undefined values.
+    if (
+      resultT?.kind === "promise" &&
+      resultT.inner.kind === "array" &&
+      primitiveElement(resultT.inner.elem) &&
+      optionalElementValue(
+        lowerer,
+        { kind: "unitLit", unit: "undefined", type: inner, loc },
+        resultT.inner.elem,
+      )
+    ) {
+      const joined = optionalPromiseAll(
+        lowerer,
+        entries,
+        inner,
+        resultT as IrType & { kind: "promise" },
+        loc,
+      );
+      if (joined) return joined;
+    }
     if (
       resultT?.kind !== "promise" ||
       (inner.kind !== "void" &&
@@ -954,3 +978,139 @@ export function lowerAwaitedUniformPromiseAll(
     loc,
   };
 }
+
+/** `Promise.all(ps)` over `Promise<T | undefined>[]` for a number, string or
+ * boolean T, whose result is a `(T | undefined)[]` (stored as T elements).
+ * The combinator joins the entries (promise.allTuple), and its builder
+ * reads each fulfilled entry into the result with its present undefined,
+ * inside the combinator's settlement, so the result settles on Node's
+ * turn. Entries of another type keep the general combinator. */
+function optionalPromiseAll(
+  lowerer: Lowerer,
+  entries: IrExpr,
+  inner: IrType,
+  resultT: IrType & { kind: "promise" },
+  loc: SrcLoc,
+): IrExpr | null {
+  const entriesT = entries.type;
+  if (entriesT.kind !== "array" || entriesT.elem.kind !== "promise") return null;
+  if (!typeEquals(entriesT.elem.inner, inner)) return null;
+  const outT = resultT.inner as IrType & { kind: "array" };
+  const voidPromise: IrType & { kind: "promise" } = { kind: "promise", inner: VOID };
+  const joinedT = arrayOf(voidPromise);
+  const key = `promise.allOptional:${typeKey(entriesT)}:${typeKey(outT)}`;
+  let helper = lowerer.arrHofHelpers.get(key);
+  if (!helper) {
+    helper = keyedHelperName(lowerer, "%promise.allOptional", key);
+    lowerer.arrHofHelpers.set(key, helper);
+    const builder = keyedHelperName(lowerer, "%promise.allOptionalValues", key);
+    const length = (arr: IrExpr): IrExpr => ({
+      kind: "arrIntrinsic",
+      method: "length",
+      receiver: arr,
+      args: [],
+      type: F64,
+      loc,
+    });
+    const joined = varRef("joined.0", joinedT, loc);
+    const out = varRef("out.0", outT, loc);
+    lowerer.liftedFns.push({
+      name: builder,
+      params: [{ localId: "joined.0", name: "joined", type: joinedT }],
+      returnType: outT,
+      locals: [
+        { id: "joined.0", name: "joined", type: joinedT, mutable: false },
+        { id: "out.0", name: "out", type: outT, mutable: false },
+        { id: "i.0", name: "i", type: F64, mutable: true },
+        { id: "v.0", name: "v", type: inner, mutable: false },
+      ],
+      body: [
+        {
+          kind: "varDecl",
+          localId: "out.0",
+          init: { kind: "arrayLit", elems: [], type: outT, loc },
+          loc,
+        },
+        countedFor(loc, length(joined), (index) => [
+          {
+            kind: "varDecl",
+            localId: "v.0",
+            init: {
+              kind: "libCall",
+              fn: "promise.fulfilledValue",
+              args: [{ kind: "arrayGet", arr: joined, index, type: voidPromise, loc }],
+              type: inner,
+              loc,
+            },
+            loc,
+          },
+          arrayValueStore(lowerer, out, index, varRef("v.0", inner, loc), outT.elem, loc),
+        ]),
+        { kind: "return", value: out, loc },
+      ],
+      loc,
+    });
+    const ps = varRef("ps.0", entriesT, loc);
+    lowerer.liftedFns.push({
+      name: helper,
+      params: [{ localId: "ps.0", name: "ps", type: entriesT }],
+      returnType: resultT,
+      locals: [
+        { id: "ps.0", name: "ps", type: entriesT, mutable: false },
+        { id: "joined.0", name: "joined", type: joinedT, mutable: false },
+        { id: "i.0", name: "i", type: F64, mutable: true },
+      ],
+      body: [
+        {
+          kind: "varDecl",
+          localId: "joined.0",
+          init: { kind: "arrayLit", elems: [], type: joinedT, loc },
+          loc,
+        },
+        countedFor(loc, length(ps), (index) => [
+          {
+            kind: "exprStmt",
+            expr: {
+              kind: "arrIntrinsic",
+              method: "push",
+              receiver: joined,
+              args: [
+                {
+                  kind: "promiseVoidWiden",
+                  value: { kind: "arrayGet", arr: ps, index, type: entriesT.elem, loc },
+                  type: voidPromise,
+                  loc,
+                },
+              ],
+              type: F64,
+              loc,
+            },
+            loc,
+          },
+        ]),
+        {
+          kind: "return",
+          value: {
+            kind: "libCall",
+            fn: "promise.allTuple",
+            args: [
+              joined,
+              {
+                kind: "closure",
+                fnName: builder,
+                captures: [],
+                type: funcOf([joinedT], outT),
+                loc,
+              },
+            ],
+            type: resultT,
+            loc,
+          },
+          loc,
+        },
+      ],
+      loc,
+    });
+  }
+  return { kind: "call", callee: helper, args: [entries], type: resultT, loc };
+}
diff --git a/packages/compiler/src/frontend/lowering/call-arguments.ts b/packages/compiler/src/frontend/lowering/call-arguments.ts
index 060ffe9d2..79e891d4f 100644
--- a/packages/compiler/src/frontend/lowering/call-arguments.ts
+++ b/packages/compiler/src/frontend/lowering/call-arguments.ts
@@ -28,6 +28,7 @@ import { checkedIterableSpread } from "./checked-iterable-spread.js";
 import { type ParamShape } from "./call-signatures.js";
 import { arrayValueRead, arrayValueType } from "./array-values.js";
 import { buildArrayConversion } from "./coercions/builders.js";
+import { lowerArraySpreadItems, lowerArrayValueItems } from "./containers/array-construction.js";
 import { restTupleElementTypes, typeKey } from "../type-mapper.js";
 
 function fixedTupleSpreadInfo(
@@ -403,6 +404,51 @@ export function completeArgs(
       }
     }
     if (restType.kind !== "array") lowerer.unsupported("SC1090", blame, "this call form");
+    // A `(T | undefined)[]` rest (stored as T elements) packs undefined
+    // arguments as present undefined elements.
+    const restNodes = sources.slice(restAt);
+    // Plain arguments and spreads, also together with spread tuple fields
+    // (pre-lowered): each item, in argument order.
+    if (
+      restType.optionalElements === true &&
+      (!restNodes.some(isIr) || restNodes.some((a) => !isIr(a) && ts.isSpreadElement(a)))
+    ) {
+      const items: (ts.Expression | IrExpr)[] = [];
+      for (const a of restNodes) {
+        if (isIr(a)) items.push(a.ir);
+        else items.push(a);
+      }
+      out.push(lowerArraySpreadItems(lowerer, items, restType.elem, restType, loc));
+      return out;
+    }
+    // Tuple-spread values (pre-lowered) and plain arguments pack the same
+    // way. Each value evaluates once, in argument order.
+    if (
+      restType.optionalElements === true &&
+      !restNodes.some((a) => !isIr(a) && ts.isSpreadElement(a))
+    ) {
+      const stmts: IrStmt[] = [];
+      const values = restNodes.map((a): IrExpr => {
+        // A tuple field is read where its spread appears: a later
+        // argument may write the tuple.
+        let value: IrExpr;
+        if (isIr(a)) {
+          value = a.ir;
+          if (value.kind === "seqExpr") {
+            stmts.push(...value.stmts);
+            value = value.result;
+          }
+        } else value = lowerer.lowerExpr(a as ts.Expression);
+        const saved = lowerer.declareHiddenLocal("%callArg", value.type);
+        stmts.push({ kind: "varDecl", localId: saved.id, init: value, loc: value.loc });
+        return { kind: "varRef", localId: saved.id, type: value.type, loc: value.loc };
+      });
+      const items = lowerArrayValueItems(lowerer, values, restType.elem, restType, loc);
+      out.push(
+        stmts.length === 0 ? items : { kind: "seqExpr", stmts, result: items, type: restType, loc },
+      );
+      return out;
+    }
     // The rest pack is a fresh array per call; surplus SPREADS copy
     // their elements in (JS-exact — `f(a, ...xs, b, ...ys)` packs in
     // order, sources untouched).
