From 33588981ae38c6a7d6380024e014dad5cc07abd0 Mon Sep 17 00:00:00 2001
From: Malte Ubl <malte@vercel.com>
Date: Mon, 19 Oct 2026 19:59:40 -0700
Subject: [PATCH 1/5] Bake mixed JSON tables and destructure unions of arrays
 and tuples

JSON documents whose rows mix strings and numbers (`(string | number)[][]`,
`(string | number[])[][]`, the character-set and code-point tables of
iconv-lite and tr46) now bake: comptime literals take union-typed elements
and pick each value's arm from its JSON kind and contents.

Array destructuring of a union of array and tuple types
(`const [m, a, b] = re.exec(s) || ["", "", ""]`) reads each position from
the arm the value holds: a tuple arm answers its field or undefined past
its end, an array arm its element or undefined past its end.

Unions of tuple types no longer pick the checker's `length` literal as a
discriminant: tuple shapes have no such slot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../src/frontend/lowering/array-values.ts     | 60 ++++++++++++---
 .../src/frontend/lowering/lower-comptime.ts   | 73 ++++++++++++++++++-
 .../src/frontend/lowering/lower-stmts.ts      | 12 ++-
 packages/compiler/src/frontend/type-mapper.ts |  3 +
 .../main.ts.json                              |  6 ++
 ...11-union-array-tuple-destructuring.ts.json |  6 ++
 .../6410-json-mixed-scalar-tables/extra.json  |  1 +
 .../6410-json-mixed-scalar-tables/main.ts     | 21 ++++++
 .../node_modules/code-table/index.d.ts        |  2 +
 .../node_modules/code-table/index.js          | 10 +++
 .../node_modules/code-table/package.json      |  1 +
 .../node_modules/code-table/table.json        |  1 +
 .../node_modules/range-map/index.d.ts         |  2 +
 .../node_modules/range-map/index.js           | 37 ++++++++++
 .../node_modules/range-map/package.json       |  1 +
 .../node_modules/range-map/table.json         |  1 +
 .../6411-union-array-tuple-destructuring.ts   | 34 +++++++++
 17 files changed, 255 insertions(+), 16 deletions(-)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6410-json-mixed-scalar-tables/main.ts.json
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6411-union-array-tuple-destructuring.ts.json
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/extra.json
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/main.ts
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/index.d.ts
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/index.js
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/package.json
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/table.json
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/index.d.ts
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/index.js
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/package.json
 create mode 100644 tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/table.json
 create mode 100644 tests/corpus/6411-union-array-tuple-destructuring.ts

diff --git a/packages/compiler/src/frontend/lowering/array-values.ts b/packages/compiler/src/frontend/lowering/array-values.ts
index 8f2b28a3f4..e9148b244d 100644
--- a/packages/compiler/src/frontend/lowering/array-values.ts
+++ b/packages/compiler/src/frontend/lowering/array-values.ts
@@ -84,8 +84,11 @@ export function arrayValueRead(
   };
 }
 
-/** Read a stabilized union of native arrays without copying either the
- * receiver or its elements. Every arm contributes its missing-value case. */
+/** Read one position of a stabilized union of native arrays and tuples
+ * (`const [a, b] = re.exec(s) || ["", ""]`) without copying the receiver
+ * or its elements. Every array arm contributes its missing-value case; a
+ * tuple arm answers its field, or undefined past its end. Tuple arms need
+ * a constant position. */
 export function unionArrayValueRead(
   lowerer: Lowerer,
   value: IrExpr,
@@ -96,17 +99,32 @@ export function unionArrayValueRead(
   const unionId = value.type.unionId;
   const arms = lowerer.unions.get(unionId)?.arms;
   if (!arms?.length) return null;
+  const position = index.kind === "numLit" ? String(index.value) : null;
+  // A tuple arm's answer at the position: its field, or undefined past
+  // its end. Null when the arm is neither an array nor a tuple.
+  const tupleField = (arm: IrType): { name: string; type: IrType } | "past" | null => {
+    if (arm.kind !== "record" || position === null) return null;
+    const shape = lowerer.shapes.get(arm.shapeId);
+    if (!shape?.tuple) return null;
+    return shape.fields.find((f) => f.name === position) ?? "past";
+  };
   // Keep the collection's IrType union layout and narrow each element at
   // its use; an inferred array predicate changes the collection's ABI.
-  for (const arm of arms) if (arm.kind !== "array") return null;
+  for (const arm of arms) if (arm.kind !== "array" && tupleField(arm) === null) return null;
   const answers = new Map<string, IrType>();
   for (const arm of arms) {
-    if (arm.kind !== "array") return null;
-    const answer = arrayValueType(lowerer, arm.elem);
+    const field = arm.kind === "array" ? null : tupleField(arm);
+    const answer =
+      arm.kind === "array"
+        ? arrayValueType(lowerer, arm.elem)
+        : field === "past"
+          ? UNDEFINED_T
+          : field!.type;
     const parts = answer.kind === "union" ? lowerer.unions.get(answer.unionId)!.arms : [answer];
     for (const part of parts) answers.set(typeKey(part), part);
   }
   const joined = [...answers.values()].sort((a, b) => (typeKey(a) < typeKey(b) ? -1 : 1));
+  if (joined.length === 1 && joined[0]!.kind === "undefinedT") return null;
   if (
     joined.length > 1 &&
     (!unionContainerArmsOk(joined) ||
@@ -118,12 +136,34 @@ export function unionArrayValueRead(
   let result: IrExpr | null = null;
   for (let tag = arms.length - 1; tag >= 0; tag--) {
     const arm = arms[tag]!;
-    if (arm.kind !== "array") return null;
     const receiver: IrExpr = { kind: "unionNarrow", unionId, tag, value, type: arm, loc };
-    const read = lowerer.coerceToExpected(
-      arrayValueRead(lowerer, receiver, index, arm.elem, loc),
-      type,
-    );
+    let read: IrExpr;
+    if (arm.kind === "array") {
+      read = lowerer.coerceToExpected(
+        arrayValueRead(lowerer, receiver, index, arm.elem, loc),
+        type,
+      );
+    } else {
+      const field = tupleField(arm);
+      if (field === null) return null;
+      if (field === "past") {
+        const missing = lowerer.wrappedUndefined(type, loc);
+        if (!missing) return null;
+        read = missing;
+      } else {
+        read = lowerer.coerceToExpected(
+          {
+            kind: "recordGet",
+            obj: receiver,
+            shapeId: (arm as IrType & { kind: "record" }).shapeId,
+            field: field.name,
+            type: field.type,
+            loc,
+          },
+          type,
+        );
+      }
+    }
     result =
       result === null
         ? read
diff --git a/packages/compiler/src/frontend/lowering/lower-comptime.ts b/packages/compiler/src/frontend/lowering/lower-comptime.ts
index 41c6704c59..31fa5afe02 100644
--- a/packages/compiler/src/frontend/lowering/lower-comptime.ts
+++ b/packages/compiler/src/frontend/lowering/lower-comptime.ts
@@ -4,7 +4,7 @@ import { InternalCompilerError } from "../../errors.js";
  * literals — records/arrays/unions included, subject to comptimeBakeable. */
 import * as ts from "../ts7/adapter.js";
 import type { Lowerer } from "./lowerer.js";
-import { type IrExpr, type IrType } from "../../ir/ir.js";
+import { isUnitType, type IrExpr, type IrType } from "../../ir/ir.js";
 import { comptimeFailedDiag } from "../../diagnostics/diagnostic.js";
 import { locOf } from "../program.js";
 import { PoisonError } from "./lowerer.js";
@@ -110,8 +110,9 @@ function comptimeFailureDetail(error: unknown): string {
 
