From 0c3811498d2cc5f04441f85213869ab699f6267d Mon Sep 17 00:00:00 2001
From: Rebecca Stevens <rebecca.stevens@outlook.co.nz>
Date: Mon, 20 May 2024 17:52:50 +1200
Subject: [PATCH] fix: type when merging index signatures

fix #459
---
 src/types/defaults.ts     | 3 ++-
 src/types/utils.ts        | 8 ++++++++
 tests/deepmerge.test-d.ts | 5 +++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/types/defaults.ts b/src/types/defaults.ts
index 18bedd49..8af0f248 100644
--- a/src/types/defaults.ts
+++ b/src/types/defaults.ts
@@ -7,6 +7,7 @@ import {
 import {
   type FilterOutNever,
   type FlattenTuples,
+  type KeyIsOptional,
   type SimplifyObject,
   type TransposeTuple,
   type TupleToIntersection,
@@ -75,7 +76,7 @@ type RecordToRecordMeta<T extends Record<PropertyKey, unknown>> = {
   [K in keyof T]-?: {
     key: K;
     value: Required<T>[K];
-    optional: {} extends Pick<T, K> ? true : false;
+    optional: KeyIsOptional<K, T>;
   };
 };
 
diff --git a/src/types/utils.ts b/src/types/utils.ts
index dd9e8bed..83a049da 100644
--- a/src/types/utils.ts
+++ b/src/types/utils.ts
@@ -55,6 +55,14 @@ export type Or<T1 extends boolean, T2 extends boolean> = T1 extends true
  */
 export type Not<T extends boolean> = T extends true ? false : true;
 
+/**
+ * Check if a key is optional in the given object.
+ */
+export type KeyIsOptional<
+  K extends PropertyKey,
+  O extends { [Key in K]?: unknown },
+> = O extends { [Key in K]: unknown } ? false : true;
+
 /**
  * Returns whether or not all the given types are never.
  */
diff --git a/tests/deepmerge.test-d.ts b/tests/deepmerge.test-d.ts
index f0a62cdd..bb9b9af2 100644
--- a/tests/deepmerge.test-d.ts
+++ b/tests/deepmerge.test-d.ts
@@ -239,3 +239,8 @@ const p: { a: true; b?: string } = { a: true, b: "n" };
 
 const test18 = deepmerge(o, p);
 expectType<{ a: true; b?: string | number }>(test18);
+
+const q: Record<string, string> = { a: "a" };
+
+const test19 = deepmerge(q, q);
+expectType<Record<string, string>>(test19);