diff --git a/packages/compiler/src/frontend/lowering/coercions/structural.ts b/packages/compiler/src/frontend/lowering/coercions/structural.ts
index 4321a5ead..93ae1c231 100644
--- a/packages/compiler/src/frontend/lowering/coercions/structural.ts
+++ b/packages/compiler/src/frontend/lowering/coercions/structural.ts
@@ -1,12 +1,13 @@
 import { keyedHelperName } from "../stable-names.js";
 import { constructedValue } from "../constructor-returns.js";
-import { dynUndefinedExpr, varRef } from "../../../ir/build.js";
+import { countedFor, dynUndefinedExpr, varRef } from "../../../ir/build.js";
 import type { FieldLift } from "./structural-plans.js";
 import { buildArrayConversion } from "./builders.js";
+import { arrayValueRead, arrayValueStore, primitiveElement } from "../array-values.js";
 import type { WidthLift } from "../width-lift.js";
 import { InternalCompilerError } from "../../../errors.js";
 import type { IrExpr, IrFunction, IrRecordShape, IrStmt, IrType, SrcLoc } from "../../../ir/ir.js";
-import { BOOL, DYN, F64, STRING, UNDEFINED_T } from "../../../ir/ir.js";
+import { BOOL, DYN, F64, STRING, UNDEFINED_T, typeEquals } from "../../../ir/ir.js";
 import { typeKey } from "../../type-mapper.js";
 import { findStaticOn, findGenericStaticOn } from "../lower-classes.js";
 import { lowerRecordOvfCaptureHelper } from "../containers/indexed-objects.js";
@@ -54,6 +55,9 @@ export function widthCoerce(lowerer: Lowerer, expr: IrExpr, expected: IrType): I
     return lowerer.classStaticsProjection(expr.type.className, expected.shapeId, expr.loc);
   }
   if (expected.kind === "array" && expr.type.kind === "array") {
+    const optional = optionalArrayHelper(lowerer, expr.type, expected, expr.loc);
+    if (optional)
+      return { kind: "call", callee: optional, args: [expr], type: expected, loc: expr.loc };
     const helper = lowerer.arrayWidthHelper(expr.type, expected, expr.loc);
     if (helper)
       return { kind: "call", callee: helper, args: [expr], type: expected, loc: expr.loc };
@@ -518,6 +522,69 @@ export function emptyArrayLiftHelper(
   return name;
 }
 
+/** Copy an array of `T | undefined` unions (a collection's values, an
+ * iterator drain) into a `(T | undefined)[]` of a number, string or
+ * boolean T, which stores T elements: undefined values become present
+ * undefined elements. Null for other element types. */
+function optionalArrayHelper(
+  lowerer: Lowerer,
+  fromT: IrType & { kind: "array" },
+  toT: IrType & { kind: "array" },
+  loc: SrcLoc,
+): string | null {
+  const from = fromT.elem;
+  const to = toT.elem;
+  if (
+    !primitiveElement(to) ||
+    from.kind !== "union" ||
+    lowerer.armTag(from.unionId, UNDEFINED_T) < 0 ||
+    !typeEquals(lowerer.stripUndefinedArm(from), to)
+  )
+    return null;
+  const key = `arrOptional:${typeKey(from)}:${typeKey(to)}`;
+  const existing = lowerer.valueHelpers.get(key);
+  if (existing) return existing;
+  const name = keyedHelperName(lowerer, "%arr.width", key);
+  lowerer.valueHelpers.set(key, name);
+  const input = varRef("a.0", fromT, loc);
+  const out = varRef("out.0", toT, loc);
+  lowerer.liftedFns.push({
+    name,
+    params: [{ localId: "a.0", name: "a", type: fromT }],
+    returnType: toT,
+    locals: [
+      { id: "a.0", name: "a", type: fromT, mutable: true },
+      { id: "out.0", name: "out", type: toT, mutable: false },
+      { id: "i.0", name: "i", type: F64, mutable: true },
+      { id: "v.0", name: "v", type: from, mutable: false },
+    ],
+    body: [
+      {
+        kind: "varDecl",
+        localId: "out.0",
+        init: { kind: "arrayLit", elems: [], type: toT, loc },
+        loc,
+      },
+      countedFor(
+        loc,
+        { kind: "arrIntrinsic", method: "length", receiver: input, args: [], type: F64, loc },
+        (index) => [
+          {
+            kind: "varDecl",
+            localId: "v.0",
+            init: arrayValueRead(lowerer, input, index, from, loc),
+            loc,
+          },
+          arrayValueStore(lowerer, out, index, varRef("v.0", from, loc), to, loc),
+        ],
+      ),
+      { kind: "return", value: out, loc },
+    ],
+    loc,
+  });
+  return name;
+}
+
 /** Copy an array through its validated element conversion. Intern before
  * building the body so recursive element conversions can reuse this helper. */
 export function arrayWidthHelper(
@@ -542,14 +609,37 @@ export function arrayWidthHelper(
       name,
       arrT,
       { kind: "arrayLit", elems: [], type: outT, loc },
-      (element, _index, result) => ({
-        kind: "arrIntrinsic",
-        method: "push",
-        receiver: result,
-        args: [lowerer.applyWidthLift(elemLift, element, toElem, loc)],
-        type: F64,
-        loc,
-      }),
+      (element, index, result) => {
+        let value = lowerer.applyWidthLift(elemLift, element, toElem, loc);
+        // A number, string or boolean array can hold present undefined
+        // elements (`(T | undefined)[]` stores T): they, and holes, copy
+        // as the target's undefined.
+        const missing = primitiveElement(fromElem) ? lowerer.wrappedUndefined(toElem, loc) : null;
+        if (missing !== null)
+          value = {
+            kind: "ternary",
+            cond: {
+              kind: "bin",
+              op: "===",
+              left: { kind: "arrayState", arr: varRef("a.0", arrT, loc), index, type: F64, loc },
+              right: { kind: "numLit", value: 1, type: F64, loc },
+              type: BOOL,
+              loc,
+            },
+            then: value,
+            else_: missing,
+            type: toElem,
+            loc,
+          };
+        return {
+          kind: "arrIntrinsic",
+          method: "push",
+          receiver: result,
+          args: [value],
+          type: F64,
+          loc,
+        };
+      },
       loc,
     ),
   );
diff --git a/packages/compiler/src/frontend/lowering/containers/array-callbacks.ts b/packages/compiler/src/frontend/lowering/containers/array-callbacks.ts
index a3335b995..46459fadb 100644
--- a/packages/compiler/src/frontend/lowering/containers/array-callbacks.ts
+++ b/packages/compiler/src/frontend/lowering/containers/array-callbacks.ts
@@ -42,6 +42,7 @@ import {
   arrayValueStore,
   arrayValueType,
   currentArrayIndexPresent,
+  primitiveElement,
 } from "../array-values.js";
 import { typeKey } from "../../type-mapper.js";
 import {
@@ -1690,6 +1691,8 @@ function flatMapUnionPlan(
   }
   const converts = (t: IrType): boolean => {
     if (typeEquals(t, outElem)) return true;
+    // A `(T | undefined)[]` result stores T elements (primitiveElement).
+    if (t.kind === "undefinedT" && primitiveElement(outElem)) return true;
     if (outElem.kind !== "union") return false;
     return lowerer.armTag(outElem.unionId, t) >= 0;
   };
@@ -1755,24 +1758,21 @@ function flatMapUnionHelper(
     };
     if (plan.arms[tag] === "empty") return [];
     if (plan.arms[tag] === "value") {
+      // A unit arm's value is the unit itself, which has no narrowed
+      // representation.
+      const value: IrExpr = isUnitType(arm)
+        ? { kind: "unitLit", unit: arm.kind === "nullT" ? "null" : "undefined", type: arm, loc }
+        : narrowed;
       return [
         arrayValueStore(
           lowerer,
           outRef(),
           outLength(),
-          lowerer.coerceToExpected(
-            // A unit arm's value is the unit itself, which has no
-            // narrowed representation.
-            isUnitType(arm)
-              ? {
-                  kind: "unitLit",
-                  unit: arm.kind === "nullT" ? "null" : "undefined",
-                  type: arm,
-                  loc,
-                }
-              : narrowed,
-            plan.outElem,
-          ),
+          // An undefined value is a present undefined element of a
+          // primitive array (arrayValueStore).
+          arm.kind === "undefinedT" && primitiveElement(plan.outElem)
+            ? value
+            : lowerer.coerceToExpected(value, plan.outElem),
           plan.outElem,
           loc,
         ),
@@ -2348,6 +2348,43 @@ function reduceHelper(
   return name;
 }
 
+/** The comparator of a sort over a `(T | undefined)[]`, which stores T
+ * elements. The comparator only receives present elements (undefined
+ * elements sort last without a call), so the unannotated parameters of an
+ * inline comparator take the element type, and the runtime calls it
+ * without an adapter. */
+function lowerOptionalElementComparator(
+  lowerer: Lowerer,
+  argNode: ts.Expression,
+  comparator: ts.Expression,
+  elem: IrType,
+  arrT: IrType & { kind: "array" },
+): IrExpr {
+  if (
+    arrT.optionalElements !== true ||
+    !(ts.isArrowFunction(comparator) || ts.isFunctionExpression(comparator)) ||
+    isJsSourceFile(ts.getSourceFileOfNode(comparator))
+  )
+    return lowerer.lowerExpr(argNode);
+  const rebound: { symbol: ts.Symbol; previous: IrType | undefined }[] = [];
+  for (const param of comparator.parameters.nodes.slice(0, 2)) {
+    if (param.type || param.initializer || param.dotDotDotToken || !ts.isIdentifier(param.name))
+      continue;
+    const symbol = lowerer.checker.getSymbolAtLocation(param.name);
+    if (!symbol) continue;
+    rebound.push({ symbol, previous: lowerer.runtimeOptionalBindingTypes.get(symbol) });
+    lowerer.runtimeOptionalBindingTypes.set(symbol, elem);
+  }
+  try {
+    return lowerer.lowerExpr(argNode);
+  } finally {
+    for (const { symbol, previous } of rebound) {
+      if (previous === undefined) lowerer.runtimeOptionalBindingTypes.delete(symbol);
+      else lowerer.runtimeOptionalBindingTypes.set(symbol, previous);
+    }
+  }
+}
+
 /** `a.sort(cmp)` / `a.toSorted(cmp)`, desugared to an interned synthetic
  * function like the other array HOFs. sort mutates and returns the receiver;
  * toSorted takes its shallow snapshot INSIDE the helper, after the receiver
@@ -2436,7 +2473,8 @@ export function lowerArraySortCall(
       loc,
     };
   }
-  let fnArg = undefinedArg ?? lowerer.lowerExpr(argNode);
+  let fnArg =
+    undefinedArg ?? lowerOptionalElementComparator(lowerer, argNode, unwrappedArg, elem, arrT);
   // Reusable JS comparators often accept checked values. Adapt the array's
   // element ABI just as other array callbacks do before sorting.