 /** True when a comptime result of this type can be written as a literal
  * expression: number/string/boolean, arrays and record shapes of those,
- * nested. Closures, class instances, unions, promises, dyn, and void
- * cannot be baked. */
+ * and unions of those with null or undefined (the mixed rows of a JSON
+ * table: `(string | number)[][]`), nested. Closures, class instances,
+ * promises, dyn, and void cannot be baked. */
 export function comptimeBakeable(lowerer: Lowerer, t: IrType): boolean {
   switch (t.kind) {
     case "f64":
@@ -124,6 +125,52 @@ export function comptimeBakeable(lowerer: Lowerer, t: IrType): boolean {
       const shape = lowerer.shapes.get(t.shapeId);
       return !!shape && shape.fields.every((f) => lowerer.comptimeBakeable(f.type));
     }
+    case "union": {
+      const arms = lowerer.unions.get(t.unionId)?.arms;
+      return (
+        arms !== undefined &&
+        arms.every(
+          (arm) => isUnitType(arm) || (arm.kind !== "union" && comptimeBakeable(lowerer, arm)),
+        )
+      );
+    }
+    default:
+      return false;
+  }
+}
+
+/** Whether `value` bakes into the bakeable type `t` without a mismatch:
+ * the arm choice of a union literal. Records are exact, as in
+ * comptimeValueToIr; an absent field reads as undefined. */
+function comptimeFits(lowerer: Lowerer, value: unknown, t: IrType): boolean {
+  switch (t.kind) {
+    case "f64":
+      return typeof value === "number" && Number.isFinite(value);
+    case "string":
+      return typeof value === "string";
+    case "bool":
+      return typeof value === "boolean";
+    case "nullT":
+      return value === null;
+    case "undefinedT":
+      return value === undefined;
+    case "array":
+      return Array.isArray(value) && value.every((el) => comptimeFits(lowerer, el, t.elem));
+    case "record": {
+      if (typeof value !== "object" || value === null || Array.isArray(value)) return false;
+      const shape = lowerer.shapes.get(t.shapeId);
+      if (!shape) return false;
+      const obj = value as Record<string, unknown>;
+      const declared = new Set(shape.fields.map((f) => f.name));
+      return (
+        Object.keys(obj).every((key) => declared.has(key)) &&
+        shape.fields.every((f) => comptimeFits(lowerer, obj[f.name], f.type))
+      );
+    }
+    case "union":
+      return (lowerer.unions.get(t.unionId)?.arms ?? []).some((arm) =>
+        comptimeFits(lowerer, value, arm),
+      );
     default:
       return false;
   }
@@ -265,6 +312,26 @@ export function comptimeValueToIr(
       }));
       return { kind: "recordLit", fields, type: expected, loc };
     }
+    case "union": {
+      // The first arm the value fits, in the union's arm order: a JSON
+      // value fits exactly one scalar arm, and array or record arms are
+      // told apart by their contents.
+      const arms = lowerer.unions.get(expected.unionId)!.arms; // bakeable-checked
+      const tag = arms.findIndex((arm) => comptimeFits(lowerer, value, arm));
+      if (tag < 0) fail(describeComptimeValue(value));
+      const arm = arms[tag]!;
+      const payload: IrExpr = isUnitType(arm)
+        ? { kind: "unitLit", unit: arm.kind === "nullT" ? "null" : "undefined", type: arm, loc }
+        : lowerer.comptimeValueToIr(value, arm, path, blame);
+      return {
+        kind: "unionWrap",
+        unionId: expected.unionId,
+        tag,
+        value: payload,
+        type: expected,
+        loc,
+      };
+    }
     default:
       // comptimeBakeable already fenced these off.
       throw new InternalCompilerError(`lowerer bug: unbakeable comptime target '${expected.kind}'`);
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index 2c36e4973a..444fb899cc 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -2379,20 +2379,26 @@ export function lowerBindingPattern(
     lowerer.fenceStaticHeadersIteration(pattern);
     if (
       srcType.kind === "union" &&
-      lowerer.unions.get(srcType.unionId)?.arms.every((arm) => arm.kind === "array")
+      lowerer.unions
+        .get(srcType.unionId)
+        ?.arms.every(
+          (arm) =>
+            arm.kind === "array" ||
+            (arm.kind === "record" && lowerer.shapes.get(arm.shapeId)?.tuple === true),
+        )
     ) {
       for (let i = 0; i < pattern.elements.nodes.length; i++) {
         const el = pattern.elements.nodes[i]!;
         if (ts.isOmittedExpression(el) || el.name === undefined) continue;
         if (el.dotDotDotToken)
-          lowerer.unsupported("SC1031", el, "rest elements over a union of array layouts");
+          lowerer.unsupported("SC1031", el, "rest elements over a union of array or tuple layouts");
         const loc = locOf(el);
         let value = unionArrayValueRead(lowerer, srcRef(), numLit(i, loc), loc);
         if (!value)
           lowerer.unsupported(
             "SC1031",
             el,
-            "array union elements without a common native representation",
+            "array or tuple union elements without a common native representation",
           );
         if (el.initializer) value = applyBindingDefault(lowerer, el, value);
         lowerer.bindPatternTarget(el.name, value, isLet, out);
diff --git a/packages/compiler/src/frontend/type-mapper.ts b/packages/compiler/src/frontend/type-mapper.ts
index 437aba2b89..eea7dd9763 100644
--- a/packages/compiler/src/frontend/type-mapper.ts
+++ b/packages/compiler/src/frontend/type-mapper.ts
@@ -4411,6 +4411,9 @@ function unionDiscriminant(
 ): IrUnionDiscriminant | undefined {
   if (arms.filter((arm) => arm.kind === "record").length < 2 || parts.length < 2) return undefined;
   const { checker } = ctx;
+  // Tuple shapes keep positions only: their checker `length` literal has
+  // no slot to compare (discriminantField refuses tuple shapes too).
+  if (parts.some((part) => ctx.shapes.get(part.mapped.shapeId)?.tuple)) return undefined;
   for (const candidate of checker.getPropertiesOfType(parts[0]!.source)) {
     if (candidate.name.startsWith("__@")) continue;
     const byTag = new Map<number, (string | number | boolean)[]>();
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6410-json-mixed-scalar-tables/main.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6410-json-mixed-scalar-tables/main.ts.json
new file mode 100644
index 0000000000..c7ec1ab567
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6410-json-mixed-scalar-tables/main.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6410-json-mixed-scalar-tables/main.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6411-union-array-tuple-destructuring.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6411-union-array-tuple-destructuring.ts.json
new file mode 100644
index 0000000000..94d8b78e74
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6411-union-array-tuple-destructuring.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6411-union-array-tuple-destructuring.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/extra.json b/tests/corpus/6410-json-mixed-scalar-tables/extra.json
new file mode 100644
index 0000000000..927c5d9680
