LastArrayElement: Fix handling of tuples with optional elements

source/last-array-element.d.ts

@@ -30,7 +30,11 @@ export type LastArrayElement<Elements extends readonly unknown[], ElementBeforeT
 				// So we need to recurse type `V` and carry over the type of the element before the spread element.
 				? LastArrayElement<V, U>
 				: Elements extends ReadonlyArray<infer U>
-					? U | ElementBeforeTailingSpreadElement
+					? number extends Elements['length']
+						// A genuine non-tuple array (e.g., `number[]`) or trailing spread element — the last element can be the rest type or the element before it.
+						? U | ElementBeforeTailingSpreadElement
+						// A tuple consisting solely of optional elements (e.g., `[string?, number?]`). The last element is the type of any of those elements (without the `| undefined` added by the optional modifier) or, when they are all absent, the element before them.
+						: Exclude<U, undefined> | ElementBeforeTailingSpreadElement
 					: never;
 
 export {};

test-d/last-array-element.ts

@@ -23,3 +23,26 @@ expectType<number | boolean>(lastOf(trailingSpreadTuple2));
 // eslint-disable-next-line @typescript-eslint/array-type
 declare const trailingSpreadTuple3: ['foo', true, ...(1 | '2')[]];
 expectType<true | 1 | '2'>(lastOf(trailingSpreadTuple3));
+
+// Tuples with optional elements
+declare const trailingOptionalTuple1: [string, number?];
+expectType<string | number>(lastOf(trailingOptionalTuple1));
+
+declare const trailingOptionalTuple2: [number?];
+expectType<number>(lastOf(trailingOptionalTuple2));
+
+declare const trailingOptionalTuple3: [string, boolean, number?];
+expectType<boolean | number>(lastOf(trailingOptionalTuple3));
+
+declare const trailingOptionalTuple4: [string, number?, boolean?];
+expectType<string | number | boolean>(lastOf(trailingOptionalTuple4));
+
+declare const leadingOptionalTuple: [string?, number?];
+expectType<string | number>(lastOf(leadingOptionalTuple));
+
+declare const readonlyOptionalTuple: readonly [string, number?];
+expectType<string | number>(lastOf(readonlyOptionalTuple));
+
+// The `| undefined` from a genuine `undefined`-containing array is preserved
+declare const undefinedArray: Array<number | undefined>;
+expectType<number | undefined>(lastOf(undefinedArray));