[nlohmann]

With JSON_DIAGNOSTICS enabled, erase() and update() can leave members of an ordered_json object with a null parent pointer. The next assert_invariant() on that object then aborts, for example when the object is copied. In NDEBUG builds nothing aborts, but diagnostic messages silently lose part of the path, e.g. (/x) instead of (/b/x). nlohmann::json isn't affected, because std::map doesn't move its entries.

Minimal reproducer (aborts on develop):

#define JSON_DIAGNOSTICS 1
#include <nlohmann/json.hpp>
using J = nlohmann::ordered_json;
int main()
{
    J a = J::parse(R"({"a":{"c":{"d":{}}}})");
    a.merge_patch(J::parse(R"({"a":{"c":{"c":"s","d":null},"e":"s"}})"));
}

Root cause

ordered_json stores its members in a vector. Two operations move members without restoring their parent pointers:

1. Erasing a member that is not the last one. ordered_map::erase rebuilds every later member in place (destroy + placement-new). The basic_json move constructor doesn't carry m_parent over, and none of the object branches of basic_json::erase (by key, iterator or range) repaired this. merge_patch() with a null member and patch() with a remove operation go through the same code.
2. update() adding a key that makes the vector grow. Reallocation copy-constructs the other members, because std::pair<const std::string, basic_json> isn't nothrow-movable. Those copies get a null parent, and update() only set the parent of the member it inserted. The it2->second.set_parents() added for json::update() with merge_objects==true may trigger JSON_ASSERT for some objects #4813 repaired this only for the nested object during a merge, not for the object update() is called on. The json::update() with merge_objects==true may trigger JSON_ASSERT for some objects #4813 test still passed because it only uses CHECK_NOTHROW and never copies the result.

In the reproducer, /a/c becomes [d, c], and erasing d shifts c into slot 0 with a null parent. The assertion fires when inserting e makes /a's vector reallocate: that reallocation copies /a/c, and the copy constructor checks the source.

Changes

• A new private helper, set_parents_after_object_erase(), runs set_parents() after an erase if the object type is vector-backed (detail::is_ordered_map). It does nothing unless JSON_DIAGNOSTICS is on. All four object erase paths call it: both erase_internal overloads, erase(iterator) and erase(iterator, iterator).
• update() now uses the existing set_parent(), which already refreshes all members for vector-backed objects. This makes the json::update() with merge_objects==true may trigger JSON_ASSERT for some objects #4813 workaround redundant, so it's removed.
• A new section in tests/src/unit-diagnostics.cpp covers erase (key, iterator, range), patch remove, update(j), update(j, true) and the reproducer above, all on ordered_json. Each case checks two things: a diagnostic path read through const access (so operator[] can't repair the pointers; this catches the bug under NDEBUG) and a deep copy (which runs assert_invariant() on every node).

Testing

• The new test aborts against the old header with asserts on. Under NDEBUG, six of its checks fail on the truncated path.
• A randomized stress test (30,000 seeds, mixing operator[], all erase forms, update, merge_patch, patch, emplace and insert on ordered_json, with a deep copy after every step) passes with the fix. The old header aborts almost immediately.
• Twenty-four related unit suites, built with and without -DJSON_DIAGNOSTICS=1, give the same results as the old header. The only difference is the new test.
• Built with Apple clang 21 under C++11, C++17 and C++20 with -Wall -Wextra -Wpedantic -Werror. GCC wasn't tested locally.

Interaction with #5547

#5547's merge_patch rewrite erases through basic_json::erase, so it picks up this fix automatically. Its update_members() and update_members_iteratively() still use the old operator[](key).m_parent = ... pattern, and the iterative version never repairs the top-level object. When #5547 is rebased onto this, both functions should switch to set_parent(...) and drop their set_parents() workarounds.

Breaking changes

None. The public API is unchanged: the new helper is private, and the behavior only changes when JSON_DIAGNOSTICS is enabled.

---

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced. (no separate issue filed; related to json::update() with merge_objects==true may trigger JSON_ASSERT for some objects #4813)
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated. (not applicable)
• The source code is amalgamated by running make amalgamate.

This pull request was written by Claude Code.

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