Detect a patch stored inside the patched value with JSON_DIAGNOSTICS

patch_inplace applied a copy only when the patch was the value itself. With JSON_DIAGNOSTICS the parent pointers make it cheap to also detect a patch nested inside the value, so that case is handled the same way. Without diagnostics it stays documented undefined behavior.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/patch_inplace.md

@@ -73,11 +73,14 @@ The JSON patch may be the JSON value itself (`#!cpp j.patch_inplace(j)`): the op
 the patch made before the first operation is applied, so the result is the same as that of
 `#!cpp j = j.patch(j)`, and the exception safety is the same as above.
 
+If [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) is enabled, the same holds for a JSON patch stored inside the
+JSON value (e.g., `#!cpp j.patch_inplace(j["patch"])`): it is detected via the parent pointers.
+
 !!! danger "Undefined behavior"
 
-    The JSON patch must not be a value stored inside the JSON value (e.g., `#!cpp j.patch_inplace(j["patch"])`), as the
-    operations may change or destroy it while it is read. Pass a copy (`#!cpp j.patch_inplace(json(j["patch"]))`) or use
-    [`patch`](patch.md) instead.
+    Without [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md), the JSON patch must not be a value stored inside the
+    JSON value (e.g., `#!cpp j.patch_inplace(j["patch"])`), as the operations may change or destroy it while it is
+    read. Pass a copy (`#!cpp j.patch_inplace(json(j["patch"]))`) or use [`patch`](patch.md) instead.
 
 ## Examples
 
@@ -132,4 +135,5 @@ the patch made before the first operation is applied, so the result is the same
 - Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) instead of
   [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) for a one-character array index that is
   not a digit (e.g., `/x`) in version 3.13.0, as it already did for longer ones.
-- Applying a JSON value as patch to itself no longer results in undefined behavior in version 3.13.0.
+- Applying a JSON value as patch to itself (or, with `JSON_DIAGNOSTICS`, to a value stored inside it) no longer results
+  in undefined behavior in version 3.13.0.

include/nlohmann/json.hpp

@@ -6951,8 +6951,16 @@ public:
     void patch_inplace(const basic_json& json_patch)
     {
         // the operations below change *this while iterating json_patch: apply
-        // a copy when a value is applied as patch to itself
-        if (JSON_HEDLEY_UNLIKELY(&json_patch == this))
+        // a copy when a value is applied as patch to itself or, with
+        // JSON_DIAGNOSTICS, to a value stored inside it
+        bool patch_aliases_this = (&json_patch == this);
+#if JSON_DIAGNOSTICS
+        for (const basic_json* p = json_patch.m_parent; !patch_aliases_this && p != nullptr; p = p->m_parent)
+        {
+            patch_aliases_this = (p == this);
+        }
+#endif
+        if (JSON_HEDLEY_UNLIKELY(patch_aliases_this))
         {
             // NOLINTNEXTLINE(performance-unnecessary-copy-initialization): *this is modified below
             const basic_json json_patch_copy = json_patch;

single_include/nlohmann/json.hpp

@@ -43253,8 +43253,16 @@ public:
     void patch_inplace(const basic_json& json_patch)
     {
         // the operations below change *this while iterating json_patch: apply
-        // a copy when a value is applied as patch to itself
-        if (JSON_HEDLEY_UNLIKELY(&json_patch == this))
+        // a copy when a value is applied as patch to itself or, with
+        // JSON_DIAGNOSTICS, to a value stored inside it
+        bool patch_aliases_this = (&json_patch == this);
+#if JSON_DIAGNOSTICS
+        for (const basic_json* p = json_patch.m_parent; !patch_aliases_this && p != nullptr; p = p->m_parent)
+        {
+            patch_aliases_this = (p == this);
+        }
+#endif
+        if (JSON_HEDLEY_UNLIKELY(patch_aliases_this))
         {
             // NOLINTNEXTLINE(performance-unnecessary-copy-initialization): *this is modified below
             const basic_json json_patch_copy = json_patch;

tests/src/unit-json_patch.cpp

@@ -855,6 +855,19 @@ TEST_CASE("JSON patch")
                 CHECK(doc == expected_copies);
             }
 
+#if JSON_DIAGNOSTICS
+            SECTION("patch stored inside the patched value")
+            {
+                // the parent pointers of JSON_DIAGNOSTICS make the aliasing detectable
+                const json op = {{"op", "replace"}, {"path", "/patch"}, {"value", 1}};
+                json doc = {{"a", 1}, {"patch", json::array({op})}};
+                const json expected = doc.patch(doc["patch"]);
+
+                doc.patch_inplace(doc["patch"]);
+                CHECK(doc == expected);
+            }
+#endif
+
             SECTION("copy operations with spare capacity")
             {
                 // reserve while the array is empty: reallocating non-empty