[hjmjohnson]

Fix two configure probes that never worked, and remove the probes that C++17 or the minimum compilers already guarantee, from the survey in #7120. Folds in #7124, which touched the same floating-point exception code.

Changes, one commit each

• BUG: itkFloatingPointExceptions_unix_feenableexcept_using_fegetenv.cxx tested ITK_HAS_STRUCT_FENV_T_CONTROL_CW, but the probe defines ITK_HAS_STRUCT_FENV_T_CW. On a C library that names the fenv_t member __cw, the build reached #error "Unknown name for 'fenv_t' struct control member".
• COMP: CMake/ITKSetStandardCompilerFlags.cmake passed /Od or -O0 to check_cxx_source_compiles as the program text, so it always failed, and nothing read CXX_HAS_DISABLE_OPTIMIZATION_FLAG. ITK_CXX_DISABLE_OPTIMIZATION_FLAG is unchanged.
• STYLE: Remove the fenv.h, fegetenv, and fesetenv probes; all three are standard C++11. The fegetenv-based code, reached only on PowerPC and x86 without feenableexcept, is now used there unconditionally. The public ITK_HAVE_FENV_H, always defined and read by nothing in ITK, is removed.
• COMP: ITK_COMPILED_CXX_STANDARD_VERSION is set to 201703L. The probe it replaces found 201703L everywhere except MSVC, where __cplusplus stays 199711L, so the older-standard warning could never fire there. The warning now reads _MSVC_LANG and uses #pragma message on MSVC.
• COMP: CMake and supported_compilers.md require AppleClang 12.0.0 (Xcode 12.4), matching the guard in itkMacro.h.
• DOC: Migration guide note for ITK_HAVE_FENV_H.

Behavior changes to review

• A project compiled with MSVC at a standard older than C++17 now gets a warning; before, it got nothing.
• AppleClang 10 and 11 now fail at configure time. They already failed to compile ITK.
• ITK_HAVE_FENV_H is no longer defined. No use was found in 309 downstream repositories.

Verification

• Linux x86_64 (glibc, GCC), full build with tests: builds with no warnings; the probe takes ITK_HAS_STRUCT_FENV_T_CONTROL_WORD; 742/742 ITKCommon tests pass, including the floating-point exception, numeric-traits, and math tests.
• macOS arm64 (earlier, per commit group): ITKCommon and its test drivers build; 91/91 floating-point, numeric-traits, and math tests pass; an x86_64 macOS target compiles the fegetenv path.
• pre-commit run --all-files passes.
• Not run: MSVC, or a C library with a __cw member (glibc uses __control_word, macOS __control).

AI assistance

Claude Code (Opus 5.5) surveyed the probes, made the changes, searched the downstream build forest, and ran the local checks. The author reviewed and is responsible for the change.

🤖 Generated with Claude Code

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable defects were found.

Summary

Fixes the floating-point control-word check and removes obsolete configure probes.

• Unix floating-point exceptions use the matching fenv_t member probe.
• MSVC projects below C++17 now get an ITK compatibility warning.
• ITK requires AppleClang 12.0.0 or newer to configure.
• ITK no longer publishes the ITK_HAVE_FENV_H macro.
• CMake no longer runs an unused optimization-flag check.

Review used source inspection; no builds or tests were run.

_{Reviews (1) · Last reviewed commit: "DOC: Note the removal of ITK_HAVE_FENV_H..." · Reviewed by Greptile}

[dzenanz]

Mostly looks good.