   if (
diff --git a/packages/compiler/src/frontend/lowering/containers/array-construction.ts b/packages/compiler/src/frontend/lowering/containers/array-construction.ts
index 1ca51f526..5029abc36 100644
--- a/packages/compiler/src/frontend/lowering/containers/array-construction.ts
+++ b/packages/compiler/src/frontend/lowering/containers/array-construction.ts
@@ -4,6 +4,7 @@ import { generatorDrain } from "../iterator-adapters.js";
 import { identityPreservingWidening } from "../coercions/identity.js";
 import { liftedFreshSpread } from "../coercions/fresh-spread.js";
 import { checkedIterableSpread } from "../checked-iterable-spread.js";
+import { AstNode } from "../../ts7/ast-node.js";
 import { InternalCompilerError } from "../../../errors.js";
 import {
   dynUndefinedExpr,
@@ -27,7 +28,9 @@ import {
   type IrType,
   STRING,
   type SrcLoc,
+  UNDEFINED_T,
   arrayOf,
+  isUnitType,
   canConvertToDyn,
   funcOf,
   isRefCounted,
@@ -35,7 +38,13 @@ import {
   typeEquals,
 } from "../../../ir/ir.js";
 import { isJsSourceFile, locOf } from "../../program.js";
-import { arrayValueRead, arrayValueStore, arrayValueType } from "../array-values.js";
+import {
+  arrayValueRead,
+  arrayValueStore,
+  arrayValueType,
+  optionalElementValue,
+  primitiveElement,
+} from "../array-values.js";
 import { typeKey } from "../../type-mapper.js";
 import { newFnCtx } from "../lowerer.js";
 import { iteratorCanStep, iteratorValue } from "../iterator-consumption.js";
@@ -51,6 +60,34 @@ import {
 } from "./callback-arguments.js";
 import { classGeneratorIterable } from "../class-iterables.js";
 
+/** A value whose type admits only null and undefined, stored into an array
+ * that keeps undefined elements: the checker admits it there only when its
+ * type is `undefined` (a tuple element typed `undefined` maps to such a
+ * union), so it is evaluated and stored as undefined. */
+function undefinedForNullish(
+  lowerer: Lowerer,
+  value: IrExpr,
+  arrType: IrType & { kind: "array" },
+): IrExpr {
+  if (arrType.optionalElements !== true || value.type.kind !== "union") return value;
+  const arms = lowerer.unions.get(value.type.unionId)?.arms ?? [];
+  if (arms.length === 0 || !arms.every(isUnitType)) return value;
+  const loc = value.loc;
+  const optional: IrType = {
+    kind: "union",
+    unionId: lowerer.unions.intern([arrType.elem, UNDEFINED_T]),
+  };
+  const missing = lowerer.wrappedUndefined(optional, loc);
+  if (!missing) return value;
+  return {
+    kind: "seqExpr",
+    stmts: [{ kind: "exprStmt", expr: value, loc }],
+    result: missing,
+    type: optional,
+    loc,
+  };
+}
+
 /** Build the argument array for a mutating/copying operation while keeping
  * the receiver's scalar payload ABI. A missing read is represented as the
  * present UNDEFINED state in this temporary, so push/unshift and toSpliced
@@ -86,9 +123,11 @@ export function lowerArrayValueItems(
       loc,
     },
   ];
-  for (const value of values) {
+  for (const item of values) {
+    const value = undefinedForNullish(lowerer, item, arrType);
     const stored =
-      lowerer.runtimeOptionalWidening(value.type, elem) !== null
+      lowerer.runtimeOptionalWidening(value.type, elem) !== null ||
+      optionalElementValue(lowerer, value, elem)
         ? value
         : lowerer.coerceToExpected(value, elem);
     body.push(arrayValueStore(lowerer, outRef, length(), stored, elem, loc));
@@ -96,16 +135,21 @@ export function lowerArrayValueItems(
   return { kind: "seqExpr", stmts: body, result: outRef, type: arrType, loc };
 }
 
+/** The items of an array built from argument nodes (a spread copies its
+ * elements in) and values already lowered (a spread tuple's fields), in
+ * order. Each value is stored when its turn comes. */
 export function lowerArraySpreadItems(
   lowerer: Lowerer,
-  nodes: readonly ts.Expression[],
+  nodes: readonly (ts.Expression | IrExpr)[],
   elem: IrType,
   arrType: IrType & { kind: "array" },
   loc: SrcLoc,
 ): IrExpr {
   const out = lowerer.declareHiddenLocal("%arrayItems", arrType);
   const outRef = varRef(out.id, arrType, loc);
-  const plainItems = nodes.filter((node) => !ts.isSpreadElement(node)).length;
+  const plainItems = nodes.filter(
+    (node) => !(node instanceof AstNode) || !ts.isSpreadElement(node),
+  ).length;
   const length = (): IrExpr => ({
     kind: "arrIntrinsic",
     method: "length",
@@ -129,7 +173,23 @@ export function lowerArraySpreadItems(
     },
   ];
   for (const node of nodes) {
-    if (ts.isSpreadElement(node)) {
+    if (!(node instanceof AstNode)) {
+      // A spread tuple's first field declares the tuple's storage, which
+      // the later fields read: its statements join this sequence, whose
+      // frame lasts until every item is stored.
+      let item = node;
+      if (item.kind === "seqExpr") {
+        body.push(...item.stmts);
+        item = item.result;
+      }
+      const value = undefinedForNullish(lowerer, item, arrType);
+      const stored =
+        lowerer.runtimeOptionalWidening(value.type, elem) !== null ||
+        optionalElementValue(lowerer, value, elem)
+          ? value
+          : lowerer.coerceToExpected(value, elem);
+      body.push(arrayValueStore(lowerer, outRef, length(), stored, elem, loc));
+    } else if (ts.isSpreadElement(node)) {
       let source = lowerer.lowerExpr(node.expression);
       if (
         source.type.kind === "set" &&
@@ -212,7 +272,8 @@ export function lowerArraySpreadItems(
     } else {
       const value = lowerer.lowerExpr(node);
       const stored =
-        lowerer.runtimeOptionalWidening(value.type, elem) !== null
+        lowerer.runtimeOptionalWidening(value.type, elem) !== null ||
+        optionalElementValue(lowerer, value, elem)
           ? value
           : lowerer.coerceInto(node, value, elem);
       body.push(arrayValueStore(lowerer, outRef, length(), stored, elem, loc));
@@ -518,6 +579,20 @@ export function lowerArrayOfCall(
   if (call.arguments.nodes.some(ts.isSpreadElement)) {
     return lowerArraySpreadItems(lowerer, call.arguments.nodes, result.elem, result, locOf(call));
   }
+  // A `(T | undefined)[]` (primitiveElement) stores undefined arguments as
+  // present undefined elements.
+  if (result.optionalElements === true) {
+    const nodes = call.arguments.nodes;
+    const values = nodes.map((arg) => lowerer.lowerExpr(arg));
+    if (values.some((value) => optionalElementValue(lowerer, value, result.elem)))
+      return lowerArrayValueItems(lowerer, values, result.elem, result, locOf(call));
+    return {
+      kind: "arrayLit",
+      elems: values.map((value, i) => lowerer.coerceInto(nodes[i]!, value, result.elem)),
+      type: result,
+      loc: locOf(call),
+    };
+  }
   return {
     kind: "arrayLit",
     elems: call.arguments.nodes.map((arg) => lowerer.lowerExprExpecting(arg, result.elem)),
@@ -959,6 +1034,31 @@ export function lowerArrayFromCall(
       const arrT = lowerer.mapTypeOf(lowerer.typeOf(call));
       if (arrT?.kind !== "array") lowerer.badType(call, lowerer.typeOf(call));
       const elem = arrT.elem;
+      // A `(T | undefined)[]` (primitiveElement): n present undefined
+      // elements.
+      const elemTs = lowerer.checker.isArrayType(lowerer.typeOf(call))
+        ? lowerer.checker.getTypeArguments(lowerer.typeOf(call) as ts.TypeReference)[0]
+        : undefined;
+      if (
+        primitiveElement(elem) &&
+        elemTs !== undefined &&
+        (elemTs.isUnionType() ? ts.constituentTypes(elemTs) : [elemTs]).some(
+          (part) => (part.flags & ts.TypeFlags.Undefined) !== 0,
+        )
+      ) {
+        return {
+          kind: "arrIntrinsic",
+          method: "fillUndefined",
+          receiver: { kind: "arrayNewLen", length: n, type: arrT, loc },
+          // To the end: fill clamps an infinite end to the length.
+          args: [
+            numLit(0, loc),
+            { kind: "bin", op: "/", left: numLit(1, loc), right: numLit(0, loc), type: F64, loc },
+          ],
+          type: arrT,
+          loc,
+        };
+      }
       const absent =
         elem.kind === "union"
           ? lowerer.wrappedUndefined(elem, loc) !== null ||
@@ -1085,7 +1185,10 @@ export function lowerArrayFromCall(
     // A generator (or an iterator adapted into one) drains in order.
     if (src.type.kind === "generator" && args.length === 1) {
       const arrT = lowerer.mapTypeOf(lowerer.typeOf(call));
-      const drained = arrT?.kind === "array" ? generatorDrain(lowerer, src, arrT.elem, loc) : null;
+      const drained =
+        arrT?.kind === "array"
+          ? generatorDrain(lowerer, src, arrT.elem, loc, arrT.optionalElements === true)
+          : null;
       if (drained) return drained;
     }
     lowerer.noLowering(
@@ -1159,6 +1262,11 @@ function buildArrayFromArrayFn(
     type: fnRet,
     loc,
   };
+  // A result of another type than the element (`T | undefined` into a
+  // `(T | undefined)[]`, which stores T elements) is held in a local, so
+  // the store tests and converts one value.
+  const held = !typeEquals(fnRet, outElem);
+  const value: IrExpr = held ? varRef("v.0", fnRet, loc) : mapped;
   return {
     name,
     params: [
@@ -1171,6 +1279,7 @@ function buildArrayFromArrayFn(
       { id: "f.0", name: "f", type: fnT, mutable: true },
       { id: "out.0", name: "out", type: outT, mutable: false },
       { id: "i.0", name: "i", type: F64, mutable: true },
+      ...(held ? [{ id: "v.0", name: "v", type: fnRet, mutable: false }] : []),
     ],
     body: [
       {
@@ -1191,7 +1300,8 @@ function buildArrayFromArrayFn(
           loc,
         },
         body: [
-          arrayValueStore(lowerer, out, i, mapped, outElem, loc),
+          ...(held ? [{ kind: "varDecl", localId: "v.0", init: mapped, loc } as const] : []),
+          arrayValueStore(lowerer, out, i, value, outElem, loc),
           {
             kind: "assign",
             localId: "i.0",
diff --git a/packages/compiler/src/frontend/lowering/containers/array-methods.ts b/packages/compiler/src/frontend/lowering/containers/array-methods.ts
index 4ef203edd..f406e57eb 100644
--- a/packages/compiler/src/frontend/lowering/containers/array-methods.ts
+++ b/packages/compiler/src/frontend/lowering/containers/array-methods.ts
@@ -32,6 +32,10 @@ import {
   arrayValueRead,
   arrayValueStore,
   arrayValueType,
+  elementPlaceholder,
+  insertedValues,
+  optionalElementValue,
+  primitiveElement,
 } from "../array-values.js";
 import { typeKey } from "../../type-mapper.js";
 import {
@@ -125,6 +129,14 @@ function lowerOptionalArrayPush(
   return { kind: "seqExpr", stmts, result: length(), type: F64, loc };
 }
 
+/** The checker type of `node` admits undefined. */
+function checkerTypeAdmitsUndefined(lowerer: Lowerer, node: ts.Expression): boolean {
+  const type = lowerer.typeOf(node);
+  return (type.isUnionType() ? ts.constituentTypes(type) : [type]).some(
+    (part) => (part.flags & (ts.TypeFlags.Undefined | ts.TypeFlags.Void)) !== 0,
+  );
+}
+
 /** `a.splice(start, count, v)` where v may hold undefined at run time.
  * Receiver, start, count, and v evaluate once, in order; a present v is
  * inserted as a plain element literal (which a statement-position splice
@@ -658,7 +670,10 @@ export function lowerArrayMethodCall(
     const hasSpread = itemNodes.some(ts.isSpreadElement);
     const itemProbes = hasSpread ? [] : itemNodes.map((arg) => tryLowerExpression(lowerer, arg));
     const statefulItems = itemProbes.some(
-      (probe) => probe !== null && lowerer.runtimeOptionalWidening(probe.type, elem) !== null,
+      (probe) =>
+        probe !== null &&
+        (lowerer.runtimeOptionalWidening(probe.type, elem) !== null ||
+          optionalElementValue(lowerer, probe, elem)),
     );
     const items: IrExpr = hasSpread
       ? lowerArraySpreadItems(lowerer, itemNodes, elem, receiverIr, loc)
@@ -875,6 +890,65 @@ export function lowerArrayMethodCall(
         loc,
       };
     }
+    // A `(T | undefined)[]` (primitiveElement) takes undefined values as
+    // present undefined elements, stored after the insertion.
