From 43904af416b277c018477dd4e4af80d33f85cd2c Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Sat, 10 Oct 2026 20:30:37 +0000
Subject: [PATCH 1/2] fix: check sparse-array holes in the isArrayOf type guard

The boolean path used Array.prototype.every(), which skips holes, while
validate() and validateAll() visit every index. The guard now visits
every index too, so a hole is checked as undefined on all paths.

Closes #450

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019BJENnU8SXcDH2Tcx5EWE4
---
 src/typeChecker.ts   | 15 +++++++++++++--
 src/validate.test.ts | 38 +++++++++++++++++++++++++++++++++++++-
 2 files changed, 50 insertions(+), 3 deletions(-)

diff --git a/src/typeChecker.ts b/src/typeChecker.ts
index 35f67e97..f681ee65 100644
--- a/src/typeChecker.ts
+++ b/src/typeChecker.ts
@@ -236,8 +236,19 @@ export const isArrayOf: <const T>(
 		const k = `isArrayOf${typeName}`;
 		return {
 			typeGuard: {
-				[k]: (v: unknown): v is Array<T> =>
-					Array.isArray(v) && v.every((i) => isT(i)),
+				// Visit every index (holes included) to match the diagnosis path;
+				// Array.prototype.every() would skip holes.
+				[k]: (v: unknown): v is Array<T> => {
+					if (!Array.isArray(v)) {
+						return false;
+					}
+					for (let index = 0; index < v.length; index++) {
+						if (!isT(v[index])) {
+							return false;
+						}
+					}
+					return true;
+				},
 			}[k],
 			*serialize(depth, done) {
 				yield "Array<";
diff --git a/src/validate.test.ts b/src/validate.test.ts
index 28dd99b2..c8de7f15 100644
--- a/src/validate.test.ts
+++ b/src/validate.test.ts
@@ -9,7 +9,7 @@ import {
 	isOptionalOf,
 	typeChecker,
 } from "./typeChecker.ts";
-import type { ValidationIssue } from "./types.ts";
+import type { TypeChecker, ValidationIssue } from "./types.ts";
 import { validate, validateAll } from "./validate.ts";
 import {
 	ValidationIssueCode,
@@ -406,3 +406,39 @@ test("exact object validation preserves its input without sanitizing it", () =>
 	assert.deepEqual(Object.getOwnPropertyDescriptors(input), before);
 	assert.equal(Object.isFrozen(input), true);
 });
+
+const assertArrayResult = <T>(
+	checker: TypeChecker<Array<T>>,
+	input: unknown,
+	expected: boolean,
+): void => {
+	const label = `${checker} ${JSON.stringify(input)}`;
+	assert.equal(checker(input), expected, label);
+	assert.equal(checker.test(input) === null, expected, label);
+	assert.equal(validate(input, checker).ok, expected, label);
+	assert.equal(validateAll(input, checker).ok, expected, label);
+};
+
+test("array holes are checked like undefined elements on every path", () => {
+	const isStrings = isArrayOf(isString);
+	const isOptionalStrings = isArrayOf(isOptionalOf(isString));
+	// biome-ignore lint/suspicious/noSparseArray: sparse arrays are the subject
+	const mixed = ["a", , "c"];
+	const cases: Array<[unknown, boolean, boolean]> = [
+		// [input, isStrings, isOptionalStrings]
+		[[], true, true],
+		[new Array(2), false, true],
+		[mixed, false, true],
+		[["a", undefined], false, true],
+		[["a", "b"], true, true],
+	];
+	for (const [input, strings, optionalStrings] of cases) {
+		assertArrayResult(isStrings, input, strings);
+		assertArrayResult(isOptionalStrings, input, optionalStrings);
+	}
+	const result = validate(mixed, isStrings);
+	assert.equal(result.ok, false);
+	if (!result.ok) {
+		assert.deepEqual(result.issue.path, [1]);
+	}
+});

From bba174251c74b8b66a07e26ba7683756ce669b24 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Sun, 11 Oct 2026 07:13:08 +0000
Subject: [PATCH 2/2] fix: keep the isArrayOf hole check within the bundle
 budget

After merging main, the explicit index loop pushed the minified UUID
checker bundle to 5924 bytes, over the 5916-byte budget enforced by
test/package.test.ts. findIndex() also visits holes as undefined, and
keeps the bundle at 5877 bytes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019BJENnU8SXcDH2Tcx5EWE4
---
 src/typeChecker.ts | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/src/typeChecker.ts b/src/typeChecker.ts
index 67c461c1..11c159b2 100644
--- a/src/typeChecker.ts
+++ b/src/typeChecker.ts
@@ -236,19 +236,10 @@ export const isArrayOf: <const T>(
 		const k = `isArrayOf${typeName}`;
 		return {
 			typeGuard: {
-				// Visit every index (holes included) to match the diagnosis path;
-				// Array.prototype.every() would skip holes.
-				[k]: (v: unknown): v is Array<T> => {
-					if (!Array.isArray(v)) {
-						return false;
-					}
-					for (let index = 0; index < v.length; index++) {
-						if (!isT(v[index])) {
-							return false;
-						}
-					}
-					return true;
-				},
+				// findIndex() visits holes as undefined, matching the diagnosis
+				// path; every() would skip them.
+				[k]: (v: unknown): v is Array<T> =>
+					Array.isArray(v) && v.findIndex((i) => !isT(i)) < 0,
 			}[k],
 			*serialize(depth, done) {
 				yield "Array<";