From bdb5a74c563d552a5dbd3e446fecadd79e89a9ce Mon Sep 17 00:00:00 2001
From: Rebecca Stevens <rebecca.stevens@outlook.co.nz>
Date: Thu, 23 Jan 2025 14:54:44 +1300
Subject: [PATCH] fix: apply filtering to types when selecting a leaf node

fixes #524
---
 src/types/merging.ts           | 48 ++++++++++++++++++++++++++++++----
 src/types/utils.ts             | 21 +++++++++++++++
 tests/deepmerge-custom.test.ts |  8 +++---
 tests/deepmerge.test-d.ts      | 20 +++++++++++++-
 4 files changed, 87 insertions(+), 10 deletions(-)

diff --git a/src/types/merging.ts b/src/types/merging.ts
index 86ac5f8e..8a21d1f5 100644
--- a/src/types/merging.ts
+++ b/src/types/merging.ts
@@ -5,7 +5,17 @@ import type {
   DeepMergeRecordsDefaultHKT,
   DeepMergeSetsDefaultHKT,
 } from "./defaults";
-import type { EveryIsArray, EveryIsMap, EveryIsRecord, EveryIsSet, IsNever, IsTuple } from "./utils";
+import type {
+  AssertType,
+  EveryIsArray,
+  EveryIsMap,
+  EveryIsRecord,
+  EveryIsSet,
+  IsNever,
+  IsTuple,
+  TupleTupleToTupleUnion,
+  UnionToTuple,
+} from "./utils";
 
 /**
  * Mapping of merge function URIs to the merge function type.
@@ -16,7 +26,7 @@ export interface DeepMergeFunctionURItoKind<
   Fs extends DeepMergeFunctionsURIs,
   in out M,
 > {
-  readonly DeepMergeLeafURI: DeepMergeLeaf<Ts>;
+  readonly DeepMergeLeafURI: DeepMergeLeaf<Ts, Fs, M>;
   readonly DeepMergeRecordsDefaultURI: DeepMergeRecordsDefaultHKT<Ts, Fs, M>;
   readonly DeepMergeArraysDefaultURI: DeepMergeArraysDefaultHKT<Ts, Fs, M>;
   readonly DeepMergeSetsDefaultURI: DeepMergeSetsDefaultHKT<Ts>;
@@ -174,18 +184,46 @@ export type DeepMergeNoFilteringURI = "DeepMergeNoFilteringURI";
 /**
  * Get the leaf type from many types that can't be merged.
  */
-export type DeepMergeLeaf<Ts extends ReadonlyArray<unknown>> = Ts extends readonly []
+export type DeepMergeLeaf<
+  Ts extends ReadonlyArray<unknown>,
+  Fs extends DeepMergeFunctionsURIs,
+  M,
+> = Ts extends readonly []
   ? never
   : Ts extends readonly [infer T]
     ? T
     : Ts extends readonly [...infer Rest, infer Tail]
       ? IsNever<Tail> extends true
         ? Rest extends ReadonlyArray<unknown>
-          ? DeepMergeLeaf<Rest>
+          ? DeepMergeLeaf<Rest, Fs, M>
           : never
-        : Tail
+        : DeepMergeLeafApplyFilter<
+            Ts,
+            AssertType<
+              ReadonlyArray<unknown>,
+              TupleTupleToTupleUnion<
+                AssertType<
+                  ReadonlyArray<ReadonlyArray<unknown>>,
+                  {
+                    [I in keyof Ts]: FilterValuesHKT<UnionToTuple<Ts[I]>, Fs, M>;
+                  }
+                >
+              >
+            >
+          >
       : never;
 
+type DeepMergeLeafApplyFilter<
+  Original extends ReadonlyArray<unknown>,
+  Filtered extends ReadonlyArray<unknown>,
+> = Original extends readonly [...infer OriginalRest, infer OriginalTail]
+  ? Filtered extends readonly [...infer FilteredRest, infer FilteredTail]
+    ? OriginalTail extends FilteredTail
+      ? FilteredTail
+      : FilteredTail | DeepMergeLeafApplyFilter<OriginalRest, FilteredRest>
+    : never
+  : never;
+
 /**
  * The meta data deepmerge is able to provide.
  */
diff --git a/src/types/utils.ts b/src/types/utils.ts
index 81a7b3b4..a3d917d4 100644
--- a/src/types/utils.ts
+++ b/src/types/utils.ts
@@ -280,3 +280,24 @@ export type UnionToTuple<T, L = LastOf<T>> = IsNever<T> extends true ? [] : [...
 type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends (k: infer I) => void ? I : never;
 
