From 3bb551f46f95e5214231611d97e8856d394b6f75 Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Sat, 5 Sep 2026 16:34:21 +0200
Subject: [PATCH] Fix swap(array_t&)/swap(object_t&) to update parent pointers
 under JSON_DIAGNOSTICS

Both overloads swapped the underlying container storage but never called
set_parents(), leaving elements moved into *this with stale m_parent
pointers (typically nullptr from the free-standing array_t/object_t).
This produced wrong JSON Pointer paths in diagnostic messages and could
trip assert_invariant() on subsequent copies. Mirrors the fix already
applied in swap(reference other).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---
 include/nlohmann/json.hpp        |  2 ++
 single_include/nlohmann/json.hpp |  2 ++
 tests/src/unit-diagnostics.cpp   | 31 +++++++++++++++++++++++++++++++
 3 files changed, 35 insertions(+)

diff --git a/include/nlohmann/json.hpp b/include/nlohmann/json.hpp
index be4ccb90f..f90999c22 100644
--- a/include/nlohmann/json.hpp
+++ b/include/nlohmann/json.hpp
@@ -3573,6 +3573,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.array), other);
+            set_parents();
         }
         else
         {
@@ -3589,6 +3590,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.object), other);
+            set_parents();
         }
         else
         {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 31bfb869d..6e38fbfa0 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -25001,6 +25001,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.array), other);
+            set_parents();
         }
         else
         {
@@ -25017,6 +25018,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.object), other);
+            set_parents();
         }
         else
         {
diff --git a/tests/src/unit-diagnostics.cpp b/tests/src/unit-diagnostics.cpp
index 1e8ed6aa3..135ecf9d6 100644
--- a/tests/src/unit-diagnostics.cpp
+++ b/tests/src/unit-diagnostics.cpp
@@ -273,5 +273,36 @@ TEST_CASE("Regression tests for extended diagnostics")
         CHECK(j1["numbers"]["two"] == 2);
         CHECK(j1["string"] == "t");
     }
+
+    SECTION("Regression test - swap(array_t&)/swap(object_t&) must update JSON_DIAGNOSTICS parent pointers")
+    {
+        // swap(array_t&)
+        {
+            json j = json::array();
+            json::array_t arr = {json::array({1})};
+            j.swap(arr);
+
+            // parent pointers of the moved-in elements must point into j, not
+            // into the now-defunct free-standing array_t
+            CHECK_THROWS_WITH_AS(j[0][0].get<std::string>(), "[json.exception.type_error.302] (/0/0) type must be string, but is number", json::type_error);
+
+            // must not trigger assert_invariant() in a debug/assert-enabled build
+            json const k = j;
+            CHECK(k == j);
+        }
+
+        // swap(object_t&)
+        {
+            json o = json::object();
+            json::object_t obj = {{"a", json::array({1})}};
+            o.swap(obj);
+
+            CHECK_THROWS_WITH_AS(o["a"][0].get<std::string>(), "[json.exception.type_error.302] (/a/0) type must be string, but is number", json::type_error);
+
+            // must not trigger assert_invariant() in a debug/assert-enabled build
+            json const p = o;
+            CHECK(p == o);
+        }
+    }
 }