fix: type when merging index signatures

fix #459

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>;
   };
 };
 

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.
  */

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);