+    if (
+      (name === "push" || name === "unshift") &&
+      primitiveElement(elem) &&
+      call.arguments.nodes.some((a) => checkerTypeAdmitsUndefined(lowerer, a))
+    ) {
+      const nodes = call.arguments.nodes;
+      const values = nodes.map((a) => lowerer.lowerExpr(a));
+      const inserted = insertedValues(
+        lowerer,
+        values,
+        elem,
+        (value, i) => lowerer.coerceInto(nodes[i]!, value, elem),
+        loc,
+      );
+      if (inserted.optional) {
+        const array = lowerer.declareHiddenLocal("%insertInto", receiver.type);
+        const arr = varRef(array.id, array.type, loc);
+        const length = lowerer.declareHiddenLocal("%insertLength", F64);
+        const count: IrExpr = numLit(values.length, loc);
+        const first: IrExpr =
+          name === "push"
+            ? {
+                kind: "bin",
+                op: "-",
+                left: varRef(length.id, F64, loc),
+                right: count,
+                type: F64,
+                loc,
+              }
+            : numLit(0, loc);
+        return {
+          kind: "seqExpr",
+          stmts: [
+            { kind: "varDecl", localId: array.id, init: receiver, loc },
+            ...inserted.stmts,
+            {
+              kind: "varDecl",
+              localId: length.id,
+              init: {
+                kind: "arrIntrinsic",
+                method: name,
+                receiver: arr,
+                args: inserted.args,
+                type: F64,
+                loc,
+              },
+              loc,
+            },
+            ...inserted.fill(arr, first),
+          ],
+          result: varRef(length.id, F64, loc),
+          type: F64,
+          loc,
+        };
+      }
+      return { kind: "arrIntrinsic", method: name, receiver, args: inserted.args, type: F64, loc };
+    }
     // Inserted values flow into the element slot like an assignment would:
     // union-element arrays wrap plain arm values (coerceInto is inert
     // when the types already agree).
@@ -996,9 +1070,12 @@ export function lowerArrayMethodCall(
     const hasSpread = itemNodes.some(ts.isSpreadElement);
     const itemProbes = hasSpread ? [] : itemNodes.map((arg) => tryLowerExpression(lowerer, arg));
     const statefulItems = itemProbes.some(
-      (probe) => probe !== null && lowerer.runtimeOptionalWidening(probe.type, elem) !== null,
+      (probe) =>
+        probe !== null &&
+        (lowerer.runtimeOptionalWidening(probe.type, elem) !== null ||
+          optionalElementValue(lowerer, probe, elem)),
     );
-    if (statefulItems && itemNodes.length === 1) {
+    if (statefulItems && itemNodes.length === 1 && itemProbes[0]?.type.kind === "union") {
       return lowerOptionalSpliceItem(
         lowerer,
         receiver,
@@ -1734,7 +1811,28 @@ function arraySearchHelper(
     type: valueT,
     loc,
   };
-  const value = directNeedle ? rawRead : optionalValue;
+  // A direct needle matches only a present value (`match` tests the state
+  // first); a present undefined of a `(T | undefined)[]` (primitiveElement)
+  // must not be read as T.
+  const value = !directNeedle
+    ? optionalValue
+    : primitiveElement(elem)
+      ? {
+          kind: "ternary" as const,
+          cond: {
+            kind: "bin" as const,
+            op: "===" as const,
+            left: stateRef,
+            right: numLit(1, loc),
+            type: BOOL,
+            loc,
+          },
+          then: rawRead,
+          else_: elementPlaceholder(elem, loc),
+          type: elem,
+          loc,
+        }
+      : rawRead;
   const readT = directNeedle ? elem : valueT;
   const equal = arraySearchEquality(
     lowerer,
diff --git a/packages/compiler/src/frontend/lowering/containers/callback-arguments.ts b/packages/compiler/src/frontend/lowering/containers/callback-arguments.ts
index d95640978..de027e08c 100644
--- a/packages/compiler/src/frontend/lowering/containers/callback-arguments.ts
+++ b/packages/compiler/src/frontend/lowering/containers/callback-arguments.ts
@@ -2,6 +2,7 @@ import { keyedHelperName } from "../stable-names.js";
 import * as ts from "../../ts7/adapter.js";
 import type { Lowerer } from "../lowerer.js";
 import { isJsSourceFile, locOf } from "../../program.js";
+import { primitiveElement } from "../array-values.js";
 import { varRef } from "../../../ir/build.js";
 import { typeKey } from "../../type-mapper.js";
 import {
@@ -144,12 +145,22 @@ export function lowerArrayCallback(
       const untyped = (checkerType.flags & (ts.TypeFlags.Any | ts.TypeFlags.Unknown)) !== 0;
       const raw = untyped ? null : lowerer.mapTypeOf(checkerType);
       const mapped = raw ? lowerer.runtimeOptionalBindingType(param.name, raw) : null;
+      // A helper that passes present number, string or boolean elements
+      // (a `(T | undefined)[]` stores T elements) binds a `T | undefined`
+      // parameter to T, so the callback takes them without an adapter.
+      const presentElement =
+        mapped !== null &&
+        primitiveElement(expected) &&
+        mapped.kind === "union" &&
+        lowerer.armTag(mapped.unionId, UNDEFINED_T) >= 0 &&
+        typeEquals(lowerer.stripUndefinedArm(mapped), expected);
       if (
         untyped
           ? !bindUntyped
           : mapped === null ||
             (lowerer.coercibleValue(expected, mapped) &&
-              !runtimeOptionalPairVariant(lowerer, expected, mapped))
+              !runtimeOptionalPairVariant(lowerer, expected, mapped) &&
+              !presentElement)
       )
         return;
       const symbol = lowerer.checker.getSymbolAtLocation(param.name);
diff --git a/packages/compiler/src/frontend/lowering/containers/collection-methods.ts b/packages/compiler/src/frontend/lowering/containers/collection-methods.ts
index e043daf68..f18291a63 100644
--- a/packages/compiler/src/frontend/lowering/containers/collection-methods.ts
+++ b/packages/compiler/src/frontend/lowering/containers/collection-methods.ts
@@ -1,4 +1,5 @@
 import { keyedHelperName } from "../stable-names.js";
+import { primitiveElement } from "../array-values.js";
 import { nodeThrowExpr, boolLit, countedFor, numLit, strLit, varRef } from "../../../ir/build.js";
 import { InternalCompilerError } from "../../../errors.js";
 import * as ts from "../../ts7/adapter.js";
@@ -1137,6 +1138,17 @@ function setFromSeedValue(
     seed = strCharsCall(lowerer, source, loc);
   } else if (source.type.kind === "array" && typeEquals(source.type.elem, setT.elem)) {
     seed = source;
+  } else if (
+    source.type.kind === "array" &&
+    primitiveElement(source.type.elem) &&
+    setT.elem.kind === "union" &&
+    lowerer.armTag(setT.elem.unionId, UNDEFINED_T) >= 0 &&
+    typeEquals(lowerer.stripUndefinedArm(setT.elem), source.type.elem)
+  ) {
+    // A `(T | undefined)[]` stores T elements: its present undefined
+    // elements copy as the Set's undefined.
+    seed = lowerer.coerceToExpected(source, arrayOf(setT.elem));
+    if (!typeEquals(seed.type, arrayOf(setT.elem))) return null;
   } else if (source.type.kind === "record") {
     const shape = lowerer.shapes.get(source.type.shapeId);
     if (!shape?.tuple || !shape.fields.every((field) => typeEquals(field.type, setT.elem)))
diff --git a/packages/compiler/src/frontend/lowering/containers/indexed-objects.ts b/packages/compiler/src/frontend/lowering/containers/indexed-objects.ts
index 668603d47..1c07885ae 100644
--- a/packages/compiler/src/frontend/lowering/containers/indexed-objects.ts
+++ b/packages/compiler/src/frontend/lowering/containers/indexed-objects.ts
@@ -1,6 +1,7 @@
 import { keyedHelperName } from "../stable-names.js";
 import { nullProtoRecordShape, recordNullProtoTest } from "../null-prototype.js";
 import { countedFor, numLit, varRef } from "../../../ir/build.js";
+import { arrayValueStore, optionalElementValue, primitiveElement } from "../array-values.js";
 import { objectEnumerationReceiver } from "../object-enumeration-receiver.js";
 import { InternalCompilerError } from "../../../errors.js";
 import * as ts from "../../ts7/adapter.js";
@@ -156,7 +157,14 @@ export function objectIterOverIndexShape(
   // also holds numbers): each overflow value checks into the element type,
   // so a value of a dropped arm throws instead of mistyping.
   let ivNarrows = false;
-  if (valueT) {
+  // `T | undefined` values into a `(T | undefined)[]`, which stores T
+  // elements (primitiveElement): each value is stored with its present
+  // undefined.
