Fix JSON_DIAGNOSTICS test crash and clang-tidy hit in patch_inplace self-aliasing

The spare-capacity test reallocated non-empty elements through get_ref(), which leaves stale parent pointers under JSON_DIAGNOSTICS; reserve while the array is empty instead. Suppress performance-unnecessary-copy-initialization for the copy that is needed because *this is modified.

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

include/nlohmann/json.hpp

@@ -6954,6 +6954,7 @@ public:
         // a copy when a value is applied as patch to itself
         if (JSON_HEDLEY_UNLIKELY(&json_patch == this))
         {
+            // NOLINTNEXTLINE(performance-unnecessary-copy-initialization): *this is modified below
             const basic_json json_patch_copy = json_patch;
             patch_inplace(json_patch_copy);
             return;

single_include/nlohmann/json.hpp

@@ -43256,6 +43256,7 @@ public:
         // a copy when a value is applied as patch to itself
         if (JSON_HEDLEY_UNLIKELY(&json_patch == this))
         {
+            // NOLINTNEXTLINE(performance-unnecessary-copy-initialization): *this is modified below
             const basic_json json_patch_copy = json_patch;
             patch_inplace(json_patch_copy);
             return;

tests/src/unit-json_patch.cpp

@@ -847,8 +847,8 @@ TEST_CASE("JSON patch")
 
             SECTION("copy operations without spare capacity")
             {
+                // a copy has exactly as much capacity as elements
                 json doc = copy_ops;
-                doc.get_ref<json::array_t&>().shrink_to_fit();
                 CHECK(doc.patch(doc) == expected_copies);
 
                 doc.patch_inplace(doc);
@@ -857,8 +857,15 @@ TEST_CASE("JSON patch")
 
             SECTION("copy operations with spare capacity")
             {
-                json doc = copy_ops;
+                // reserve while the array is empty: reallocating non-empty
+                // elements behind the library's back leaves stale parent
+                // pointers with JSON_DIAGNOSTICS
+                json doc = json::array();
                 doc.get_ref<json::array_t&>().reserve(100);
+                for (int i = 0; i < 5; ++i)
+                {
+                    doc.push_back(copy_op);
+                }
                 CHECK(doc.patch(doc) == expected_copies);
 
                 doc.patch_inplace(doc);