Adarsh-Me opened a new pull request, #51315:
URL: https://github.com/apache/arrow/pull/51315
### Rationale for this change
GH-51267 tracks removing the vendored datetime fallback once all supported
toolchains provide working C++20 chrono timezone support. The backend selection
is currently hardcoded in `chrono_internal.h` (Windows-only) with no
build-level switch, and the vendored tz implementation is compiled into every
binary — including standard-backend ones.
### What changes are included in this PR?
- New `ARROW_USE_STD_CHRONO` CMake option (`AUTO`/`ON`/`OFF`, default
`AUTO`) resolved by a toolchain feature probe (`__cpp_lib_chrono >= 201907L`)
in `cpp/cmake_modules/CheckStdChrono.cmake`; new matching `use_std_chrono`
Meson feature option with the same semantics.
- `AUTO` preserves the current platform default (std::chrono on Windows
toolchains with timezone support, vendored fallback elsewhere) until the
minimum-toolchain prerequisites in GH-51267 are met. Explicit opt-in fails
loudly at configure time on unsupported toolchains.
- The resolved backend is recorded in `arrow/util/config.h` and honored by
`chrono_internal.h`; standard-backend binaries no longer build
`vendored/datetime.cpp`, and `config.cc` / the `SetTimezoneConfig` test honor
the selected backend (OS tzdb, no vendored `set_install`/`reload_tzdb`).
### Are these changes tested?
- CMake 3.31.6: verified the new module resolves `AUTO`→`OFF` on Linux/GCC
11, passes `OFF` through, fails configure on `ON` without toolchain support and
on invalid values; verified `config.h.cmake` renders `ARROW_USE_STD_CHRONO 0/1`
via real `configure_file`.
- Meson 1.7.2: verified `auto`/`disabled` keep the fallback and render `0`,
and `enabled` errors out on an unsupported toolchain.
- g++ 11 (C++20): compiled the real `vendored/datetime.cpp` plus a
functional test against the modified `chrono_internal.h` fallback path —
calendar round-trip, floor/ceil/trunc, `format`, `make_time`, `OffsetZone`, and
a live `locate_zone("America/New_York")` lookup against system zoneinfo all
pass; also confirmed the header selects the std branch when
`ARROW_USE_STD_CHRONO=1`.
- Full `ctest` matrix (notably Windows/MSVC and opt-in toolchains) left to
CI: this environment has no CMake project build deps and GCC 11 lacks
`<format>`/chrono-tz.
### Are there any user-facing changes?
New opt-in build options only; default builds behave exactly as before.
* GitHub Issue: #51267
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