From 07d057cb63ff29bee7aa1e4a4c42bf058a6c2206 Mon Sep 17 00:00:00 2001
From: Adam Buechler <butchler@gmail.com>
Date: Fri, 21 Jun 2024 15:05:50 +0900
Subject: [PATCH 1/2] Add failing type tests for first() and last()

---
 type-definitions/flow-tests/immutable-flow.js | 10 ++++++++++
 type-definitions/ts-tests/list.ts             | 20 +++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/type-definitions/flow-tests/immutable-flow.js b/type-definitions/flow-tests/immutable-flow.js
index 4b32d64341..c203962052 100644
--- a/type-definitions/flow-tests/immutable-flow.js
+++ b/type-definitions/flow-tests/immutable-flow.js
@@ -130,6 +130,16 @@ var item: number = numberList.get(4);
 var nullableItem: ?number = numberList.get(4);
 var itemOrDefault: number = numberList.get(4, 10);
 
+// $FlowExpectedError[incompatible-type]
+var item: number = numberList.first();
+// $FlowExpectedError[incompatible-type]
+var func: () => number = () => numberList.first();
+
+// $FlowExpectedError[incompatible-type]
+var item: number = numberList.last();
+// $FlowExpectedError[incompatible-type]
+var func: () => number = () => numberList.last();
+
 numberList = List().insert(0, 0);
 numberOrStringList = List.of(0).insert(1, 'a');
 // $FlowExpectedError[incompatible-call]
diff --git a/type-definitions/ts-tests/list.ts b/type-definitions/ts-tests/list.ts
index 1697d62042..08ea834568 100644
--- a/type-definitions/ts-tests/list.ts
+++ b/type-definitions/ts-tests/list.ts
@@ -76,6 +76,26 @@ import {
   get(List<number>(), 4, 'a');
 }
 
+{
+  // #first
+
+  // $ExpectType number | undefined
+  List<number>().first();
+
+  // $ExpectError
+  (): number => List<number>().first();
+}
+
+{
+  // #last
+
+  // $ExpectType number | undefined
+  List<number>().last();
+
+  // $ExpectError
+  (): number => List<number>().last();
+}
+
 {
   // #set
 

From 8b3cd075d55f6488c63c7e51fda06d02c376302a Mon Sep 17 00:00:00 2001
From: Adam Buechler <butchler@gmail.com>
Date: Fri, 21 Jun 2024 15:06:10 +0900
Subject: [PATCH 2/2] Fix types of first() and last()

---
 type-definitions/immutable.d.ts    | 6 ++++--
 type-definitions/immutable.js.flow | 6 ++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/type-definitions/immutable.d.ts b/type-definitions/immutable.d.ts
index dc2b24547c..3fd60eecb9 100644
--- a/type-definitions/immutable.d.ts
+++ b/type-definitions/immutable.d.ts
@@ -4208,7 +4208,8 @@ declare namespace Immutable {
      * In case the `Collection` is empty returns the optional default
      * value if provided, if no default value is provided returns undefined.
      */
-    first<NSV = undefined>(notSetValue?: NSV): V | NSV;
+    first<NSV>(notSetValue: NSV): V | NSV;
+    first(): V | undefined;
 
     /**
      * In case the `Collection` is not empty returns the last element of the
@@ -4216,7 +4217,8 @@ declare namespace Immutable {
      * In case the `Collection` is empty returns the optional default
      * value if provided, if no default value is provided returns undefined.
      */
-    last<NSV = undefined>(notSetValue?: NSV): V | NSV;
+    last<NSV>(notSetValue: NSV): V | NSV;
+    last(): V | undefined;
 
     // Reading deep values
 
diff --git a/type-definitions/immutable.js.flow b/type-definitions/immutable.js.flow
index 67a496f29c..901960b873 100644
--- a/type-definitions/immutable.js.flow
+++ b/type-definitions/immutable.js.flow
@@ -76,8 +76,10 @@ declare class _Collection<K, +V> implements ValueObject {
   has(key: K): boolean;
   includes(value: V): boolean;
   contains(value: V): boolean;
-  first<NSV>(notSetValue?: NSV): V | NSV;
-  last<NSV>(notSetValue?: NSV): V | NSV;
+  first(): V | void;
+  first<NSV>(notSetValue: NSV): V | NSV;
+  last(): V | void;
+  last<NSV>(notSetValue: NSV): V | NSV;
 
   hasIn(keyPath: Iterable<mixed>): boolean;