forward mapped value in object::insert_or_assign assign branch

[Ramya-9353]

Repro: insert_or_assign(key, std::move(v)) on an existing key, with v on the object's own resource, deep-copies v and leaves it intact instead of moving it. For a 100-element array the assign branch runs 101 allocations and the source array is still populated afterwards.
Cause: the assign branch casts the mapped value to M rather than M&&, so for an rvalue argument it selects value(value const&), which allocates a copy on the source's storage before the copy into sp_. The insert branch one line above already forwards with M&&.
Fix: cast to M&& to match the insert branch. The stored value is unchanged; the redundant copy and the ignored move go away.

[cppalliance-bot]

An automated preview of the documentation is available at https://1186.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.

2026-08-19 06:03:58 UTC

[cppalliance-bot]

GCOVR code coverage report https://1186.json.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://1186.json.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff Report https://1186.json.prtest2.cppalliance.org/diff-report/index.html

Build time: 2026-08-19 06:37:59 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (e2f976c) to head (08e993d).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1186   +/-   ##
========================================
  Coverage    93.71%   93.71%           
========================================
  Files           85       85           
  Lines         8971     8980    +9     
========================================
+ Hits          8407     8416    +9     
  Misses         564      564           

Files with missing lines	Coverage Δ
include/boost/json/impl/object.hpp	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1186/pullrequest.html

[grisumbras]

Ouch. Thanks for catching this.

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1186/pullrequest.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1186/pullrequest.html