From 8e724e372c95d9bb20d8a043f8c32babd3e6d109 Mon Sep 17 00:00:00 2001
From: unional <homawong@gmail.com>
Date: Fri, 10 Apr 2026 09:10:36 -0700
Subject: [PATCH 1/2] fix: improve higher order type inference for
 WritableDraft

---
 __tests__/produce.ts        | 12 ++++++++++++
 src/types/types-external.ts |  2 +-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/__tests__/produce.ts b/__tests__/produce.ts
index c3913873..a7667b13 100644
--- a/__tests__/produce.ts
+++ b/__tests__/produce.ts
@@ -758,3 +758,15 @@ it("allows for mixed property value types", () => {
 		}
 	})
 })
+
+it("allows higher order type inference", () => {
+	function _test<A>() {
+		type S = {prop: A | undefined}
+		produce(
+			draft => {
+				draft.prop = undefined
+			},
+			{prop: undefined} as S
+		)
+	}
+})
diff --git a/src/types/types-external.ts b/src/types/types-external.ts
index d3e7df2c..16db28a8 100644
--- a/src/types/types-external.ts
+++ b/src/types/types-external.ts
@@ -37,7 +37,7 @@ export type WritableDraft<T> = T extends any[]
 	: WritableNonArrayDraft<T>
 
 type WritableNonArrayDraft<T> = {
-	-readonly [K in keyof T]: T[K] extends infer V
+	-readonly [K in keyof T]: {_: T[K]} extends {_: infer V}
 		? V extends object
 			? Draft<V>
 			: V

From 3c296633d5128f6bf8dd00b92632cb1a0b9fa30c Mon Sep 17 00:00:00 2001
From: unional <homawong@gmail.com>
Date: Fri, 10 Apr 2026 09:10:36 -0700
Subject: [PATCH 2/2] fix: support tuples with spread in WritableDraft

---
 __tests__/produce.ts        |  6 ++++++
 src/types/types-external.ts | 15 ++++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/__tests__/produce.ts b/__tests__/produce.ts
index a7667b13..8da9c2b2 100644
--- a/__tests__/produce.ts
+++ b/__tests__/produce.ts
@@ -770,3 +770,9 @@ it("allows higher order type inference", () => {
 		)
 	}
 })
+
+it("supports tuples with spread", () => {
+	type State = [boolean, ...number[]]
+	const foo = (x: State) => x
+	produce<State>(x => foo(x))
+})
diff --git a/src/types/types-external.ts b/src/types/types-external.ts
index 16db28a8..5c98be89 100644
--- a/src/types/types-external.ts
+++ b/src/types/types-external.ts
@@ -32,10 +32,23 @@ type WeakReferences = IfAvailable<WeakMap<any, any>> | IfAvailable<WeakSet<any>>
 
 export type WritableDraft<T> = T extends any[]
 	? number extends T["length"]
-		? Draft<T[number]>[]
+		? IsPlainArray<T> extends true
+			? Draft<T[number]>[]
+			: WritableNonArrayDraft<T>
 		: WritableNonArrayDraft<T>
 	: WritableNonArrayDraft<T>
 
+/**
+ * Distinguishes plain array types (`number[]`, `readonly string[]`, etc.) from
+ * tuple types — including variadic tuples like `[boolean, ...number[]]` — whose
+ * `length` is also `number` but must not be widened to `Element[]`.
+ */
+type IsPlainArray<T extends readonly any[]> = T extends readonly (infer U)[]
+	? U[] extends T
+		? true
+		: false
+	: false
+
 type WritableNonArrayDraft<T> = {
 	-readonly [K in keyof T]: {_: T[K]} extends {_: infer V}
 		? V extends object