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/extra.json
@@ -0,0 +1 @@
+[["x", 1, "two", 3], [4, "five"], ["six"]]
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/main.ts b/tests/corpus/6410-json-mixed-scalar-tables/main.ts
new file mode 100644
index 0000000000..54e337b02d
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/main.ts
@@ -0,0 +1,21 @@
+// JSON tables whose rows mix strings and numbers, the shape of a
+// character-set table (`[["8140", 12288, 65292], ...]`) and of a
+// code-point range map (`[[[0, 44], "disallowed"], [[65, 65], "mapped",
+// [97]], ...]`). A package requires them at load or inside a function, and
+// an ES module imports one; each row keeps its strings and numbers.
+import { lookup, rowCount } from "code-table";
+import { mapChars, status } from "range-map";
+import extra from "./extra.json" with { type: "json" };
+
+console.log(lookup("8150"), lookup("a1"), lookup("zzzz"), rowCount());
+const mapped = mapChars("AB-c9");
+console.log(mapped.text, mapped.error);
+console.log(JSON.stringify(mapChars("a,b")), JSON.stringify(mapChars("{")));
+console.log(status(10), status(46), status(66), status(1000));
+
+for (const row of extra) {
+  const head = row[0];
+  const rest = row.slice(1);
+  console.log(typeof head, head, rest.length, rest.map((v) => (typeof v === "number" ? v + 1 : v.toUpperCase())).join(","));
+}
+console.log(JSON.stringify(extra));
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/index.d.ts b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/index.d.ts
new file mode 100644
index 0000000000..ae7db1d983
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/index.d.ts
@@ -0,0 +1,2 @@
+export declare function lookup(code: string): number | string;
+export declare function rowCount(): number;
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/index.js b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/index.js
new file mode 100644
index 0000000000..dc10bb817c
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/index.js
@@ -0,0 +1,10 @@
+'use strict';
+const table = require('./table.json');
+exports.lookup = function (code) {
+  for (const row of table) if (row[0] === code) return row[1];
+  return -1;
+};
+exports.rowCount = function () {
+  // A second require of the same document answers the same rows.
+  return require('./table.json').length;
+};
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/package.json b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/package.json
new file mode 100644
index 0000000000..e15622d878
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/package.json
@@ -0,0 +1 @@
+{ "name": "code-table", "version": "1.0.0", "main": "index.js", "types": "index.d.ts" }
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/table.json b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/table.json
new file mode 100644
index 0000000000..5abaa6772a
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/code-table/table.json
@@ -0,0 +1 @@
+[["8140", 12288, 65292], ["8150", 65507, 65340], ["a1", "｡", 62]]
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/index.d.ts b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/index.d.ts
new file mode 100644
index 0000000000..322bd20e1c
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/index.d.ts
@@ -0,0 +1,2 @@
+export declare function mapChars(s: string): { text: string; error: boolean };
+export declare function status(cp: number): string;
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/index.js b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/index.js
new file mode 100644
index 0000000000..481d93ad43
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/index.js
@@ -0,0 +1,37 @@
+'use strict';
+var table = require('./table.json');
+function findStatus(val) {
+  var start = 0;
+  var end = table.length - 1;
+  while (start <= end) {
+    var mid = Math.floor((start + end) / 2);
+    var target = table[mid];
+    if (target[0][0] <= val && target[0][1] >= val) {
+      return target;
+    } else if (target[0][0] > val) {
+      end = mid - 1;
+    } else {
+      start = mid + 1;
+    }
+  }
+  return null;
+}
+exports.status = function (cp) {
+  var found = findStatus(cp);
+  return found === null ? 'unknown' : found[1];
+};
+exports.mapChars = function (s) {
+  var error = false;
+  var out = '';
+  for (var i = 0; i < s.length; ++i) {
+    var cp = s.codePointAt(i);
+    var found = findStatus(cp);
+    if (found === null) { error = true; continue; }
+    switch (found[1]) {
+      case 'disallowed': error = true; out += String.fromCodePoint(cp); break;
+      case 'mapped': out += String.fromCodePoint.apply(String, found[2]); break;
+      case 'valid': out += String.fromCodePoint(cp); break;
+    }
+  }
+  return { text: out, error: error };
+};
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/package.json b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/package.json
new file mode 100644
index 0000000000..bc877e1e80
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/package.json
@@ -0,0 +1 @@
+{ "name": "range-map", "version": "1.0.0", "main": "index.js", "types": "index.d.ts" }
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/table.json b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/table.json
new file mode 100644
index 0000000000..900f9e3165
--- /dev/null
+++ b/tests/corpus/6410-json-mixed-scalar-tables/node_modules/range-map/table.json
@@ -0,0 +1 @@
+[[[0,44],"disallowed"],[[45,46],"valid"],[[47,47],"disallowed"],[[48,57],"valid"],[[65,65],"mapped",[97]],[[66,67],"mapped",[98,99]],[[97,122],"valid"]]
diff --git a/tests/corpus/6411-union-array-tuple-destructuring.ts b/tests/corpus/6411-union-array-tuple-destructuring.ts
new file mode 100644
index 0000000000..1dd88fa921
--- /dev/null
+++ b/tests/corpus/6411-union-array-tuple-destructuring.ts
@@ -0,0 +1,34 @@
+// Array destructuring of a value whose type is a union of array and tuple
+// types: a regex match or a default row (`re.exec(s) || ["", "", ""]`), and
+// functions answering either a fixed tuple or a list. Each position reads
+// the arm the value holds; positions past a tuple's end and past an array's
+// end read undefined, and defaults fire exactly there.
+function scan(text: string): string {
+  const [match, head, word] = /(\w*)-(\w+)/.exec(text) || ["", "", ""];
+  return `${match}|${head}|${word.toUpperCase()}`;
+}
+console.log(scan("ab-cd"), scan("none"), scan("-x"));
+
+function pick(flag: boolean): [string, number] | string[] {
+  return flag ? ["a", 1] : ["b", "c"];
+}
+const [p, q] = pick(true);
+const [r, s = "dflt", t = "z"] = pick(false);
+const [u, , w = "w"] = pick(true);
+console.log(p, q, r, s, t, u, w);
+console.log(typeof q, typeof s);
+
+function pair(n: number): number[] | [number, number] {
+  return n > 1 ? [n, n * 2] : [n];
+}
+const [x, y] = pair(7);
+const [x1, y1 = -1] = pair(1);
+console.log(x, y, x1, y1);
+
+function row(kind: string): [string, boolean] | [string, boolean, number] {
+  return kind === "long" ? [kind, true, 3] : [kind, false];
+}
+for (const k of ["long", "short"]) {
+  const [name, flag, count = 0] = row(k);
+  console.log(name, flag, count);
+}

