macOS Release: 17 unit tests fail; macOS Debug and other platforms unaffected

[barendgehrels]

After a macOS / Xcode update, 17 unit tests fail on the same machine only when built in Release mode.

The exact same source built in Debug mode on the same toolchain passes.

Building against an older Boost (1.89) instead of master produces the same failures, so this is not a regression in Boost master.

Environment

• OS: macOS (post-update; reproduced on Apple Silicon)

Darwin Kernel Version 25.4.0

• Compiler: Apple clang (default Xcode toolchain after update)

Apple clang version 17.0.0 (clang-1700.6.3.2)
Target: arm64-apple-darwin25.4.0

• Build mode: Failures only in Release (-O2); Debug passes the same source
• Boost.Geometry: develop branch
• Other Boost libraries: master and 1.89.0 — both produce identical failures

FMA:
Rebuilt Release with -ffp-contract=off: all 17 failures unchanged. FMA contraction ruled out as cause.

Flags:
No single compiler flag eliminates the failures, and there's no monotonic "worse with more optimization" pattern:

• -O0 is clean
• -O2 -fno-vectorize -fno-slp-vectorize fails for 17 tests
• -O1 fails for 22 tests
• -O2 fails for 17 tests
• -O1 and -O2 fail different subsets, it seems the failure set isn't caused by any single optimization that -O2 adds

Failing tests (for Release):

96% tests passed, 17 tests failed out of 389

The following tests FAILED:
	 12 - boost_geometry_algorithms_densify (Failed)
	 43 - boost_geometry_algorithms_area (Failed)
	 53 - boost_geometry_algorithms_buffer_multi_polygon (Failed)
	 55 - boost_geometry_algorithms_buffer_point_geo (Failed)
	 59 - boost_geometry_algorithms_buffer_multi_linestring_geo (Failed)
	 71 - boost_geometry_algorithms_convex_hull_robust (Failed)
	 77 - boost_geometry_algorithms_closest_points_pl_l (Failed)
	108 - boost_geometry_algorithms_distance_se_geo_ar_ar (Failed)
	111 - boost_geometry_algorithms_distance_se_geo_pl_ar (Failed)
	115 - boost_geometry_algorithms_distance_se_pl_pl (Failed)
	143 - boost_geometry_algorithms_get_distance_measure (Failed)
	172 - boost_geometry_algorithms_set_ops_areal_areal (Failed)
	194 - boost_geometry_algorithms_difference (Failed)
	195 - boost_geometry_algorithms_difference_multi (Failed)
	210 - boost_geometry_algorithms_discrete_hausdorff_distance (Failed)
	255 - boost_geometry_formulas_intersection (Failed)
	298 - boost_geometry_srs_projection_selftest (Failed)