recreate insert iterator after clear in parse_into sequence handler

[Ramya-9353]

Repro: parse_into a boost::container::flat_set<int> that already holds elements ([1,2,3], then [4] into the same object), or a std::vector<flat_set<int>> from [[1,2],[3,4]]. Debug builds stop on Boost.Container's priv_in_range_or_end(hint) assertion; with NDEBUG, ASan reports a heap-buffer-overflow read in flat_tree::priv_insert_unique_prepare, reached from signal_value. C++23 std::flat_set fails the same way.

Cause: the sequence handler builds its inserter in the constructor and in signal_end, but the target is cleared later, in on_array_begin, and a nested target (next_value_) is also moved from and reassigned by the parent between elements. Without push_back the inserter is std::inserter(c, c.end()), so the stored hint is already invalid when the first element arrives. std::set still works (checked with libc++) because its end() survives both, which is why this went unnoticed.

Fix: build the inserter in on_array_begin, right after the clear, instead of in signal_end. The added test aborts on develop and passes with the change.

[cppalliance-bot]

An automated preview of the documentation is available at https://1207.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-10-05 08:37:46 UTC

[cppalliance-bot]

GCOVR code coverage report https://1207.json.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://1207.json.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff Report https://1207.json.prtest2.cppalliance.org/diff-report/index.html

Build time: 2026-10-05 09:13:10 UTC

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1207/pullrequest.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.98%. Comparing base (fc70d23) to head (1fd61ae).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1207   +/-   ##
========================================
  Coverage    93.98%   93.98%           
========================================
  Files           85       85           
  Lines         8973     8973           
========================================
  Hits          8433     8433           
  Misses         540      540           

Files with missing lines	Coverage Δ
include/boost/json/detail/parse_into.hpp	99.33% <100.00%> (ø)

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