From 52f9bf284f7174c3e0f0163aa676f57591ec25b4 Mon Sep 17 00:00:00 2001
From: Malte Ubl <malte@vercel.com>
Date: Mon, 19 Oct 2026 20:40:56 -0700
Subject: [PATCH 2/5] Destructure a match-or-empty-string union by code point

`const [hit] = re.exec(s) || ""` destructures `RegExpExecArray | string`:
the string arm answers its code point at each position, or undefined past
its end, as the string iterator does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../src/frontend/lowering/array-values.ts     | 25 ++++++++++++++-----
 .../src/frontend/lowering/lower-stmts.ts      |  8 ++++--
 .../6411-union-array-tuple-destructuring.ts   |  8 ++++++
 3 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/packages/compiler/src/frontend/lowering/array-values.ts b/packages/compiler/src/frontend/lowering/array-values.ts
index e9148b244d..0ca8503df3 100644
--- a/packages/compiler/src/frontend/lowering/array-values.ts
+++ b/packages/compiler/src/frontend/lowering/array-values.ts
@@ -8,6 +8,7 @@ import {
   type IrType,
   isIntegerBytesElem,
   type SrcLoc,
+  STRING,
   UNDEFINED_T,
   typeEquals,
   typeKey,
@@ -88,12 +89,15 @@ export function arrayValueRead(
  * (`const [a, b] = re.exec(s) || ["", ""]`) without copying the receiver
  * or its elements. Every array arm contributes its missing-value case; a
  * tuple arm answers its field, or undefined past its end. Tuple arms need
- * a constant position. */
+ * a constant position. With `stringChars` (the string iterator's code
+ * points as a fresh string[]), a string arm (`re.exec(s) || ""`) answers
+ * its code point at the position, or undefined past its end. */
 export function unionArrayValueRead(
   lowerer: Lowerer,
   value: IrExpr,
   index: IrExpr,
   loc: SrcLoc,
+  stringChars?: (text: IrExpr) => IrExpr,
 ): IrExpr | null {
   if (value.type.kind !== "union") return null;
   const unionId = value.type.unionId;
@@ -110,16 +114,20 @@ export function unionArrayValueRead(
   };
   // Keep the collection's IrType union layout and narrow each element at
   // its use; an inferred array predicate changes the collection's ABI.
-  for (const arm of arms) if (arm.kind !== "array" && tupleField(arm) === null) return null;
+  const stringArm = (arm: IrType): boolean => arm.kind === "string" && stringChars !== undefined;
+  for (const arm of arms)
+    if (arm.kind !== "array" && !stringArm(arm) && tupleField(arm) === null) return null;
   const answers = new Map<string, IrType>();
   for (const arm of arms) {
-    const field = arm.kind === "array" ? null : tupleField(arm);
+    const field = arm.kind === "array" || stringArm(arm) ? null : tupleField(arm);
     const answer =
       arm.kind === "array"
         ? arrayValueType(lowerer, arm.elem)
-        : field === "past"
-          ? UNDEFINED_T
-          : field!.type;
+        : stringArm(arm)
+          ? arrayValueType(lowerer, STRING)
+          : field === "past"
+            ? UNDEFINED_T
+            : field!.type;
     const parts = answer.kind === "union" ? lowerer.unions.get(answer.unionId)!.arms : [answer];
     for (const part of parts) answers.set(typeKey(part), part);
   }
@@ -143,6 +151,11 @@ export function unionArrayValueRead(
         arrayValueRead(lowerer, receiver, index, arm.elem, loc),
         type,
       );
+    } else if (stringArm(arm)) {
+      read = lowerer.coerceToExpected(
+        arrayValueRead(lowerer, stringChars!(receiver), index, STRING, loc),
+        type,
+      );
     } else {
       const field = tupleField(arm);
       if (field === null) return null;
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index 444fb899cc..737c2e9ae1 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -2384,8 +2384,10 @@ export function lowerBindingPattern(
         ?.arms.every(
           (arm) =>
             arm.kind === "array" ||
+            arm.kind === "string" ||
             (arm.kind === "record" && lowerer.shapes.get(arm.shapeId)?.tuple === true),
-        )
+        ) &&
+      lowerer.unions.get(srcType.unionId)!.arms.some((arm) => arm.kind !== "string")
     ) {
       for (let i = 0; i < pattern.elements.nodes.length; i++) {
         const el = pattern.elements.nodes[i]!;
@@ -2393,7 +2395,9 @@ export function lowerBindingPattern(
         if (el.dotDotDotToken)
           lowerer.unsupported("SC1031", el, "rest elements over a union of array or tuple layouts");
         const loc = locOf(el);
-        let value = unionArrayValueRead(lowerer, srcRef(), numLit(i, loc), loc);
+        let value = unionArrayValueRead(lowerer, srcRef(), numLit(i, loc), loc, (text) =>
+          strCharsCall(lowerer, text, loc),
+        );
         if (!value)
           lowerer.unsupported(
             "SC1031",
diff --git a/tests/corpus/6411-union-array-tuple-destructuring.ts b/tests/corpus/6411-union-array-tuple-destructuring.ts
index 1dd88fa921..f62eba996c 100644
--- a/tests/corpus/6411-union-array-tuple-destructuring.ts
+++ b/tests/corpus/6411-union-array-tuple-destructuring.ts
@@ -32,3 +32,11 @@ for (const k of ["long", "short"]) {
   const [name, flag, count = 0] = row(k);
   console.log(name, flag, count);
 }
+
+// A match or an empty string (`re.exec(s) || ""`): a string destructures
+// by code point, so the empty string binds undefined.
+function firstHit(text: string): string {
+  const [hit] = /\d+/.exec(text) || "";
+  return hit ? `hit ${hit}` : "none";
+}
+console.log(firstHit("ab12"), firstHit("ab"));