+  const optionalValues =
+    member === "values" &&
+    valueT !== null &&
+    optionalElementValue(lowerer, { kind: "unitLit", unit: "undefined", type: iv, loc }, valueT);
+  if (valueT && !optionalValues) {
     const ivSurfaces =
       typeEquals(iv, valueT) ||
       (valueT.kind === "union" && lowerer.armTag(valueT.unionId, iv) >= 0) ||
@@ -350,7 +358,25 @@ export function objectIterOverIndexShape(
               loc,
             },
             then: [fieldStmt],
-            else_: undefinedElement ? [push(pushedOf(undefinedElement))] : null,
+            else_: undefinedElement
+              ? [push(pushedOf(undefinedElement))]
+              : member === "values" && valueT && primitiveElement(valueT)
+                ? [
+                    {
+                      kind: "arraySetUndefined",
+                      arr: outRef,
+                      index: {
+                        kind: "arrIntrinsic",
+                        method: "length",
+                        receiver: outRef,
+                        args: [],
+                        type: F64,
+                        loc,
+                      },
+                      loc,
+                    },
+                  ]
+                : null,
             loc,
           };
         }
@@ -374,35 +400,45 @@ export function objectIterOverIndexShape(
     const kRef = varRef("k.0", STRING, loc);
     const readValue: IrExpr | null = !valueT
       ? null
-      : ivNarrows
+      : optionalValues
         ? {
-            kind: "dynCheck",
-            value: {
-              kind: "dynFrom",
-              value: {
-                kind: "recordKeyGet",
-                obj: rRef,
-                shapeId: argIr.shapeId,
-                key: kRef,
-                overflowOnly: true,
-                type: iv,
-                loc,
-              },
-              type: DYN,
-              loc,
-            },
-            type: valueT,
-            loc,
-          }
-        : {
             kind: "recordKeyGet",
             obj: rRef,
             shapeId: argIr.shapeId,
             key: kRef,
             overflowOnly: true,
-            type: valueT,
+            type: iv,
             loc,
-          };
+          }
+        : ivNarrows
+          ? {
+              kind: "dynCheck",
+              value: {
+                kind: "dynFrom",
+                value: {
+                  kind: "recordKeyGet",
+                  obj: rRef,
+                  shapeId: argIr.shapeId,
+                  key: kRef,
+                  overflowOnly: true,
+                  type: iv,
+                  loc,
+                },
+                type: DYN,
+                loc,
+              },
+              type: valueT,
+              loc,
+            }
+          : {
+              kind: "recordKeyGet",
+              obj: rRef,
+              shapeId: argIr.shapeId,
+              key: kRef,
+              overflowOnly: true,
+              type: valueT,
+              loc,
+            };
     const loopPushed: IrExpr =
       member === "keys"
         ? kRef
@@ -440,7 +476,26 @@ export function objectIterOverIndexShape(
             },
             loc,
           },
-          push(loopPushed),
+          ...(optionalValues
+            ? [
+                { kind: "varDecl" as const, localId: "v.0", init: readValue!, loc },
+                arrayValueStore(
+                  lowerer,
+                  outRef,
+                  {
+                    kind: "arrIntrinsic",
+                    method: "length",
+                    receiver: outRef,
+                    args: [],
+                    type: F64,
+                    loc,
+                  },
+                  varRef("v.0", iv, loc),
+                  valueT!,
+                  loc,
+                ),
+              ]
+            : [push(loopPushed)]),
         ],
       ),
       { kind: "return", value: outRef, loc },
@@ -457,6 +512,7 @@ export function objectIterOverIndexShape(
         { id: "ks.0", name: "ks", type: ksT, mutable: false },
         { id: "i.0", name: "i", type: F64, mutable: true },
         { id: "k.0", name: "k", type: STRING, mutable: false },
+        ...(optionalValues ? [{ id: "v.0", name: "v", type: iv, mutable: false }] : []),
       ],
       body,
       loc,
diff --git a/packages/compiler/src/frontend/lowering/expressions/array-literals.ts b/packages/compiler/src/frontend/lowering/expressions/array-literals.ts
index d8e855c5d..903f0884d 100644
--- a/packages/compiler/src/frontend/lowering/expressions/array-literals.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/array-literals.ts
@@ -31,7 +31,7 @@ import { strCharsCall } from "../containers/array-construction.js";
 import { generatorDrain } from "../iterator-adapters.js";
 import { isSearchParamsIterable } from "../builtins/url.js";
 import { dynUndefinedExpr, varRef } from "../../../ir/build.js";
-import { arrayValueStore } from "../array-values.js";
+import { arrayValueStore, primitiveElement } from "../array-values.js";
 import { classGeneratorIterable } from "../class-iterables.js";
 import { iteratorCanStep } from "../iterator-consumption.js";
 
@@ -103,6 +103,13 @@ function emptyNeverLiteral(lowerer: Lowerer, expr: ts.ArrayLiteralExpression): b
  * tsc's API doesn't surface it (ternary arms under an array context —
  * tsc accepts the arm covariantly, but a tagged element representation
  * must be BUILT as the slot's element type). */
+/** The checker type of `node` admits undefined. */
+function mayHoldUndefined(lowerer: Lowerer, node: ts.Expression): boolean {
+  const type = lowerer.typeOf(node);
+  const parts = type.isUnionType() ? ts.constituentTypes(type) : [type];
+  return parts.some((part) => (part.flags & (ts.TypeFlags.Undefined | ts.TypeFlags.Void)) !== 0);
+}
+
 export function lowerArrayLiteral(
   lowerer: Lowerer,
   expr: ts.ArrayLiteralExpression,
@@ -524,7 +531,13 @@ export function lowerArrayLiteral(
       // `[...it]` over a generator (or an iterator adapted into one):
       // drain it once, in order, into a fresh element array.
       if (src.type.kind === "generator") {
-        const drained = generatorDrain(lowerer, src, type.elem, locOf(el));
+        const drained = generatorDrain(
+          lowerer,
+          src,
+          type.elem,
+          locOf(el),
+          type.optionalElements === true,
+        );
         if (drained) src = drained;
       }
       // `[...sp]` and `[...sp.entries()]` over a URLSearchParams: its
@@ -615,6 +628,9 @@ export function lowerArrayLiteral(
     let lowered: IrExpr;
     const directOptionalCandidate =
       runtimeOptionalElements.has(i) ||
+      // An element that can be undefined in a `(T | undefined)[]`
+      // (primitiveElement) is stored as a present undefined.
+      (primitiveElement(type.elem) && mayHoldUndefined(lowerer, el)) ||
       ts.isElementAccessExpression(el) ||
       (ts.isCallExpression(el) &&
         ts.isPropertyAccessExpression(el.expression) &&
@@ -692,7 +708,8 @@ export function lowerArrayLiteral(
       continue;
     }
     let value = el;
-    if (!typeEquals(value.type, type.elem)) {
+    // An undefined element is stored directly (arrayValueStore).
+    if (!typeEquals(value.type, type.elem) && value.type.kind !== "undefinedT") {
       const temp = lowerer.declareHiddenLocal("%arrayValue", value.type);
       body.push({ kind: "varDecl", localId: temp.id, init: value, loc });
       value = varRef(temp.id, temp.type, loc);
diff --git a/packages/compiler/src/frontend/lowering/expressions/element-access.ts b/packages/compiler/src/frontend/lowering/expressions/element-access.ts
index 9ac6a04b6..a2f2ebdb6 100644
--- a/packages/compiler/src/frontend/lowering/expressions/element-access.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/element-access.ts
@@ -61,6 +61,8 @@ import {
   arrayValueType,
   lowerSafeBytesRead,
   lowerSafeIndexRead,
+  primitiveElement,
+  unionArrayValueRead,
 } from "../array-values.js";
 import { lowerOptionalStringIndex, lowerUnionStringIndex } from "../string-index.js";
 import { InternalCompilerError } from "../../../errors.js";
@@ -594,6 +596,8 @@ export function lowerElementAccess(lowerer: Lowerer, expr: ts.ElementAccessExpre
         }
       }
       if (key.type.kind === "f64") {
+        const optional = optionalArmIndexRead(lowerer, value, key, locOf(expr));
+        if (optional) return lowerer.maybeNarrow(optional, expr);
         const safe = lowerOptionalArrayIndexRead(lowerer, expr, value, key);
         if (safe) return safe;
       }
@@ -884,9 +888,14 @@ export function lowerElementAccess(lowerer: Lowerer, expr: ts.ElementAccessExpre
     // must be the element type (library ABI checks and the catchable
     // RangeError contract for an out-of-bounds assert). Where undefined is
     // representable, the assertion keeps JavaScript's unchecked read.
+    // A declared `(T | undefined)[]` element (stored as T) keeps its
+    // optional read: the assertion then narrows it with a catchable
+    // TypeError, as for a union element.
+    const declaredOptional = arr.type.optionalElements === true;
     const provenPresent =
       ts.isNonNullExpression(expr.parent) &&
       expr.parent.expression === expr &&
+      !declaredOptional &&
       !assertedElementKeepsUndefined(lowerer, expr.parent);
     if (provenPresent) {
       return lowerer.maybeNarrow(
@@ -2437,3 +2446,46 @@ export function lowerEnvironmentKey(lowerer: Lowerer, node: ts.Expression): IrEx
     return { kind: "libCall", fn: "dyn.toStringCoerce", args: [key], type: STRING, loc: key.loc };
   return lowerRecordPropertyKey(lowerer, key, node);
 }
+
+/** A number-keyed read of a union of arrays with a `(T | undefined)[]` arm
+ * (stored as T elements, primitiveElement) whose element facts admit
+ * missing values: each arm reads its element by its state
+ * (unionArrayValueRead), where the keyed read would trap on a present
+ * undefined. The receiver and key evaluate once. Null for other unions. */
+function optionalArmIndexRead(
+  lowerer: Lowerer,
+  value: IrExpr,
+  key: IrExpr,
+  loc: SrcLoc,
+): IrExpr | null {
+  if (value.type.kind !== "union") return null;
+  const arms = lowerer.unions.get(value.type.unionId)?.arms ?? [];
+  if (
+    !arms.some(
+      (arm) =>
+        arm.kind === "array" &&
+        primitiveElement(arm.elem) &&
+        arrayValueType(lowerer, arm.elem).kind === "union",
+    )
+  )
+    return null;
+  if (unionArrayValueRead(lowerer, value, key, loc) === null) return null;
+  const stmts: IrStmt[] = [];
+  const stable = (e: IrExpr, name: string, keep: boolean): IrExpr => {
+    if (keep) return e;
+    const local = lowerer.declareHiddenLocal(name, e.type);
+    stmts.push({ kind: "varDecl", localId: local.id, init: e, loc });
+    return varRef(local.id, e.type, loc);
+  };
+  // A receiver local stays a direct read only when the key cannot assign
+  // it (`a[(a = other, 0)]` reads the array `a` held before the key).
+  const plainKey = key.kind === "varRef" || key.kind === "numLit";
+  const receiver = stable(
+    value,
+    "%indexReceiver",
+    value.kind === "numLit" || (value.kind === "varRef" && plainKey),
+  );
+  const index = stable(key, "%indexKey", plainKey);
+  const read = unionArrayValueRead(lowerer, receiver, index, loc)!;
+  return stmts.length === 0 ? read : { kind: "seqExpr", stmts, result: read, type: read.type, loc };
+}
diff --git a/packages/compiler/src/frontend/lowering/iterator-adapters.ts b/packages/compiler/src/frontend/lowering/iterator-adapters.ts
index 9f2b1e31f..4909e7f45 100644
--- a/packages/compiler/src/frontend/lowering/iterator-adapters.ts
+++ b/packages/compiler/src/frontend/lowering/iterator-adapters.ts
@@ -1,3 +1,4 @@
+import { appendElement, optionalElementValue, primitiveElement } from "./array-values.js";
 import { keyedHelperName } from "./stable-names.js";
 import { dynUndefinedExpr, varRef } from "../../ir/build.js";
 import {
@@ -964,16 +965,27 @@ function generatorRetypeAdapter(
   return { kind: "call", callee: name, args: [expr], type: expected, loc };
 }
 
+/** A checked-dynamic yielded value for a `(T | undefined)[]` of a
+ * number, string or boolean T: converted to `T | undefined`, which the
+ * drain stores as a T element or a present undefined. */
+function optionalYield(lowerer: Lowerer, value: IrExpr, elem: IrType, optional: boolean): IrExpr {
+  if (!optional || value.type.kind !== "dyn" || !primitiveElement(elem)) return value;
+  return lowerer.coerceToExpected(value, lowerer.withUndefinedArm(elem));
+}
+
 /** Drain a synchronous generator into a fresh `elem[]` — the eager
  * consumers of an iterator (`[...it]`, `Array.from(it)`). Each step resumes
  * the generator once and appends its value; completion stops the drain
  * without appending the return value, as in JS. Null when the yielded
- * values cannot be extracted into the element type. */
+ * values cannot be extracted into the element type. `optional`: the
+ * result is a `(T | undefined)[]`, which stores T elements, so a
+ * checked-dynamic value may also be undefined. */
 export function generatorDrain(
   lowerer: Lowerer,
   source: IrExpr,
   elem: IrType,
   loc: SrcLoc,
+  optional = false,
 ): IrExpr | null {
   const gen = source.type;
   if (gen.kind !== "generator" || gen.async || gen.yieldT.kind === "void") return null;
@@ -982,14 +994,16 @@ export function generatorDrain(
   const valueT = shape?.fields.find((field) => field.name === "value")?.type;
   if (!resultType || !valueT) return null;
   const arrT: IrType = { kind: "array", elem };
-  const key = `genDrain:${typeKey(gen)}:${typeKey(elem)}`;
+  const key = `genDrain:${typeKey(gen)}:${typeKey(elem)}${optional ? "?" : ""}`;
   let name = lowerer.valueHelpers.get(key);
   if (!name && stepDrivable(gen)) {
     // The record-free drive: test done, then take the yielded value.
