[C++] std::bit_width in ALP compression fails to compile with CRAN's macOS 11.3 SDK #53945

Generated based on analysis of failing nightlies runs.

Describe the bug, including details regarding any error messages, version, and platform.

The nightly test-r-macos-as-cran job has failed since October 13, 2026 because Arrow C++ no longer compiles with the MacOSX 11.3 SDK that CRAN uses. The libc++ in that SDK has std::log2p1 but not std::bit_width.

Failing run: https://github.com/ursacomputing/crossbow/actions/runs/38682228949

cpp/src/arrow/util/alp/alp_compression.cc:203:65: error: no member named 'bit_width' in namespace 'std'; did you mean 'internal::UnpackOptions::bit_width'?
cpp/src/arrow/util/alp/alp_compression.cc:448:38: error: no member named 'bit_width' in namespace 'std'; did you mean simply 'bit_width'?

The calls were added in #48345 (GH-48701):

The existing call sites in bit_util.h, chunker_internal.cc and encoder.cc guard this with #if defined(__apple_build_version__) && !defined(__cpp_lib_bitops) and fall back to std::log2p1, e.g.:

Regular macOS C++ CI does not catch this because only the as-cran job uses the 11.3 SDK. Rather than a fourth copy of the guard, a small bit_util helper wrapping it would let all call sites share one definition. Related: #49841.

Component(s)

C++