From fe322ae901cbc4914f527c756d8f71235f2454f4 Mon Sep 17 00:00:00 2001
From: Malte Ubl <malte@vercel.com>
Date: Mon, 19 Oct 2026 21:16:25 -0700
Subject: [PATCH 3/5] Keep the JSON-shape diagnostics fixture refused now that
 null fields bake

A null-valued JSON field bakes as null, so the fixture's document now
compiles. It keeps its purpose with a value no literal spells (1e400 parses
to Infinity), and the mixed-table corpus program covers null elements.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 tests/corpus/6410-json-mixed-scalar-tables/extra.json | 2 +-
 tests/corpus/6410-json-mixed-scalar-tables/main.ts    | 7 ++++++-
 tests/diagnostics/json-shape/main.ts                  | 8 ++++----
 tests/diagnostics/json-shape/nulled.json              | 2 +-
 tests/harness/__snapshots__/json-shape/main.ts.txt    | 4 ++--
 5 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/tests/corpus/6410-json-mixed-scalar-tables/extra.json b/tests/corpus/6410-json-mixed-scalar-tables/extra.json
index 927c5d9680..7af99c22f2 100644
--- a/tests/corpus/6410-json-mixed-scalar-tables/extra.json
+++ b/tests/corpus/6410-json-mixed-scalar-tables/extra.json
@@ -1 +1 @@
-[["x", 1, "two", 3], [4, "five"], ["six"]]
+[["x", 1, "two", 3], [4, "five", null], ["six"], [null]]
diff --git a/tests/corpus/6410-json-mixed-scalar-tables/main.ts b/tests/corpus/6410-json-mixed-scalar-tables/main.ts
index 54e337b02d..c60f962dcf 100644
--- a/tests/corpus/6410-json-mixed-scalar-tables/main.ts
+++ b/tests/corpus/6410-json-mixed-scalar-tables/main.ts
@@ -16,6 +16,11 @@ console.log(status(10), status(46), status(66), status(1000));
 for (const row of extra) {
   const head = row[0];
   const rest = row.slice(1);
-  console.log(typeof head, head, rest.length, rest.map((v) => (typeof v === "number" ? v + 1 : v.toUpperCase())).join(","));
+  console.log(
+    typeof head,
+    head,
+    rest.length,
+    rest.map((v) => (v === null ? "null" : typeof v === "number" ? v + 1 : v.toUpperCase())).join(","),
+  );
 }
 console.log(JSON.stringify(extra));
diff --git a/tests/diagnostics/json-shape/main.ts b/tests/diagnostics/json-shape/main.ts
index 1d95525068..a46338d528 100644
--- a/tests/diagnostics/json-shape/main.ts
+++ b/tests/diagnostics/json-shape/main.ts
@@ -1,7 +1,7 @@
-/* A JSON document whose shape doesn't bake: a null-valued field has no
- * standalone STATIC type, so the import reports the dynamic-family type
- * diagnostic (SC2011 at the import site — the shape maps in the embedded
- * engine) and uses of the binding poison. */
+/* A JSON document whose value doesn't bake: `1e400` parses to Infinity,
+ * which no number literal spells, so the import reports the bake failure
+ * at the import site and uses of the binding poison. (Its null-valued
+ * field bakes as null.) */
 import bad from "./nulled.json" with { type: "json" };
 
