[mariobob]

What

Since #4558 the relinker-only build no longer compiles on macOS 15. ParamJsonParser.cpp converts JSON numbers with floating-point std::from_chars, which Apple's libc++ marks unavailable below a macOS 26 deployment target:

core/libs/prx/libkernel/AppMetadata/src/ParamJsonParser.cpp:234:33: error: 'from_chars' is unavailable: introduced in macOS 26.0

Both relinker and param_json_utf8_tests include that file, so the build stops there. CI does not see it because the macos-latest relinker job now runs on macOS 26.6, where the deployment target already satisfies the annotation.

The conversion moves into parseJsonNumberValue:

• Where the standard library defines __cpp_lib_to_chars it uses std::from_chars exactly as on main. libstdc++ defines the macro and gates its own floating from_chars declarations on it, so the Linux and Windows builds keep the current code.
• Otherwise it reads the number through an std::istringstream imbued with std::locale::classic(). libc++ does not define __cpp_lib_to_chars (it is commented out in libc++ 20's <version>), so this is the macOS path. libc++ converts with strtod_l and the C locale, so the result does not depend on LC_NUMERIC or the global C++ locale. libc++ already defines __cpp_lib_format and __cpp_lib_print only when floating-point to_chars is available for the deployment target, so a later version that defines __cpp_lib_to_chars can be expected to gate it the same way.
• Both paths require the whole number to be consumed and throw number outside supported range in param.json when it does not convert to a double, as before.

A stream is not used everywhere because libstdc++'s generic locale model, which MinGW-w64 uses, switches the process-wide C locale to "C" around each stream conversion, and parseParamJson is reached from ensureTitleLoaded while guest threads run. from_chars stays where it is available.

This follows the same feature-test pattern as AtomicSharedPtr in #4629.

Tested

macOS 15.8 (24H23), Apple M4 Max, Apple clang 17.0.0 with the Xcode 26 SDK (libc++ 20.1), CMake 4.4.3, Unix Makefiles, Release, -DANYPS5_RELINKER_ONLY=ON -DBUILD_TESTING=ON. No title or PS5 hardware needed.

• main (c13dc7e): the build fails with the error above for relinker and param_json_utf8_tests.
• This branch: the build succeeds and ctest passes 134 of 134.
• param_json_utf8: fr_FR.UTF-8 is available on this host, so CheckNumericLocale runs the comma-decimal regression from fix(kernel): parse JSON numbers independently of host locale #4558 instead of printing the skip message, and it passes. As a negative control, the same test built against a copy of the parser that uses std::stod fails with JSON number depends on host locale: 1.25e3.
• The from_chars branch was syntax-checked with clang against a macOS 26 target. It was not built with GCC locally; it is the code main already builds on Linux and Windows.
• python3 tools/check_conventions.py --base origin/main passes.
• fix(kernel): honor the declared flexible memory budget #4508 and fix(libkernel): take the flexible memory size from param.json #4617 also change ParamJsonParser.cpp; both merge cleanly with this branch.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw (NotImplemented_nid_no_patch); silent stubs are listed under Silent stubs in their library's technical debt file
• No notes, investigation .md files or images added to the repository (attach them to this PR)
• New third-party code is a submodule built from source
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: none
• AI-assisted: yes

[simonbronk]

Tested with a newer toolchain than the one above: macOS 27.0.1, Apple silicon, Apple clang 21 with the macOS 27 SDK (libc++ 22.1), CMake 4.4, Ninja, relinker-only Release. This PR (e10d7f1a8) merged into main 18c438b10, built with CMAKE_OSX_DEPLOYMENT_TARGET 15.0 and 27.0:

• main, target 15.0: fails with the same 'from_chars' is unavailable: introduced in macOS 26.0 for relinker and param_json_utf8_tests. Target 27.0 builds and passes 146/146.
• This PR: both targets build and pass 146/146. fr_FR locales are installed, so param_json_utf8 runs the comma-decimal check.
• libc++ 22.1 still does not define __cpp_lib_to_chars at any of these targets, so the stream path is the one taken on macOS here too.

AI-assisted: yes (Claude Code).