From fac37ba1575a0e8715832e447afe54e045f56fb0 Mon Sep 17 00:00:00 2001
From: Francesco Trotta <github@fasttime.org>
Date: Fri, 29 Dec 2023 11:15:13 +0100
Subject: [PATCH] fix an edge case with non-enumerable properties

---
 lib/config/flat-config-schema.js       |  2 +-
 tests/lib/config/flat-config-schema.js | 30 ++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/lib/config/flat-config-schema.js b/lib/config/flat-config-schema.js
index 83c010ea0e3..6b64319c8fc 100644
--- a/lib/config/flat-config-schema.js
+++ b/lib/config/flat-config-schema.js
@@ -114,7 +114,7 @@ function deepMerge(first, second, mergeMap = new Map()) {
     for (const key of Object.keys(second)) {
 
         // avoid hairy edge case
-        if (key === "__proto__") {
+        if (key === "__proto__" || !Object.prototype.propertyIsEnumerable.call(first, key)) {
             continue;
         }
 
diff --git a/tests/lib/config/flat-config-schema.js b/tests/lib/config/flat-config-schema.js
index ff94d6f910a..b1b7303af62 100644
--- a/tests/lib/config/flat-config-schema.js
+++ b/tests/lib/config/flat-config-schema.js
@@ -199,6 +199,36 @@ describe("merge", () => {
         assert.deepStrictEqual(result, { foo: void 0, bar: void 0, baz: void 0 });
     });
 
+    it("considers only own enumerable properties", () => {
+        const first = {
+            __proto__: { inherited1: "A" }, // non-own properties are not considered
+            included1: "B",
+            notMerged1: { first: true }
+        };
+        const second = {
+            __proto__: { inherited2: "C" }, // non-own properties are not considered
+            included2: "D",
+            notMerged2: { second: true }
+        };
+
+        // non-enumerable properties are not considered
+        Object.defineProperty(first, "notMerged2", { enumerable: false, value: { first: true } });
+        Object.defineProperty(second, "notMerged1", { enumerable: false, value: { second: true } });
+
+        const result = merge(first, second);
+
+        assert.deepStrictEqual(
+            result,
+            {
+                included1: "B",
+                included2: "D",
+                notMerged1: { first: true },
+                notMerged2: { second: true }
+            }
+        );
+        confirmLegacyMergeResult(first, second, result);
+    });
+
     it("merges objects with self-references", () => {
         const first = { foo: 42 };