align caller-supplied buffer in value_stack::stack

[Ramya-9353]

Repro: hand any of the buffer-taking parser / stream_parser / value_stack constructors a buffer that is not aligned for value (buf + 1, a heap block, a sub-buffer) and parse a small array. UBSan reports constructor call on misaligned address ... for type boost::json::value, which requires 8 byte alignment at detail/value.hpp:229, via value_stack::stack::push.

Cause: value_stack::stack stores value objects in the caller-owned buffer but reinterprets it as value* without aligning the pointer. static_resource::do_allocate and monotonic_resource both align a caller buffer through std::align; this constructor was the one that skipped it, and the buffer overloads document the parameter only as "a pointer to valid storage".

Fix: align the buffer up to alignof(value) with std::align, keeping min_size_ * sizeof(value) usable bytes, and fall back to the memory resource when the aligned region is too small.

Regression test walks every misalignment of a value-sized buffer and parses [1,2,3]; the odd offsets trip UBSan before the change and pass after.

[cppalliance-bot]

An automated preview of the documentation is available at https://1193.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.

2026-09-08 16:28:23 UTC

[cppalliance-bot]

GCOVR code coverage report https://1193.json.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://1193.json.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff Report https://1193.json.prtest2.cppalliance.org/diff-report/index.html

Build time: 2026-09-08 16:48:01 UTC

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1193/pullrequest.html

[Ramya-9353]

Rebased onto develop to pick up the GCC 16 CI fix (82398f9); the failing Drone stages were the null_resource.cpp -Wfree-nonheap-object error, not this change. No code changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.72%. Comparing base (82398f9) to head (37912e9).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1193   +/-   ##
========================================
  Coverage    93.72%   93.72%           
========================================
  Files           85       85           
  Lines         8981     8982    +1     
========================================
+ Hits          8417     8418    +1     
  Misses         564      564           

Files with missing lines	Coverage Δ
include/boost/json/impl/value_stack.ipp	99.50% <100.00%> (+<0.01%)	⬆️

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1193/pullrequest.html