SetOSX_SDK.cmake overrides requested `CMAKE_OSX_SYSROOT`

[cho-m]

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Description

Seen while building 6.40.00 in Homebrew

• root 6.40.00 Homebrew/homebrew-core#283822

At Homebrew, we pass in a specific -DCMAKE_OSX_SYSROOT=... to select the correct SDK we need to build for our distribution. This avoiding using incorrect CLT SDKs and/or Xcode SDKs that are installed on runner and aligns with all other packages we ship.

It looks like the new SetOSX_SDK.cmake does not respect this, even though comment claims it does:

root/cmake/modules/SetOSX_SDK.cmake

Lines 7 to 10 in 9d249c3

	# To choose an SDK, one has the following options:
	# - Let xcrun choose the latest installed SDK: "cmake ..."
	# - Configure an SDK using "SDKROOT=<path> cmake ..."
	# - Set a cache variable: "cmake -DCMAKE_OSX_SYSROOT=<path> ..."

Main issue is the usage of ${...} to check if defined rather than the variable itself. Pretty much an if(TRUE) right now:

root/cmake/modules/SetOSX_SDK.cmake

Line 19 in 9d249c3

if(NOT DEFINED "${CMAKE_OSX_SYSROOT}")

Reproducer

1. Unpack https://root.cern/download/root_v6.40.00.source.tar.gz
2. cmake -Bbuild -DCMAKE_OSX_SYSROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk (or other installed SDK, ignoring other options as visible without them)
3. See output. In my case, I also have Xcode.app installed. In Homebrew CI, the wrong CLT SDK gets picked which causes build to fail.

   -- Mac OS SDK version: '26.5' /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.5.sdk
   

ROOT version

6.40.00

Installation method

Building from source (for Homebrew distribution)

Operating system

macOS

Additional context

When wrong SDK is picked, the build fails on handling <filesystem> detection - https://github.com/Homebrew/homebrew-core/actions/runs/26187130966/job/77067037892#step:3:155

  CMake Error at cmake/modules/SearchInstalledSoftware.cmake:1775 (message):
    Could not determine how to use C++17 <filesystem>
  Call Stack (most recent call first):
    CMakeLists.txt:134 (include)

[vepadulano]

Dear @cho-m ,

Thanks for the report! I agree the current logic is wrong, it was probably an oversight, apologies. This will probably come up also on the conda-forge build, which will start very soon testing for 6.40, to be confirmed. In any case, I have opened a PR to fix it.

[dpiparo]

Thanks @cho-m . Maybe a trivial question: was this problem also visible with the HEAD version of ROOT in the Homebrew infrastructure?

[cho-m]

The CMAKE_OSX_SYSROOT modification is visible with HEAD:

==> cmake -S . -B builddir -DCLING_CXX_PATH=clang++ -DCMAKE_CXX_STANDARD=17 -DCMAKE_INSTALL_ELISPDIR=/opt/homebrew/Cellar/root/HEAD-b71a7d8/share/emacs/site-lisp/root -DPYTHON_EXECUTABLE=/opt/homebrew/opt/python@3.14/bin/python3.14 -DXROOTD_ROOT_DIR=/opt/homebrew/opt/xrootd -Dbuiltin_cfitsio=OFF -Dbuiltin_clang=ON -Dbuiltin_cling=ON -Dbuiltin_cppzmq=OFF -Dbuiltin_davix=OFF -Dbuiltin_fftw3=OFF -Dbuiltin_freetype=OFF -Dbuiltin_ftgl=OFF -Dbuiltin_gl2ps=OFF -Dbuiltin_glew=OFF -Dbuiltin_gsl=OFF -Dbuiltin_llvm=ON -Dbuiltin_lz4=OFF -Dbuiltin_lzma=OFF -Dbuiltin_nlohmannjson=OFF -Dbuiltin_openssl=OFF -Dbuiltin_openui5=ON -Dbuiltin_pcre=OFF -Dbuiltin_tbb=OFF -Dbuiltin_unuran=OFF -Dbuiltin_vc=OFF -Dbuiltin_vdt=ON -Dbuiltin_veccore=OFF -Dbuiltin_xrootd=OFF -Dbuiltin_xxhash=OFF -Dbuiltin_zeromq=OFF -Dbuiltin_zlib=OFF -Dbuiltin_zstd=OFF -Dcfitsio=ON -Ddavix=ON -Ddev=OFF -Dfail-on-missing=ON -Dfftw3=ON -Dfitsio=ON -Dfortran=ON -Dfreetype=ON -Dgdml=ON -Dgfal=OFF -Dgnuinstall=ON -Dimt=ON -Dmathmore=ON -Docaml=OFF -Doracle=OFF -Dpgsql=OFF -Dpyroot=ON -Dpythia6=OFF -Dpythia8=OFF -Droofit=ON -Dssl=ON -Dtmva=ON -Dvdt=ON -Dxrootd=ON -GNinja -DCMAKE_INSTALL_PREFIX=/opt/homebrew/Cellar/root/HEAD-b71a7d8 -DCMAKE_INSTALL_LIBDIR=lib/root -DCMAKE_BUILD_TYPE=Release -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_PROJECT_TOP_LEVEL_INCLUDES=/opt/homebrew/Library/Homebrew/cmake/trap_fetchcontent_provider.cmake -Wno-dev -DBUILD_TESTING=OFF -DCMAKE_OSX_SYSROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk
-- Mac OS SDK version: '26.5' /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
-- The C compiler identification is AppleClang 21.0.0.21000101

To bypass build failures and distribute release in Homebrew, I did add a workaround to ignore the extra CMake file:

• https://github.com/Homebrew/homebrew-core/blob/main/Formula/r/root.rb#L90-L91

Build failures mainly happen on older macOS where the latest installable CLT SDK is from the next Xcode major version, e.g. macOS 15 is compatible with Xcode 26.3 and latest CLT SDK is 26.3.0.0.1.1771626560