 console.log(bad.name);
diff --git a/tests/diagnostics/json-shape/nulled.json b/tests/diagnostics/json-shape/nulled.json
index fa47384497..5e506a6aff 100644
--- a/tests/diagnostics/json-shape/nulled.json
+++ b/tests/diagnostics/json-shape/nulled.json
@@ -1 +1 @@
-{ "name": "x", "deprecated": null }
+{ "name": "x", "deprecated": null, "limit": 1e400 }
diff --git a/tests/harness/__snapshots__/json-shape/main.ts.txt b/tests/harness/__snapshots__/json-shape/main.ts.txt
index b03f098a60..85f5d2917f 100644
--- a/tests/harness/__snapshots__/json-shape/main.ts.txt
+++ b/tests/harness/__snapshots__/json-shape/main.ts.txt
@@ -1,6 +1,6 @@
-json-shape/main.ts:5:8 - error SC2001: values of type '{ name: string; deprecated: null; }' cannot be compiled yet (supported: number, string, boolean, arrays, Maps, Sets, RegExp, read-only Date values, functions, classes, records, unions of those, and 'unknown')
+json-shape/main.ts:5:8 - error SC1110: comptime evaluation failed: expected 'number' at $.limit, got Infinity (only finite numbers can be written as literals)
 
-  4 |  * engine) and uses of the binding poison. */
+  4 |  * field bakes as null.) */
   5 | import bad from "./nulled.json" with { type: "json" };
     |        ^~~
   6 | 

From 45431aec320c53e12e9b664e80d8d48ffbb4f547 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Tue, 20 Oct 2026 02:01:48 -0500
Subject: [PATCH 4/5] Choose comptime union arms by discriminant and refuse
 recursive records

A comptime record value whose union has a discriminant takes the arm its
discriminant field names, so a value that fits two arms with overlapping
shapes gets that arm's tag and layout instead of the first arm it fits.
comptimeBakeable reports a record type reachable from its own fields as
not bakeable instead of recursing without end, so such a comptime result
reports SC1090, and the message names unions and that limit.
---
 .../src/frontend/lowering/lower-comptime.ts   | 54 ++++++++++++++-----
 tests/corpus/1052-comptime-records.ts         | 17 ++++++
 tests/diagnostics/comptime.ts                 |  7 +++
 tests/harness/__snapshots__/comptime.ts.txt   |  9 +++-
 4 files changed, 74 insertions(+), 13 deletions(-)

diff --git a/packages/compiler/src/frontend/lowering/lower-comptime.ts b/packages/compiler/src/frontend/lowering/lower-comptime.ts
index 31fa5afe02..bdd020127f 100644
--- a/packages/compiler/src/frontend/lowering/lower-comptime.ts
+++ b/packages/compiler/src/frontend/lowering/lower-comptime.ts
@@ -76,7 +76,7 @@ export function lowerComptime(lowerer: Lowerer, expr: ts.CallExpression): IrExpr
       "SC1090",
       expr,
       `baking comptime results of type '${lowerer.fmt(target)}' into the binary ` +
-        `(only number, string, boolean, arrays, and records lower to literals)`,
+        `(number, string, boolean, and arrays, records and unions of those lower to literals; a record type reachable from its own fields does not)`,
     );
   }
   // 5. The host executes the extracted callback under a finite budget.
@@ -112,25 +112,36 @@ function comptimeFailureDetail(error: unknown): string {
  * expression: number/string/boolean, arrays and record shapes of those,
  * and unions of those with null or undefined (the mixed rows of a JSON
  * table: `(string | number)[][]`), nested. Closures, class instances,
- * promises, dyn, and void cannot be baked. */
+ * promises, dyn, void, and recursive record types (a record reachable
+ * from its own fields) cannot be baked. */
 export function comptimeBakeable(lowerer: Lowerer, t: IrType): boolean {
+  return bakeableWithin(lowerer, t, new Set());
+}
+
+/** comptimeBakeable below the records in `open`, whose fields are being
+ * checked: reaching one of them again means the type is recursive. */
+function bakeableWithin(lowerer: Lowerer, t: IrType, open: Set<string>): boolean {
   switch (t.kind) {
     case "f64":
     case "string":
     case "bool":
       return true;
     case "array":
-      return lowerer.comptimeBakeable(t.elem);
+      return bakeableWithin(lowerer, t.elem, open);
     case "record": {
       const shape = lowerer.shapes.get(t.shapeId);
-      return !!shape && shape.fields.every((f) => lowerer.comptimeBakeable(f.type));
+      if (!shape || open.has(t.shapeId)) return false;
+      open.add(t.shapeId);
+      const bakeable = shape.fields.every((f) => bakeableWithin(lowerer, f.type, open));
+      open.delete(t.shapeId);
+      return bakeable;
     }
     case "union": {
       const arms = lowerer.unions.get(t.unionId)?.arms;
       return (
         arms !== undefined &&
         arms.every(
-          (arm) => isUnitType(arm) || (arm.kind !== "union" && comptimeBakeable(lowerer, arm)),
+          (arm) => isUnitType(arm) || (arm.kind !== "union" && bakeableWithin(lowerer, arm, open)),
         )
       );
     }
@@ -139,6 +150,30 @@ export function comptimeBakeable(lowerer: Lowerer, t: IrType): boolean {
   }
 }
 
