fix: improve higher order type inference for WritableDraft (#1205)

* fix: improve higher order type inference for WritableDraft

* fix: support tuples with spread in WritableDraft

---------

Co-authored-by: Michel Weststrate <mweststrate@gmail.com>

Author: unional

__tests__/produce.ts

@@ -771,6 +771,24 @@ 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
+		)
+	}
+})
+
+it("supports tuples with spread", () => {
+	type State = [boolean, ...number[]]
+	const foo = (x: State) => x
+	produce<State>(x => foo(x))
+})
+
 it("#877 - produce with typed state generic requires initial state", () => {
 	const reducerNoInitial = produce<{count: number}, [{type: "inc"}]>(
 		(draft, action: {type: "inc"}) => {

src/types/types-external.ts

@@ -30,12 +30,25 @@ 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
+	-readonly [K in keyof T]: {_: T[K]} extends {_: infer V}
 		? V extends object
 			? Draft<V>
 			: V