From ecaa87d39c1aa86ffebef1d6b43f2b507fb4ba00 Mon Sep 17 00:00:00 2001
From: daishuge <play13661948263@gmail.com>
Date: Sun, 5 Jul 2026 16:38:39 +0800
Subject: [PATCH] fix(lib): preserve readonly narrowing in Array.isArray type
 guard

Adds an overload to ArrayConstructor.isArray that narrows
readonly array types correctly, fixing a 9-year-old issue where
Array.isArray() would lose readonly type information.

Fixes #17002

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 src/lib/es5.d.ts                              |  1 +
 .../compiler/arrayIsArrayReadonlyNarrowing.ts | 26 +++++++++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 tests/cases/compiler/arrayIsArrayReadonlyNarrowing.ts

diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 673ac55e47612..d99c3a651c213 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -1499,6 +1499,7 @@ interface ArrayConstructor {
     (arrayLength?: number): any[];
     <T>(arrayLength: number): T[];
     <T>(...items: T[]): T[];
+    isArray(arg: readonly any[] | any): arg is readonly any[];
     isArray(arg: any): arg is any[];
     readonly prototype: any[];
 }
diff --git a/tests/cases/compiler/arrayIsArrayReadonlyNarrowing.ts b/tests/cases/compiler/arrayIsArrayReadonlyNarrowing.ts
new file mode 100644
index 0000000000000..7e38b6af4d5c1
--- /dev/null
+++ b/tests/cases/compiler/arrayIsArrayReadonlyNarrowing.ts
@@ -0,0 +1,26 @@
+// @strict: true
+
+function test1(x: readonly string[] | string) {
+    if (Array.isArray(x)) {
+        x; // should be readonly string[]
+    }
+}
+
+function test2(x: readonly number[] | number) {
+    if (Array.isArray(x)) {
+        x; // should be readonly number[]
+        x[0]; // should be number
+    }
+}
+
+function test3(x: string[] | string) {
+    if (Array.isArray(x)) {
+        x; // should still be string[] (not readonly)
+    }
+}
+
+function test4(x: unknown) {
+    if (Array.isArray(x)) {
+        x; // should still be any[] (existing behavior)
+    }
+}