+/** The tag of the arm of union `unionId` that `value` bakes into, or -1.
+ * A record value whose discriminant field holds one of the union's
+ * discriminant values takes that case's arm, since arms with overlapping
+ * shapes can both fit it. Any other value takes the first arm it fits, in
+ * the union's arm order: a JSON value fits exactly one scalar arm, and
+ * array or record arms are told apart by their contents. */
+function comptimeUnionArm(lowerer: Lowerer, unionId: string, value: unknown): number {
+  const def = lowerer.unions.get(unionId);
+  if (!def) return -1;
+  const discriminant = def.discriminant;
+  if (
+    discriminant !== undefined &&
+    typeof value === "object" &&
+    value !== null &&
+    !Array.isArray(value)
+  ) {
+    const key = (value as Record<string, unknown>)[discriminant.field];
+    const named = discriminant.cases.find((c) => c.values.some((v) => v === key));
+    if (named !== undefined)
+      return comptimeFits(lowerer, value, def.arms[named.tag]!) ? named.tag : -1;
+  }
+  return def.arms.findIndex((arm) => comptimeFits(lowerer, value, arm));
+}
+
 /** Whether `value` bakes into the bakeable type `t` without a mismatch:
  * the arm choice of a union literal. Records are exact, as in
  * comptimeValueToIr; an absent field reads as undefined. */
@@ -168,9 +203,7 @@ function comptimeFits(lowerer: Lowerer, value: unknown, t: IrType): boolean {
       );
     }
     case "union":
-      return (lowerer.unions.get(t.unionId)?.arms ?? []).some((arm) =>
-        comptimeFits(lowerer, value, arm),
-      );
+      return comptimeUnionArm(lowerer, t.unionId, value) >= 0;
     default:
       return false;
   }
@@ -313,11 +346,8 @@ export function comptimeValueToIr(
       return { kind: "recordLit", fields, type: expected, loc };
     }
     case "union": {
-      // The first arm the value fits, in the union's arm order: a JSON
-      // value fits exactly one scalar arm, and array or record arms are
-      // told apart by their contents.
       const arms = lowerer.unions.get(expected.unionId)!.arms; // bakeable-checked
-      const tag = arms.findIndex((arm) => comptimeFits(lowerer, value, arm));
+      const tag = comptimeUnionArm(lowerer, expected.unionId, value);
       if (tag < 0) fail(describeComptimeValue(value));
       const arm = arms[tag]!;
       const payload: IrExpr = isUnitType(arm)
diff --git a/tests/corpus/1052-comptime-records.ts b/tests/corpus/1052-comptime-records.ts
index 47cccf7b32..658aaed32a 100644
--- a/tests/corpus/1052-comptime-records.ts
+++ b/tests/corpus/1052-comptime-records.ts
@@ -43,3 +43,20 @@ const meta = comptime(() => {
   return { kind: "meta", tags, count: tags.length };
 });
 console.log(meta.kind, meta.tags.join("|"), meta.count);
+
+// A discriminated union whose arms both fit a "labeled" value without a
+// label: the discriminant picks the arm.
+type Item = { kind: "plain"; v: number } | { kind: "labeled"; v: number; label?: string };
+const items: Item[] = comptime(
+  () =>
+    [
+      { kind: "plain", v: 1 },
+      { kind: "labeled", v: 2 },
+      { kind: "labeled", v: 3, label: "x" },
+    ] as Item[],
+);
+for (const item of items) {
+  if (item.kind === "labeled") console.log("labeled", item.v, item.label);
+  else console.log("plain", item.v);
+}
+console.log(JSON.stringify(items));
diff --git a/tests/diagnostics/comptime.ts b/tests/diagnostics/comptime.ts
index 9ba882aafd..9c82394643 100644
--- a/tests/diagnostics/comptime.ts
+++ b/tests/diagnostics/comptime.ts
@@ -20,3 +20,10 @@ const nested = comptime(() => comptime(() => 1));
 const eventually = comptime(async () => 1);
 
 const lied = comptime(() => 1 as unknown as string);
+
+// A record type reachable from its own fields has no literal form.
+interface Chain {
+  value: number;
+  next: Chain | null;
+}
+const chain: Chain = comptime(() => ({ value: 1, next: null }) as Chain);
diff --git a/tests/harness/__snapshots__/comptime.ts.txt b/tests/harness/__snapshots__/comptime.ts.txt
index 2b1411ae5d..603c297a83 100644
--- a/tests/harness/__snapshots__/comptime.ts.txt
+++ b/tests/harness/__snapshots__/comptime.ts.txt
@@ -45,4 +45,11 @@ comptime.ts:22:23 - error SC1110: comptime evaluation failed: expected 'string'
   21 | 
   22 | const lied = comptime(() => 1 as unknown as string);
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
-  23 | 
\ No newline at end of file
+  23 | 
+
+comptime.ts:29:22 - error SC1090: baking comptime results of type '{ next: null | ...; value: number }' into the binary (number, string, boolean, and arrays, records and unions of those lower to literals; a record type reachable from its own fields does not) is not supported yet
+
+  28 | }
+  29 | const chain: Chain = comptime(() => ({ value: 1, next: null }) as Chain);
+     |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  30 | 
\ No newline at end of file

From 437dc1163f77466486a63d759c57d15b70077b8e Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Tue, 20 Oct 2026 11:39:23 -0500
Subject: [PATCH 5/5] Refuse recursive unions in comptime baking

