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.

2026-08-28 19:43:54 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: 2026-08-28 19:58:43 UTC

[cppalliance-bot]

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