 type LastOf<T> = UnionToIntersection<T extends any ? () => T : never> extends () => infer R ? R : never;
+
+/**
+ * Convert a tuple of tuples to a tuple of unions.
+ */
+export type TupleTupleToTupleUnion<T extends ReadonlyArray<ReadonlyArray<unknown>>> = {
+  [I in keyof T]: TupleToUnion<T[I]>;
+};
+
+/**
+ * Convert a tuple to a union.
+ */
+export type TupleToUnion<T extends ReadonlyArray<unknown>> = T extends readonly []
+  ? never
+  : T extends readonly [infer Head, ...infer Rest]
+    ? Head | TupleToUnion<Rest>
+    : never;
+
+/**
+ * Assert that a type is of a given type.
+ */
+export type AssertType<Expected, T> = T extends Expected ? T : never;
diff --git a/tests/deepmerge-custom.test.ts b/tests/deepmerge-custom.test.ts
index 9515e3a5..fb606092 100644
--- a/tests/deepmerge-custom.test.ts
+++ b/tests/deepmerge-custom.test.ts
@@ -41,13 +41,13 @@ type Entries<T> = Array<
 
 declare module "../src/types" {
   interface DeepMergeFunctionURItoKind<Ts extends ReadonlyArray<unknown>, Fs extends DeepMergeFunctionsURIs, M> {
-    readonly NoArrayMerge1: DeepMergeLeaf<Ts>;
+    readonly NoArrayMerge1: DeepMergeLeaf<Ts, Fs, M>;
   }
 }
 
 declare module "../src/types" {
   interface DeepMergeFunctionURItoKind<Ts extends ReadonlyArray<unknown>, Fs extends DeepMergeFunctionsURIs, M> {
-    readonly MergeDates1: EveryIsDate<Ts> extends true ? Ts : DeepMergeLeaf<Ts>;
+    readonly MergeDates1: EveryIsDate<Ts> extends true ? Ts : DeepMergeLeaf<Ts, Fs, M>;
   }
 }
 
@@ -60,7 +60,7 @@ declare module "../src/types" {
     Fs extends DeepMergeFunctionsURIs,
     M,
   > {
-    readonly KeyPathBasedMerge: Ts[number] extends number ? Ts[number] | string : DeepMergeLeaf<Ts>;
+    readonly KeyPathBasedMerge: Ts[number] extends number ? Ts[number] | string : DeepMergeLeaf<Ts, Fs, M>;
   }
 }
 
@@ -76,7 +76,7 @@ declare module "../src/types" {
           Fs,
           M
         >
-      : DeepMergeLeaf<Ts>;
+      : DeepMergeLeaf<Ts, Fs, M>;
   }
 }
 
diff --git a/tests/deepmerge.test-d.ts b/tests/deepmerge.test-d.ts
index 18a1db16..a808d2f6 100644
--- a/tests/deepmerge.test-d.ts
+++ b/tests/deepmerge.test-d.ts
@@ -1,6 +1,12 @@
 import { expectAssignable, expectType } from "tsd";
 
-import { type DeepMergeMapsDefaultHKT, type DeepMergeSetsDefaultHKT, deepmerge } from "../src";
+import {
+  type DeepMergeMapsDefaultHKT,
+  type DeepMergeNoFilteringURI,
+  type DeepMergeSetsDefaultHKT,
+  deepmerge,
+  deepmergeCustom,
+} from "../src";
 
 const a = {
   foo: "abc",
@@ -252,3 +258,15 @@ expectType<unknown>(test22);
 const r: { a?: string; b?: number; c?: boolean } = { a: "a", b: 1 };
 const test23 = deepmerge(r, r);
 expectType<{ a?: string; b?: number; c?: boolean }>(test23);
+
+const s: { foo: number | undefined } = { foo: undefined };
+const test24 = deepmerge(a, s);
+expectType<{ foo: string | number; baz: { quux: string[] }; garply: number }>(test24);
+
+const test25 = deepmergeCustom<
+  unknown,
+  {
+    DeepMergeFilterValuesURI: DeepMergeNoFilteringURI;
+  }
+>({ filterValues: false })(a, s);
+expectType<{ foo: number | undefined; baz: { quux: string[] }; garply: number }>(test25);