The bakeability check tracks the unions it is inside as well as the
records, so a union reachable from itself through an array
(`type Tree = number | Tree[]`) reports SC1090 instead of recursing without
end. The message names both recursive records and recursive unions.
---
 .../src/frontend/lowering/lower-comptime.ts   | 28 +++++++++++--------
 tests/diagnostics/comptime.ts                 |  4 +++
 tests/harness/__snapshots__/comptime.ts.txt   | 11 ++++++--
 3 files changed, 29 insertions(+), 14 deletions(-)

diff --git a/packages/compiler/src/frontend/lowering/lower-comptime.ts b/packages/compiler/src/frontend/lowering/lower-comptime.ts
index bdd020127f..f239eea826 100644
--- a/packages/compiler/src/frontend/lowering/lower-comptime.ts
+++ b/packages/compiler/src/frontend/lowering/lower-comptime.ts
@@ -76,7 +76,7 @@ export function lowerComptime(lowerer: Lowerer, expr: ts.CallExpression): IrExpr
       "SC1090",
       expr,
       `baking comptime results of type '${lowerer.fmt(target)}' into the binary ` +
-        `(number, string, boolean, and arrays, records and unions of those lower to literals; a record type reachable from its own fields does not)`,
+        `(number, string, boolean, and arrays, records and unions of those lower to literals; a record or union type reachable from itself does not)`,
     );
   }
   // 5. The host executes the extracted callback under a finite budget.
@@ -112,13 +112,14 @@ function comptimeFailureDetail(error: unknown): string {
  * expression: number/string/boolean, arrays and record shapes of those,
  * and unions of those with null or undefined (the mixed rows of a JSON
  * table: `(string | number)[][]`), nested. Closures, class instances,
- * promises, dyn, void, and recursive record types (a record reachable
- * from its own fields) cannot be baked. */
+ * promises, dyn, void, and recursive types (a record or union reachable
+ * from itself, directly or through arrays) cannot be baked. */
 export function comptimeBakeable(lowerer: Lowerer, t: IrType): boolean {
   return bakeableWithin(lowerer, t, new Set());
 }
 
-/** comptimeBakeable below the records in `open`, whose fields are being
+/** comptimeBakeable below the records and unions in `open` (keyed
+ * `record:<shapeId>` and `union:<unionId>`), whose members are being
  * checked: reaching one of them again means the type is recursive. */
 function bakeableWithin(lowerer: Lowerer, t: IrType, open: Set<string>): boolean {
   switch (t.kind) {
@@ -130,20 +131,23 @@ function bakeableWithin(lowerer: Lowerer, t: IrType, open: Set<string>): boolean
       return bakeableWithin(lowerer, t.elem, open);
     case "record": {
       const shape = lowerer.shapes.get(t.shapeId);
-      if (!shape || open.has(t.shapeId)) return false;
-      open.add(t.shapeId);
+      const key = `record:${t.shapeId}`;
+      if (!shape || open.has(key)) return false;
+      open.add(key);
       const bakeable = shape.fields.every((f) => bakeableWithin(lowerer, f.type, open));
-      open.delete(t.shapeId);
+      open.delete(key);
       return bakeable;
     }
     case "union": {
       const arms = lowerer.unions.get(t.unionId)?.arms;
-      return (
-        arms !== undefined &&
-        arms.every(
-          (arm) => isUnitType(arm) || (arm.kind !== "union" && bakeableWithin(lowerer, arm, open)),
-        )
+      const key = `union:${t.unionId}`;
+      if (arms === undefined || open.has(key)) return false;
+      open.add(key);
+      const bakeable = arms.every(
+        (arm) => isUnitType(arm) || (arm.kind !== "union" && bakeableWithin(lowerer, arm, open)),
       );
+      open.delete(key);
+      return bakeable;
     }
     default:
       return false;
diff --git a/tests/diagnostics/comptime.ts b/tests/diagnostics/comptime.ts
index 9c82394643..645779ea77 100644
--- a/tests/diagnostics/comptime.ts
+++ b/tests/diagnostics/comptime.ts
@@ -27,3 +27,7 @@ interface Chain {
   next: Chain | null;
 }
 const chain: Chain = comptime(() => ({ value: 1, next: null }) as Chain);
+
+// A union reachable from itself through an array has no literal form either.
+type Tree = number | Tree[];
+const tree: Tree = comptime((): Tree => [1, [2, [3]]]);
diff --git a/tests/harness/__snapshots__/comptime.ts.txt b/tests/harness/__snapshots__/comptime.ts.txt
index 603c297a83..757c30e6ce 100644
--- a/tests/harness/__snapshots__/comptime.ts.txt
+++ b/tests/harness/__snapshots__/comptime.ts.txt
@@ -47,9 +47,16 @@ comptime.ts:22:23 - error SC1110: comptime evaluation failed: expected 'string'
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
   23 | 
 
-comptime.ts:29:22 - error SC1090: baking comptime results of type '{ next: null | ...; value: number }' into the binary (number, string, boolean, and arrays, records and unions of those lower to literals; a record type reachable from its own fields does not) is not supported yet
+comptime.ts:29:22 - error SC1090: baking comptime results of type '{ next: null | ...; value: number }' into the binary (number, string, boolean, and arrays, records and unions of those lower to literals; a record or union type reachable from itself does not) is not supported yet
 
   28 | }
   29 | const chain: Chain = comptime(() => ({ value: 1, next: null }) as Chain);
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-  30 | 
\ No newline at end of file
+  30 | 
+
+comptime.ts:33:20 - error SC1090: baking comptime results of type '(...)[] | number' into the binary (number, string, boolean, and arrays, records and unions of those lower to literals; a record or union type reachable from itself does not) is not supported yet
+
+  32 | type Tree = number | Tree[];
+  33 | const tree: Tree = comptime((): Tree => [1, [2, [3]]]);
+     |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  34 | 
\ No newline at end of file