fix overlapping memcpy in string::assign

[Ramya-9353]

Summary

string::assign(char const* s, size_type count) uses
char_traits::copy (memcpy) unconditionally. When s points into
the string's own buffer — e.g. via s.assign(string_view(s).substr(3)) —
the source and destination regions overlap, which is undefined behaviour
per [mem.res]/[char.traits.require].

Under AddressSanitizer this fires memcpy-param-overlap; without
instrumentation it silently corrupts the result on some compilers/optimisation
levels.

Fix

Detect aliasing with detail::ptr_in_range. When the source is internal,
use char_traits::move (memmove) directly followed by impl_.term(count).
The non-aliasing path remains unchanged.

The aliasing case guarantees count <= size() <= capacity(), so no
reallocation is needed and bypassing impl_.assign is safe.

Reproducer

boost::json::string s("abcdefghijklmnopqrstuvwxyz0123456789");
s.assign(boost::json::string_view(s).substr(3));
// ASAN: memcpy-param-overlap
// Without ASAN: silent data corruption (null terminator overwrites source)

Labels

• component: boost/json/string
• type: memcpy-param-overlap / undefined behaviour
• severity: medium — data corruption on common self-referencing patterns
• affected: string::assign(char const*, size_type) and all callers
  (operator=(string_view), assign(string_view), construction from
  self-referencing string_view)

[cppalliance-bot]

An automated preview of the documentation is available at https://1175.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.

2026-07-19 14:08:59 UTC

[cppalliance-bot]

GCOVR code coverage report https://1175.json.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://1175.json.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff Report https://1175.json.prtest2.cppalliance.org/diff-report/index.html

Build time: 2026-07-19 14:22:50 UTC

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1175/pullrequest.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.91%. Comparing base (ea90668) to head (f54d353).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1175   +/-   ##
========================================
  Coverage    93.91%   93.91%           
========================================
  Files           91       91           
  Lines         9265     9269    +4     
========================================
+ Hits          8701     8705    +4     
  Misses         564      564           

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

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