fix: improve higher order type inference for WritableDraft

Author: unional

__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
+		)
+	}
+})

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