     const genRef = varRef("gen.0", gen, loc) as IrExpr & { type: typeof gen };
     const outRef = varRef("out.0", arrT, loc);
-    const value = lowerer.coerceToExpected(genValue(genRef, loc), elem);
-    if (!typeEquals(value.type, elem)) return null;
+    const yielded = optionalYield(lowerer, genValue(genRef, loc), elem, optional);
+    const present = optionalElementValue(lowerer, yielded, elem);
+    const value = present ? yielded : lowerer.coerceToExpected(yielded, elem);
+    if (!present && !typeEquals(value.type, elem)) return null;
     name = keyedHelperName(lowerer, "%gen.drain", key);
     lowerer.valueHelpers.set(key, name);
     lowerer.liftedFns.push({
@@ -999,6 +1013,7 @@ export function generatorDrain(
       locals: [
         { id: "gen.0", name: "gen", type: gen, mutable: false },
         { id: "out.0", name: "out", type: arrT, mutable: false },
+        ...(present ? [{ id: "v.0", name: "v", type: value.type, mutable: false }] : []),
       ],
       body: [
         {
@@ -1010,20 +1025,7 @@ export function generatorDrain(
         {
           kind: "while",
           cond: { kind: "unary", op: "!", operand: genStep(genRef, loc), type: BOOL, loc },
-          body: [
-            {
-              kind: "exprStmt",
-              expr: {
-                kind: "arrIntrinsic",
-                method: "push",
-                receiver: outRef,
-                args: [value],
-                type: F64,
-                loc,
-              },
-              loc,
-            },
-          ],
+          body: appendElement(lowerer, outRef, value, elem, "v.0", loc),
           loc,
         },
         { kind: "return", value: outRef, loc },
@@ -1050,8 +1052,10 @@ export function generatorDrain(
       loc,
     );
     if (!raw) return null;
-    const value = lowerer.coerceToExpected(raw, elem);
-    if (!typeEquals(value.type, elem)) return null;
+    const yielded = optionalYield(lowerer, raw, elem, optional);
+    const present = optionalElementValue(lowerer, yielded, elem);
+    const value = present ? yielded : lowerer.coerceToExpected(yielded, elem);
+    if (!present && !typeEquals(value.type, elem)) return null;
     name = keyedHelperName(lowerer, "%gen.drain", key);
     lowerer.valueHelpers.set(key, name);
     lowerer.liftedFns.push({
@@ -1062,6 +1066,7 @@ export function generatorDrain(
         { id: "gen.0", name: "gen", type: gen, mutable: false },
         { id: "out.0", name: "out", type: arrT, mutable: false },
         { id: "step.0", name: "step", type: resultType, mutable: false },
+        ...(present ? [{ id: "v.0", name: "v", type: value.type, mutable: false }] : []),
       ],
       body: [
         {
@@ -1101,18 +1106,7 @@ export function generatorDrain(
               else_: null,
               loc,
             },
-            {
-              kind: "exprStmt",
-              expr: {
-                kind: "arrIntrinsic",
-                method: "push",
-                receiver: outRef,
-                args: [value],
-                type: F64,
-                loc,
-              },
-              loc,
-            },
+            ...appendElement(lowerer, outRef, value, elem, "v.0", loc),
           ],
           loc,
         },
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index fc8f10fa8..624597832 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -6585,6 +6585,17 @@ export class Lowerer {
       mayBeUndefined: (node) => {
         if (explicitlyNonNull(node)) return assertedReadMayBeUndefined(node);
         if (mayBeOptional(node)) return true;
+        // A value the checker types as possibly undefined: `(T | undefined)[]`
+        // stores T elements (the type mapper), so its undefined values are
+        // facts of T's arrays.
+        const checked = typeOrNull(node);
+        if (
+          checked !== null &&
+          (checked.isUnionType() ? ts.constituentTypes(checked) : [checked]).some(
+            (part) => (part.flags & (ts.TypeFlags.Undefined | ts.TypeFlags.Void)) !== 0,
+          )
+        )
+          return true;
         // `undefined as unknown as T` keeps the undefined value.
         for (
           let inner: ts.Expression = node;
@@ -6603,7 +6614,49 @@ export class Lowerer {
         const symbol = symbolOf(e);
         return symbol !== null && iterationSymbols.has(symbol) && !provenPresent(e, symbol);
       },
-      callbackReturnsUndefined: (node) => callbackReturnsOptional(node),
+      iterationMayYieldUndefined: (node) => {
+        const type = typeOrNull(node);
+        if (type === null || this.checker.isArrayType(type)) return false;
+        // A tuple yields its elements, in order.
+        if (this.checker.isTupleType(type))
+          return panicSafe(
+            () =>
+              this.checker
+                .getTypeArguments(type as ts.TypeReference)
+                .some((element) =>
+                  (element.isUnionType() ? ts.constituentTypes(element) : [element]).some(
+                    (part) => (part.flags & (ts.TypeFlags.Undefined | ts.TypeFlags.Void)) !== 0,
+                  ),
+                ),
+            true,
+          );
+        // The first type argument of Set<T>, Map and Set iterators,
+        // generators and iterables is the value they yield.
+        const element =
+          (type.flags & ts.TypeFlags.Object) !== 0
+            ? panicSafe(() => this.checker.getTypeArguments(type as ts.TypeReference)[0], undefined)
+            : undefined;
+        if (element === undefined) return false;
+        return (element.isUnionType() ? ts.constituentTypes(element) : [element]).some(
+          (part) => (part.flags & (ts.TypeFlags.Undefined | ts.TypeFlags.Void)) !== 0,
+        );
+      },
+      callbackReturnsUndefined: (node) => {
+        if (callbackReturnsOptional(node)) return true;
+        // A callback the checker types as possibly returning undefined
+        // (`(m) => map.get(m)`): its results are present undefined
+        // elements of a `(T | undefined)[]`, which stores T elements.
+        const type = typeOrNull(node);
+        return (
+          type !== null &&
+          this.checker.getCallSignatures(type).some((signature) => {
+            const result = this.checker.getReturnTypeOfSignature(signature);
+            return (result.isUnionType() ? ts.constituentTypes(result) : [result]).some(
+              (part) => (part.flags & (ts.TypeFlags.Undefined | ts.TypeFlags.Void)) !== 0,
+            );
+          })
+        );
+      },
       isGlobal: (node, name) => this.isStdlibGlobal(node, name),
       isString: (node) => {
         const type = typeOrNull(node);
@@ -6630,6 +6683,14 @@ export class Lowerer {
         const rest = sig.params.find((shape) => shape.mode === "rest");
         return rest?.type.kind === "array" ? [typeKey(rest.type.elem)] : [];
       },
+      restStart: (call) => {
+        const callee = peel(call.expression);
+        if (!ts.isIdentifier(callee) && !ts.isPropertyAccessExpression(callee)) return null;
+        const symbol = callableSymbolOf(callee);
+        const sig = symbol ? signatureBySymbol.get(symbol) : undefined;
+        const at = sig ? sig.params.findIndex((shape) => shape.mode === "rest") : -1;
+        return at >= 0 ? at : null;
+      },
       markIterationBinding: (node, known) => {
         const symbol = symbolOf(node);
         if (!symbol) return false;
@@ -6664,6 +6725,7 @@ export class Lowerer {
       const cached = scanSites.get(sf);
       if (cached !== undefined) return cached;
       const nodes: ts.Node[] = [];
+      const values: ts.Node[] = [];
       ts.walkPreorder(sf, (node) => {
         if (
           ts.isVariableDeclaration(node) ||
@@ -6679,7 +6741,21 @@ export class Lowerer {
           ts.isPostfixUnaryExpression(node)
         )
           nodes.push(node);
+        // The values these sites store, whose checker types mayBeUndefined
+        // reads: batched here, since dead bodies are scanned too.
+        if (ts.isArrayLiteralExpression(node))
+          for (const element of node.elements.nodes)
+            values.push(ts.isSpreadElement(element) ? element.expression : element);
+        else if (ts.isCallExpression(node) || ts.isNewExpression(node))
+          for (const arg of node.arguments?.nodes ?? [])
+            values.push(ts.isSpreadElement(arg) ? arg.expression : arg);
+        else if (
+          ts.isBinaryExpression(node) &&
+          node.operatorToken.kind === ts.SyntaxKind.EqualsToken
+        )
+          values.push(node.right);
       });
+      this.checker.prefetchCollectionTypes(values);
       scanSites.set(sf, nodes);
       return nodes;
     };
diff --git a/packages/compiler/src/frontend/lowering/runtime-optional-elements.ts b/packages/compiler/src/frontend/lowering/runtime-optional-elements.ts
index 96d20d2c4..7561f4a65 100644
--- a/packages/compiler/src/frontend/lowering/runtime-optional-elements.ts
+++ b/packages/compiler/src/frontend/lowering/runtime-optional-elements.ts
@@ -16,6 +16,9 @@ export interface ElementStateHost {
   elementAdmitsUndefined(key: string): boolean;
   /** The value stored by `node` can be `undefined` at runtime. */
   mayBeUndefined(node: ts.Expression): boolean;
+  /** Iterating `node`, an iterable other than an array (a Set, a Map's
+   * values, a generator), can yield `undefined`. */
+  iterationMayYieldUndefined?(node: ts.Expression): boolean;
   /** A callback argument can return `undefined` at runtime. */
   callbackReturnsUndefined(node: ts.Expression): boolean;
   /** The value may be a plain array-like object, whose missing indices
@@ -28,6 +31,9 @@ export interface ElementStateHost {
   /** Element keys of the rest parameter that the spread arguments of
    * `call` fill: `[]` when the callee has none, null when it is unknown. */
   restKeys(call: ts.CallExpression | ts.NewExpression): readonly string[] | null;
+  /** The argument index where the callee's rest parameter starts, or null
+   * when the callee has none or is unknown. */
+  restStart?(call: ts.CallExpression | ts.NewExpression): number | null;
   /** Record that iterating `node`'s binding can observe `undefined`;
    * `known` when the iterated array's own facts (not only those shared by
    * its element ABI) say so. Returns true when the binding's record grew. */
@@ -353,7 +359,11 @@ export class ArrayElementStates {
 
   /** One stored value, or the elements a spread copies. */
   private valueMayBeUndefined(value: ts.Expression): boolean {
-    if (ts.isSpreadElement(value)) return this.mayYieldUndefined(value.expression);
+    if (ts.isSpreadElement(value))
+      return (
+        this.mayYieldUndefined(value.expression) ||
+        (this.host.iterationMayYieldUndefined?.(value.expression) ?? false)
+      );
     return this.host.mayBeUndefined(value);
   }
 
@@ -511,14 +521,26 @@ export class ArrayElementStates {
   private scanCall(node: ts.CallExpression | ts.NewExpression): boolean {
     let changed = false;
     const args = node.arguments?.nodes ?? [];
-    // Spreading a sparse array into a rest parameter fills the holes.
