Fix: update() parent pointers not updated after recursive merge with JSON_DIAGNOSTICS

[VasuBhakt]

Issue

Fixes #4813

Overview

Added a set_parents() call under #if JSON_DIAGNOSTICS in the update() iterator overload after a recursive merge.

When update() is called with merge_objects=true, nested objects are recursively merged via it2->second.update(it.value(), true). After this recursive call, the child nodes inside the merged object have stale m_parent pointers, causing the parent consistency assertion to fire when JSON_DIAGNOSTICS=1 is enabled. Calling set_parents() after the recursive merge walks the subtree and fixes all parent pointers correctly.

Changes

• include/nlohmann/json.hpp — 3 line fix in update() iterator overload
• single_include/nlohmann/json.hpp — regenerated via make amalgamate

The changes were tested locally which gave the results:

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• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced.
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated.
• The source code is amalgamated by running make amalgamate.

[nlohmann]

Can you please add a regression test (best the example from #4813) to unit-regression2.cpp?

[VasuBhakt]

sure, working on it.

[VasuBhakt]

added the regression test @nlohmann, please review it

[nlohmann]

Looks good to me.

[VasuBhakt]

Thanks a lot @nlohmann! You have been very patient and responsive throughout the review process. Really appreciate the guidance!

[nlohmann]

Thanks!