COMP: Emit GoogleTest export code for install-tree consumers

[hjmjohnson]

Follow-on to #6776, same class of defect: a module whose consumer-side dependency is not declared in what ITK exports. ITKGoogleTest set only ITKGoogleTest_EXPORT_CODE_BUILD, so the module file installed with ITK carried no GTest resolution, and a project built against an installed ITK configured with ITK_USE_SYSTEM_GOOGLETEST=ON failed at CreateGoogleTestDriver with GTest::gtest ... but the target was not found.

It is the only asymmetric module: on main, seventeen modules declare export code and the other sixteen set both _BUILD and _INSTALL.

Red/green, verified at configure time

-DITK_BUILD_DEFAULT_MODULES=ON -DBUILD_TESTING=ON -DITK_USE_SYSTEM_GOOGLETEST=ON, comparing the generated install-tree module file Modules/ThirdParty/GoogleTest/CMakeFiles/ITKGoogleTest.cmake:

	tail of the install-tree ITKGoogleTest.cmake
before	(nothing after the set(ITKGoogleTest_FACTORY_NAMES "") block)
after	set(GTEST_ROOT "") then find_package(GTest REQUIRED)

The build-tree variant already carried the equivalent block and is unchanged. Both variants are written at configure time, so this needs no build or install to check.

Audit of the other targets named in #6776 — no change needed

#6776's description claimed DCMTK::*, GTest::gtest and MPI::MPI_C were latent leaks of the same kind. Each was checked against real build trees; that claim was wrong, and #6776's body has been corrected.

target	finding
DCMTK::* (19)	Already handled. ITKDCMTK sets export code on both the system and FetchContent paths. An external project linking ITK::ITKIODCMTK against a Module_ITKIODCMTK=ON build configures cleanly.
GTest::gtest, GTest::gtest_main	Self-contained on the vendored path — ITK exports them as real imported targets in ITKTargets.cmake. The install-tree gap this PR fixes is on the system-GTest path, where they are absent from the export instead.
VTK::* (10, via ITKVtkGlue)	Already handled, both variants.
MPI::MPI_C, and Threads::Threads from the vendored HDF5	Unreachable. Both sit inside $<$<BOOL:${HDF5_ENABLE_PARALLEL}>:...>-style genexes, and vendored HDF5 refuses to configure with HDF5_ENABLE_PARALLEL=ON ("Parallel and C++ options are mutually exclusive"), which ITK requires. HDF5_ENABLE_THREADS is referenced by the vendored snapshot but never defined.

Unrelated defect found while testing, not fixed here

-DITK_BUILD_DEFAULT_MODULES=OFF -DModule_ITKIONRRD=ON -DBUILD_TESTING=ON fails to configure: ITKGoogleTest is not enabled, but Modules/IO/NRRD/test/CMakeLists.txt still calls CreateGoogleTestDriver, so GTest::gtest is undefined. That is a module-enablement problem rather than an export problem, and can be filed separately.

[greptile-apps]

Greptile Summary

This change adds install-tree GTest package discovery for one system-GTest configuration. When GTEST_ROOT points to a valid GoogleTest source directory, the install export is still omitted, so an installed ITK package cannot provide the GTest:: targets required by downstream projects using CreateGoogleTestDriver. A producer/install/consumer CMake reproduction confirmed the configuration failure and confirmed that emitting the export for this branch resolves it.

Confidence Score: 4/5

Not safe to merge until installed ITK consumers using a system GoogleTest source root can restore the required GTest targets.

The failure was reproduced with an installed producer and an independent CMake consumer. The same consumer configured, built, and ran when the install export was emitted outside the source-root conditional.

Files Needing Attention: Modules/ThirdParty/GoogleTest/CMakeLists.txt

T-Rex Logs

What T-Rex did

• T-Rex reproduced the P1 finding by using a CMake install-tree reproduction harness, coordinated a consumer configure flow, observed an initial failure with a valid GTest source root, and achieved a successful configure and build after relocating the test-only export.
• T-Rex posted a second P1 finding proof and prepared corresponding review context.
• T-Rex performed contract validation around GoogleTest integration, identified the affected CMakeLists.txt lines and the nested conditional behavior, confirmed no repository changes were needed, and noted that CMake 3.31.10 was installed locally to run the test.

_{Ran code and verified through T-Rex}

Comments Outside Diff (1)

1. General comment

   Install-tree GoogleTest export is omitted for a valid system source root

   • Bug
     • An ITK install configured with ITK_USE_SYSTEM_GOOGLETEST=ON and GTEST_ROOT pointing to a source tree containing CMakeLists.txt exports no find_package(GTest) call. A downstream consumer invoking CreateGoogleTestDriver therefore fails to configure because GTest::gtest is absent.
   • Cause
     • ITKGoogleTest_EXPORT_CODE_INSTALL is assigned only inside if(NOT DEFINED GTEST_ROOT OR NOT EXISTS "${GTEST_ROOT}/CMakeLists.txt"); a valid source tree makes that condition false, even though the installed module still needs restored imported GTest targets.
   • Fix
     • Emit the install export code for all ITK_USE_SYSTEM_GOOGLETEST configurations (outside the source-root/no-source conditional), so install-tree consumers call find_package(GTest REQUIRED) before using CreateGoogleTestDriver.

   _{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "COMP: Emit GoogleTest export code for in..." | Re-trigger Greptile}

[greptile-apps]

Install-tree GoogleTest export is omitted for a valid system source root

When ITK_USE_SYSTEM_GOOGLETEST=ON and GTEST_ROOT points to a GoogleTest source directory containing CMakeLists.txt, the condition at line 5 is false and this install export code is never emitted. The installed module then does not restore GTest::gtest and GTest::gtest_main; consequently, downstream projects that use CreateGoogleTestDriver fail to configure because those targets are missing. Emit the install-side find_package(GTest REQUIRED) export for both system-GTest branches.

Artifacts

CMake install-tree reproduction harness source

• The executable shell harness creates a valid fake GTest source root and package, installs a producer, and configures/builds an independent consumer; it is the exact source used for both captured runs.

Consumer configure failure with valid GTest source root

• The repository's exact module file was installed and consumed with `GTEST_ROOT` containing `CMakeLists.txt`; CMake failed because `GTest::gtest` was not found, confirming the broken install export path.

Consumer configure and build success after test-only export relocation

• A temporary test-only variant emits the install export outside the source-root condition; the consumer found GTest, built `consumer_driver`, and exited successfully, demonstrating the repair.

_{Ran code and verified through T-Rex}

[blowekamp]

The initial implementation of the google test third party library was that is should not be installed and only available in the build tree.

[hjmjohnson]

Closing: rejected by the maintainer. Not vetted beyond a configure-time check.

[hjmjohnson]

Understood — GoogleTest is deliberately build-tree-only, so the missing install-tree export is the design, not a gap. Correcting my closing note above: that, not the verification depth, is why this is wrong.