+    // Spreading a sparse array, or a tuple with undefined elements, into a
+    // rest parameter fills the holes.
     if (args.some((arg) => ts.isSpreadElement(arg))) {
       for (const arg of args) {
-        if (!ts.isSpreadElement(arg) || !this.mayYieldUndefined(arg.expression)) continue;
+        if (!ts.isSpreadElement(arg) || !this.valueMayBeUndefined(arg)) continue;
         const rest = this.host.restKeys(node) ?? this.host.arrayKeys(arg.expression);
         if (this.addUndefined(rest)) changed = true;
       }
     }
+    // An argument in a rest position that may be undefined is a present
+    // undefined element of the rest array.
+    const restStart = this.host.restStart?.(node) ?? null;
+    if (restStart !== null)
+      for (const arg of args.slice(restStart))
+        if (
+          !ts.isSpreadElement(arg) &&
+          this.host.mayBeUndefined(arg) &&
+          this.addUndefined(this.host.restKeys(node))
+        )
+          changed = true;
     if (this.record(node)) changed = true;
     const callee = peel(node.expression);
     if (!ts.isPropertyAccessExpression(callee)) return changed;
diff --git a/packages/compiler/src/frontend/lowering/sanitize-class-types.ts b/packages/compiler/src/frontend/lowering/sanitize-class-types.ts
index 77478d76d..6b89b446a 100644
--- a/packages/compiler/src/frontend/lowering/sanitize-class-types.ts
+++ b/packages/compiler/src/frontend/lowering/sanitize-class-types.ts
@@ -16,6 +16,8 @@ export function sanitizeUnregisteredClassTypes(
       case "object":
         return hasClass(type.className) ? type : F64;
       case "array":
+        type.elem = rewrite(type.elem);
+        break;
       case "set":
         type.elem = rewrite(type.elem);
         break;
diff --git a/packages/compiler/src/frontend/type-mapper.ts b/packages/compiler/src/frontend/type-mapper.ts
index 63f421dab..17f9d0d11 100644
--- a/packages/compiler/src/frontend/type-mapper.ts
+++ b/packages/compiler/src/frontend/type-mapper.ts
@@ -2349,6 +2349,23 @@ function mapTypeInner(type: ts.Type, ctx: TypeMapperCtx): IrType | null {
       elem = unitOnlyUnion(unions);
 
     if (!elem) return null;
+    // `(T | undefined)[]` of a number, string or boolean T stores T
+    // elements: an array slot holds a present undefined by its own state,
+    // so a T[] seen as `(T | undefined)[]` stays the same array.
+    let optionalElements = false;
+    if (elem.kind === "union") {
+      const arms = unions.get(elem.unionId)?.arms;
+      const present =
+        arms?.length === 2 ? arms.find((arm) => arm.kind !== "undefinedT") : undefined;
+      if (
+        present !== undefined &&
+        arms!.some((arm) => arm.kind === "undefinedT") &&
+        (present.kind === "f64" || present.kind === "string" || present.kind === "bool")
+      ) {
+        elem = present;
+        optionalElements = true;
+      }
+    }
     // A dyn ELEMENT makes the WHOLE array the checked-dynamic value:
     // `unknown[]`, `object[]`, and the collapsed `(string | object)[]`
     // (the plugins-slot shape) — the checked-dynamic tree has real arrays, so length/
@@ -2370,7 +2387,7 @@ function mapTypeInner(type: ts.Type, ctx: TypeMapperCtx): IrType | null {
     // drain of the auxiliary-server registries): handles are ordinary
     // refcounted REF elements (their `_v` adapters, no trace — both drop
     // their closures at their terminal event).
-    return arrayOf(elem);
+    return optionalElements ? { kind: "array", elem, optionalElements: true } : arrayOf(elem);
   }
   // Tuples: `[string, string]` maps to an interned RECORD shape flagged
   // `tuple`, with one field per position named by its index ("0", "1", ...)
diff --git a/packages/compiler/src/ir/ir.ts b/packages/compiler/src/ir/ir.ts
index 251d14904..147f769a1 100644
--- a/packages/compiler/src/ir/ir.ts
+++ b/packages/compiler/src/ir/ir.ts
@@ -87,7 +87,16 @@ export type IrType =
   | { kind: "date" }
   | { kind: "string" } // heap, refcounted, UTF-8
   | { kind: "bool" }
-  | { kind: "array"; elem: IrType } // heap, refcounted, monomorphic elements
+  | {
+      kind: "array";
+      elem: IrType;
+      /** A `(T | undefined)[]` of a number, string or boolean T, stored as
+       * T elements (which hold a present undefined by their state). Not
+       * part of the type's identity (typeEquals, typeKey): it only lets a
+       * checked conversion from a dynamic value accept undefined
+       * elements. */
+      optionalElements?: true;
+    } // heap, refcounted, monomorphic elements
   /** ES `Map<K, V>` — heap, refcounted, insertion-ordered hash map with ONE
    * runtime representation (ScrMap) and type-directed key/value handling,
    * exactly the array pattern (never per-instantiation structs). Keys use
diff --git a/packages/compiler/src/ir/validate.ts b/packages/compiler/src/ir/validate.ts
index eba319f85..3d97ac533 100644
--- a/packages/compiler/src/ir/validate.ts
+++ b/packages/compiler/src/ir/validate.ts
@@ -2072,12 +2072,18 @@ function validateFunction(
       }
       case "arrayNewLen": {
         // Mapper-less Array.from({length: n}): absent-slot fill exists
-        // only for refcounted element kinds (frontend fences scalars).
+        // only for refcounted element kinds. Number and boolean slots are
+        // filled with present undefined values right after
+        // (`(T | undefined)[]`); the frontend fences other scalar uses.
         if (e.type.kind !== "array") {
           err(`arrayNewLen must be array-typed, got ${e.type.kind}`, e.loc);
           break;
         }
-        if (!isRefCounted(e.type.elem)) {
+        if (
+          !isRefCounted(e.type.elem) &&
+          e.type.elem.kind !== "f64" &&
+          e.type.elem.kind !== "bool"
+        ) {
           err(
             `arrayNewLen with non-refcounted ${e.type.elem.kind} elements (no absent value)`,
             e.loc,
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8000-optional-element-arrays-share-storage.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8000-optional-element-arrays-share-storage.ts.json
new file mode 100644
index 000000000..8cdd32baf
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8000-optional-element-arrays-share-storage.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8000-optional-element-arrays-share-storage.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8001-optional-element-writers.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8001-optional-element-writers.ts.json
new file mode 100644
index 000000000..fe45249bd
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8001-optional-element-writers.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8001-optional-element-writers.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8004-optional-element-conversions.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8004-optional-element-conversions.ts.json
new file mode 100644
index 000000000..2a1bc7c66
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8004-optional-element-conversions.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8004-optional-element-conversions.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/8000-optional-element-arrays-share-storage.ts b/tests/corpus/8000-optional-element-arrays-share-storage.ts
new file mode 100644
index 000000000..fa31fcad2
--- /dev/null
+++ b/tests/corpus/8000-optional-element-arrays-share-storage.ts
@@ -0,0 +1,61 @@
+// A number[] or string[] seen as `(T | undefined)[]` is the same array, as
+// in Node: undefined written through either name is visible through both,
+// whether the wider view is a cast, a binding or a parameter.
+
+const c: number[] = [1.5, 2];
+(c as (number | undefined)[])[1] = undefined;
+console.log(c);
+const e: number[] = [1.5, 2];
+const f = e as (number | undefined)[];
+f[1] = undefined;
+console.log(e, f, e === f);
+
+function clearFirst(values: (number | undefined)[]): void {
+  values[0] = undefined;
+  values.push(undefined, 4);
+}
+const a: number[] = [1, 2, 3];
+clearFirst(a);
+console.log(a, a.length, a.indexOf(4), a.includes(undefined as unknown as number));
+
+const names: string[] = ["x"];
+const wider: (string | undefined)[] = names;
+wider.push(undefined);
+wider.unshift(undefined);
+console.log(names, wider === names, names.length, JSON.stringify(names));
+
+// The wider arrays themselves: literals, pushes, splices and copies keep
+// their undefined elements.
+const lit: (number | undefined)[] = [1, undefined, 3];
+const maybe = (i: number): number | undefined => (i % 2 ? i : undefined);
+const built: (number | undefined)[] = [];
+for (let i = 0; i < 4; i++) built.push(maybe(i));
+built.splice(1, 0, undefined);
+console.log(lit, built, lit.indexOf(undefined), built.filter((x): boolean => x === undefined).length);
+console.log(built.map((x) => x ?? -1), built.find((x) => x !== undefined), [...built].reverse());
+let sum = 0;
+for (const x of built) sum += x ?? 0;
+console.log(sum, built.join("-"), JSON.stringify(built));
+
+const flags: (boolean | undefined)[] = [true, undefined, false];
+const strs: (string | undefined)[] = ["a", undefined];
+console.log(flags, flags.filter(Boolean).length, strs.map((s) => s?.toUpperCase()));
+
+// Values from other containers keep their undefined.
+const m = new Map<string, number | undefined>([["a", undefined], ["b", 2]]);
+const fromMap = [...m.values()];
+const fromSet = Array.from(new Set<string | undefined>(["s", undefined]));
+const sized = Array.from<number | undefined>({ length: 2 });
+console.log(fromMap, fromSet, sized, sized.length);
+const dict: { [key: string]: number | undefined } = { a: 1, b: undefined };
+console.log(Object.values(dict));
+
+// An asserted element that is undefined throws a catchable TypeError.
+function lengthAt(items: (string | undefined)[], i: number): number {
+  try {
+    return items[i]!.length;
+  } catch {
+    return -1;
+  }
+}
+console.log(lengthAt(strs, 0), lengthAt(strs, 1));
diff --git a/tests/corpus/8001-optional-element-writers.ts b/tests/corpus/8001-optional-element-writers.ts
new file mode 100644
index 000000000..5dff693c3
--- /dev/null
+++ b/tests/corpus/8001-optional-element-writers.ts
@@ -0,0 +1,103 @@
+// Every way a value reaches a `(T | undefined)[]` of numbers, strings or
+// booleans keeps its undefined: callbacks, generators, Array.of, rest
+// packs, destructuring, class fields and copies into Sets and Maps.
