From 689091a4311a5545a01f2ed283050b15322d4e9c Mon Sep 17 00:00:00 2001
From: huymobile <kingonwork@gmail.com>
Date: Sat, 3 Oct 2026 08:50:04 +0700
Subject: [PATCH] fix: avoid excessive Draft recursion for readonly arrays in
 recursive types

WritableDraft only took the `Draft<T[number]>[]` shortcut for mutable
arrays, so readonly arrays went through the mapped type and recursive
types such as type-fest's JsonValue (which uses `readonly JsonValue[]`)
still hit TS2589. Match `readonly any[]` so readonly plain arrays take
the same path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 __tests__/type-external.ts  | 19 ++++++++++++++++++-
 src/types/types-external.ts |  2 +-
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/__tests__/type-external.ts b/__tests__/type-external.ts
index fcf3b2df9..c5eef3ffc 100644
--- a/__tests__/type-external.ts
+++ b/__tests__/type-external.ts
@@ -1,6 +1,6 @@
 import {isType, JSONArray, JSONObject, JSONTypes} from "type-plus"
 import {Draft} from "../src/types/types-external"
-import {createDraft, current, original} from "../src/immer"
+import {createDraft, current, original, produce} from "../src/immer"
 
 describe("Draft<T>", () => {
 	test("can use JSONTypes as T", () => {
@@ -17,6 +17,23 @@ describe("Draft<T>", () => {
 		type A = Draft<[string, number, JSONArray, JSONObject]>
 		isType.equal<true, [string, number, JSONArray, JSONObject], A>()
 	})
+
+	it("can use recursive types containing readonly arrays as T (#839)", () => {
+		// Same shape as type-fest's JsonValue
+		type JsonValue = string | number | boolean | null | JsonObject | JsonArray
+		type JsonObject = {[Key in string]: JsonValue} & {
+			[Key in string]?: JsonValue | undefined
+		}
+		type JsonArray = JsonValue[] | readonly JsonValue[]
+
+		type A = Draft<readonly JsonValue[]>
+		isType.equal<true, Draft<JsonValue>[], A>()
+
+		const base: {readonly items: readonly JsonValue[]} = {items: []}
+		produce(base, draft => {
+			draft.items.push({a: [1, "b"]})
+		})
+	})
 })
 
 describe("current() typings", () => {
diff --git a/src/types/types-external.ts b/src/types/types-external.ts
index d42ab8f24..e599b8217 100644
--- a/src/types/types-external.ts
+++ b/src/types/types-external.ts
@@ -28,7 +28,7 @@ export type IfAvailable<T, Fallback = void> =
  */
 type WeakReferences = IfAvailable<WeakMap<any, any>> | IfAvailable<WeakSet<any>>
 
-export type WritableDraft<T> = T extends any[]
+export type WritableDraft<T> = T extends readonly any[]
 	? number extends T["length"]
 		? IsPlainArray<T> extends true
 			? Draft<T[number]>[]