BUG: Eigen3 update (64ddc666) breaks downstream FetchContent+find_package use of ITK — eigen_internal removed from main export set

[blowekamp]

Summary

The Eigen3 update in commit 64ddc666 ("Merge branch 'upstream-Eigen3' into eigen-5.0.1-update", May 1, 2026) breaks any downstream project that uses FetchContent with FIND_PACKAGE_ARGS to consume ITK, including SimpleITK.

This was identified via git bisect against the SimpleITK continuous integration dashboard failure reported at:
https://open.cdash.org/builds/11263985/build

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Root Cause

Before commit 64ddc666, eigen_internal was exported as part of ITK's main target export set:

install(TARGETS eigen_internal EXPORT ${ITK3P_INSTALL_EXPORT_NAME})

The bad commit changed this to a separate, standalone export set (ITKInternalEigen3Targets) that is never referenced from ITKConfig.cmake:

set_target_properties(eigen_internal PROPERTIES EXPORT_NAME Eigen)
install(TARGETS eigen_internal EXPORT ITKInternalEigen3Targets)
install(EXPORT ITKInternalEigen3Targets NAMESPACE ITKInternalEigen3:: DESTINATION ${CMAKEPACKAGE_INSTALL_DIR})

Because ITKInternalEigen3Targets.cmake is not loaded by ITKConfig.cmake, find_package(ITK) partially fails: ITKTargets.cmake is loaded (registering all other IMPORTED targets globally), but the configure step then errors because ITKEigen3 lists eigen_internal as a library target that doesn't exist as an imported target.

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Error Symptoms

When find_package(ITK) is used (directly or via FetchContent ... FIND_PACKAGE_ARGS):

1. ITKTargets.cmake loads successfully — all ITK IMPORTED targets (itkdouble-conversion, itksys, etc.) are registered into CMake's global namespace.
2. ITKConfig.cmake then fails because eigen_internal is missing from the loaded targets.
3. If using FetchContent with a FIND_PACKAGE_ARGS fallback (as SimpleITK does in CMake/sitkITKFetchContent.cmake), CMake falls back to add_subdirectory on the ITK source.
4. The source build then tries add_library(itkdouble-conversion ...) — collision with the already-registered IMPORTED target — causing a hard configure error.

The cascade of CMake errors seen in the dashboard:

CMake Error at .../DoubleConversion/CMakeLists.txt:40 (add_library):
  add_library cannot create target "itkdouble-conversion" because an
  imported target with the same name already exists.

CMake Error at .../FetchContent.cmake:2476 (add_subdirectory):
  EXPORT_NAME property can't be set on imported targets ("itkdouble-conversion")

CMake Error at .../ITKModuleMacros.cmake:778 (export):
  export given target "itkdouble-conversion" which is not built by this project.

CMake Error at .../KWSys/CMakeLists.txt:749 (add_library):
  add_library cannot create target "itksys" because an imported
  target with the same name already exists.

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How to Reproduce (with an installed ITK)

Build and install ITK at commit 64ddc666 or later (with the bad Eigen3 update), then configure a project that calls find_package(ITK):

# Build ITK at the bad commit
git clone https://github.com/InsightSoftwareConsortium/ITK.git itk-src
cd itk-src && git checkout 64ddc666
cmake -S . -B ../itk-build -G Ninja \
  -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_INSTALL_PREFIX=../itk-install \
  -DBUILD_TESTING=OFF -DBUILD_EXAMPLES=OFF \
  -DITK_BUILD_DEFAULT_MODULES=ON \
  -DITK_INSTALL_PACKAGE_DIR=lib/cmake/ITK
cmake --build ../itk-build
cmake --install ../itk-build

# Now try to configure SimpleITK against the installed ITK
git clone https://github.com/SimpleITK/SimpleITK.git sitk-src
cmake -S sitk-src -B sitk-build -G Ninja \
  -DITK_DIR=../itk-install/lib/cmake/ITK \
  -DBUILD_TESTING=OFF -DBUILD_EXAMPLES=OFF \
  -DWRAP_DEFAULT=OFF
# ^ This will fail with the target collision errors above

The same error also reproduces on the SimpleITK continuous dashboard: https://open.cdash.org/builds/11263985/build

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Bisect Result

Commit	Result
c79274e4 (2026-03-09, previous known-good ITK tag in SimpleITK)	✅ Good
64ddc6660c (Merge branch 'upstream-Eigen3' into eigen-5.0.1-update, 2026-05-01)	❌ First bad commit

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Suggested Fix

Commit 7aa8ae2bd2 from March 12, 2026 appears to contain the correct fix — it reverts eigen_internal back to the ITK main export set:

# 7aa8ae2: the correct approach
install(TARGETS eigen_internal EXPORT ${ITK3P_INSTALL_EXPORT_NAME})

and removes the separate ITKInternalEigen3Targets install/export machinery. It appears these changes from 7aa8ae2 were not carried forward into the May 1 Eigen3 update (64ddc666), which introduced the regression.

Applying the equivalent of 7aa8ae2 to the current state of Modules/ThirdParty/Eigen3/src/itkeigen/CMakeLists.txt — i.e., putting eigen_internal back into ${ITK3P_INSTALL_EXPORT_NAME} and removing the separate ITKInternalEigen3Targets export — is recommended as the starting point for the fix.

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Environment

• CMake 3.28–3.31 (reproduced on multiple versions per CDash)
• Linux x86_64, GCC
• SimpleITK main using sitkITKFetchContent.cmake with FetchContent FIND_PACKAGE_ARGS

[blowekamp]

@hjmjohnson Can you please look into this issue, and restore ITK working from an installation and check the CMake code changes between the upstream fork and ITK?

[hjmjohnson]

Yes. I'll be looking into it later on Saturday May 9, 2026,