+
+const slots = new Map<string, number>([["a", 1]]);
+const looked = ["x", "a", "y"].map((key) => slots.get(key));
+console.log(looked[0] === undefined, looked, looked.length);
+const fromMapped = Array.from(["a", "b"], (key) => slots.get(key));
+console.log(fromMapped[1] === undefined, fromMapped);
+const objects: { n?: number }[] = [{ n: 1 }, {}];
+const fields = objects.map((o) => o.n);
+function pick(o: { n?: number }): number | undefined {
+  return o.n;
+}
+console.log(fields[1] === undefined, objects.map(pick)[1] === undefined, fields);
+function wrap<T>(value: T): T[] {
+  return [value];
+}
+console.log(wrap<number | undefined>(undefined)[0] === undefined);
+
+function* maybe(): Generator<number | undefined> {
+  yield undefined;
+  yield 2;
+}
+const drained = [...maybe()];
+console.log(drained[0] === undefined, drained, Array.from(maybe()));
+const yielding = function* (): Generator<string | undefined> {
+  yield "s";
+  yield undefined;
+};
+console.log([...yielding()]);
+console.log(Array.of<number | undefined>(undefined, 1), Array.of<boolean | undefined>(true));
+
+function sum(...values: (number | undefined)[]): number {
+  return values.reduce<number>((total, v) => total + (v ?? 0), 0);
+}
+const spread: (number | undefined)[] = [undefined, 4];
+console.log(sum(1, undefined, 2), sum(...spread), sum(...spread, undefined, 1));
+
+const [first, second = 7, third] = [3, undefined, 1] as (number | undefined)[];
+console.log(first, second, third);
+const strings: (string | undefined)[] = ["a", undefined, "a"];
+const seen = new Set(strings);
+console.log(seen.size, seen.has(undefined), [...seen]);
+for (const [i, s] of strings.entries()) console.log(i, s);
+for (const key in strings) console.log("key", key);
+
+class Holder {
+  values: (number | undefined)[] = [];
+  add(value: number | undefined): void {
+    this.values.push(value);
+  }
+  first(): number | undefined {
+    return this.values[0];
+  }
+}
+const holder = new Holder();
+holder.add(undefined);
+holder.add(2);
+console.log(holder.first(), holder.values, holder.values.length);
+
+const xs: (number | undefined)[] = [3, undefined, 1];
+xs.forEach((x, i) => console.log(i, x, typeof x));
+console.log(xs.findIndex((x) => x === undefined), xs.findLast((x) => x !== undefined));
+console.log(xs.some((x) => x === undefined), xs.every((x) => x !== undefined));
+console.log(xs.reduce<number>((acc, x) => acc + (x ?? 10), 0), xs.join("/"), xs.at(1), xs.at(-1));
+console.log(xs.toSorted((a, b) => (a ?? 0) - (b ?? 0)), [...xs].sort());
+xs[1] ??= 5;
+console.log(xs);
+const flags: (boolean | undefined)[] = [undefined, true];
+console.log(flags.map((b) => !b), flags.indexOf(undefined), flags.includes(true), String(flags));
+const nested: (number | undefined)[][] = [[undefined], [1]];
+console.log(nested.flat(), nested[0]![0]);
+const byKey = new Map<string, (number | undefined)[]>([["k", [undefined, 1]]]);
+console.log(byKey.get("k")![0], byKey);
+
+// Promise.all over `Promise<T | undefined>[]` settles on Node's turn, with
+// its undefined values.
+const lookup = async (key: string): Promise<number | undefined> => {
+  if (key === "x") throw new Error("missing " + key);
+  return slots.get(key);
+};
+const turns: string[] = [];
+const pending: Promise<number | undefined>[] = [lookup("a"), lookup("b")];
+void Promise.all(pending).then((vs) => turns.push("all " + vs.join(",") + " " + (vs[1] === undefined)));
+const failing: Promise<number | undefined>[] = [lookup("a"), lookup("x")];
+Promise.all(failing).then(
+  () => turns.push("unexpected"),
+  (error: unknown) => turns.push("rejected " + (error as Error).message),
+);
+void Promise.all([] as Promise<number | undefined>[]).then((vs) => turns.push("empty " + vs.length));
+void Promise.resolve()
+  .then(() => turns.push("t1"))
+  .then(() => turns.push("t2"))
+  .then(() => turns.push("t3"))
+  .then(() => console.log(turns.join(" ")));
+
+// Index reads through a union of arrays, and searches with a number
+// needle, see the present undefined elements.
+const pairs: ((null | undefined)[] | (number | undefined)[])[] = [[null, undefined], [undefined, 0]];
+for (const pair of pairs) console.log(pair[0], pair[1], pair[2]);
+const sparse: (number | undefined)[] = [1, , undefined, 1];
+console.log(sparse.lastIndexOf(undefined), sparse.lastIndexOf(1, 2), sparse.indexOf(1, 1));
diff --git a/tests/corpus/8004-optional-element-conversions.ts b/tests/corpus/8004-optional-element-conversions.ts
new file mode 100644
index 000000000..a592a4f32
--- /dev/null
+++ b/tests/corpus/8004-optional-element-conversions.ts
@@ -0,0 +1,82 @@
+// `(T | undefined)[]` values built by checked conversions, rest parameters
+// and union index reads: nested arrays converted beside their non-optional
+// form, a union arm that is an optional-element array, undefined arguments
+// and tuple spreads packed into a rest parameter, and an index expression
+// that reassigns the receiver.
+
+const plainRows = JSON.parse("[[1, 2], [3]]") as number[][];
+console.log(plainRows);
+const optionalSource: unknown = (JSON.parse("[[1, null], [null, 4]]") as unknown[][]).map((row) =>
+  row.map((v) => (v === null ? undefined : v)),
+);
+const optionalRows = optionalSource as (number | undefined)[][];
+console.log(optionalRows, optionalRows[0]![1], optionalRows[1]!.length);
+
+const armSource: unknown = [1, undefined, 3];
+const arm = armSource as (number | undefined)[] | string;
+console.log(typeof arm === "string" ? arm : arm.length, arm);
+const stringArm = ["a", undefined] as unknown as (string | undefined)[] | number;
+console.log(stringArm);
+const textArm = "text" as unknown as (string | undefined)[] | string;
+console.log(textArm);
+
+function flags(...values: (boolean | undefined)[]): string {
+  let text = "";
+  for (const value of values) text += value === undefined ? "u" : value ? "t" : "f";
+  return `${values.length}:${text}:${String(values[1])}`;
+}
+console.log(flags(true, undefined));
+const flagPair: [boolean, boolean | undefined] = [false, undefined];
+console.log(flags(...flagPair), flags(true, ...flagPair), flags(...flagPair, true));
+function label(first: string, ...rest: (string | undefined)[]): string {
+  return `${first}/${rest.length}/${rest.map((v) => v ?? "-").join("")}`;
+}
+const words: [string, string | undefined, string | undefined] = ["a", undefined, "c"];
+console.log(label(...words), label("b", undefined));
+// Spread tuple fields are read before later arguments run.
+const changing: [boolean, boolean | undefined] = [true, undefined];
+function flip(pair: [boolean, boolean | undefined], second: boolean | undefined): boolean {
+  pair[0] = !pair[0];
+  pair[1] = second;
+  return pair[0];
+}
+console.log(flags(...changing, flip(changing, true)));
+console.log(flags(...changing, flip(changing, undefined), ...changing));
+const swapped: [string, string | undefined] = ["x", "y"];
+function swap(pair: [string, string | undefined]): string {
+  pair[0] = "w";
+  pair[1] = undefined;
+  return "v";
+}
+console.log(label("z", ...swapped, swap(swapped), ...swapped));
+// Tuple spreads together with array and Set spreads keep undefined
+// elements, and every item is read in argument order.
+const mixedPair: [boolean, boolean | undefined] = [true, undefined];
+const mixedMore: (boolean | undefined)[] = [false, undefined];
+console.log(flags(...mixedPair, ...mixedMore), flags(...mixedMore, ...mixedPair), flags(true, ...mixedPair, ...mixedMore, undefined));
+const mixedChanging: [boolean, boolean | undefined] = [true, undefined];
+console.log(flags(...mixedChanging, ...mixedMore, flip(mixedChanging, true), ...mixedChanging));
+const mixedWords: [string, string | undefined] = ["a", undefined];
+console.log(label("z", ...["b", undefined, "c"], ...mixedWords), label("y", ...mixedWords, ...new Set(["s", undefined])));
+// A tuple returned by a call keeps its string fields alive until every
+// field is packed beside the other spreads.
+function makeTuple(n: number): [string, string | undefined, string] {
+  return ["p" + n.toString(16).repeat(4), n % 2 ? undefined : "q" + n, "r".repeat((n % 7) + 3) + n];
+}
+for (let i = 0; i < 3; i++) {
+  console.log(label("z", ...["m", undefined], ...makeTuple(i), ...mixedWords), label("y", ...makeTuple(i + 10), ...new Set(["s" + i])));
+}
+
+function pick(start: (number | undefined)[] | string[], other: (number | undefined)[] | string[]): string {
+  let a = start;
+  const read = a[((a = other), 0)];
+  return `${String(read)} ${String(a[0])}`;
+}
+console.log(pick([1, undefined, 3], ["x", "y"]), pick([undefined, 2], ["z"]));
+function counted(start: (number | undefined)[] | string[]): string {
+  let b = start;
+  let i = 1;
+  const read = b[((b = ["q"]), i--)];
+  return `${String(read)} ${b.length} ${i}`;
+}
+console.log(counted([4, undefined]), counted(["s", "t"]));
diff --git a/tests/harness/corpus-costs/8000-optional-element-arrays-share-storage.ts.json b/tests/harness/corpus-costs/8000-optional-element-arrays-share-storage.ts.json
new file mode 100644
index 000000000..b3db55643
--- /dev/null
+++ b/tests/harness/corpus-costs/8000-optional-element-arrays-share-storage.ts.json
@@ -0,0 +1 @@
+{ "plain": 6206, "san": 8487 }
diff --git a/tests/harness/corpus-costs/8001-optional-element-writers.ts.json b/tests/harness/corpus-costs/8001-optional-element-writers.ts.json
new file mode 100644
index 000000000..31695bc47
--- /dev/null
+++ b/tests/harness/corpus-costs/8001-optional-element-writers.ts.json
@@ -0,0 +1 @@
+{ "plain": 3690, "san": 4000 }
diff --git a/tests/harness/corpus-costs/8004-optional-element-conversions.ts.json b/tests/harness/corpus-costs/8004-optional-element-conversions.ts.json
new file mode 100644
index 000000000..9f8eb9e05
--- /dev/null
+++ b/tests/harness/corpus-costs/8004-optional-element-conversions.ts.json
@@ -0,0 +1 @@
+{ "plain": 4882, "san": 6379 }