[testing][temp] Re-enable clad for testing

We also need to update `CLANG_CMAKE_DIR` after the changes in:
llvm/llvm-project@88b77073

We also need to add a patch to clad to get this building.

Author: devajithvs

.github/workflows/root-ci-config/buildconfig/global.txt

@@ -28,7 +28,7 @@ builtin_zstd=OFF
 ccache=ON
 cefweb=OFF
 check_connection=OFF
-clad=OFF
+clad=ON
 clingtest=OFF
 cocoa=OFF
 coverage=OFF

cmake/modules/RootBuildOptions.cmake

@@ -112,7 +112,7 @@ ROOT_BUILD_OPTION(builtin_zstd OFF "Build included libzstd, or use system libzst
 ROOT_BUILD_OPTION(ccache OFF "Enable ccache usage for speeding up builds")
 ROOT_BUILD_OPTION(cefweb OFF "Enable support for CEF (Chromium Embedded Framework) web-based display")
 ROOT_BUILD_OPTION(check_connection ON "Fail the configuration step if there is no internet connection, but it's required for the build")
-ROOT_BUILD_OPTION(clad OFF "Build clad, the cling automatic differentiation plugin (requires network, or existing source directory indicated with -DCLAD_SOURCE_DIR=<clad_src_path>)")
+ROOT_BUILD_OPTION(clad ON "Build clad, the cling automatic differentiation plugin (requires network, or existing source directory indicated with -DCLAD_SOURCE_DIR=<clad_src_path>)")
 ROOT_BUILD_OPTION(cocoa OFF "Use native Cocoa/Quartz graphics backend (MacOS X only)")
 ROOT_BUILD_OPTION(coverage OFF "Enable compile flags for coverage testing")
 ROOT_BUILD_OPTION(cuda OFF "Enable support for CUDA (requires CUDA toolkit >= 7.5)")

interpreter/CMakeLists.txt

@@ -437,7 +437,7 @@ if (builtin_clang)
     ${CMAKE_CURRENT_SOURCE_DIR}/llvm-project/clang/include
     ${CLANG_BINARY_DIR}/include
     CACHE STRING "Clang include directories.")
-  set(CLANG_CMAKE_DIR "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/clang/")
+  set(CLANG_CMAKE_DIR "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/clang/")
 else()
   find_package(Clang REQUIRED CONFIG)
   message(STATUS "Found Clang ${CLANG_PACKAGE_VERSION} in ${CLANG_CMAKE_DIR}")

interpreter/cling/tools/plugins/clad/CMakeLists.txt

@@ -74,17 +74,17 @@ if (DEFINED CLAD_SOURCE_DIR)
   list(APPEND _clad_extra_settings SOURCE_DIR ${CLAD_SOURCE_DIR})
 else()
   list(APPEND _clad_extra_settings GIT_REPOSITORY https://github.com/vgvassilev/clad.git)
-  list(APPEND _clad_extra_settings GIT_TAG v2.3)
+  list(APPEND _clad_extra_settings GIT_TAG 5b0fbe0)
 endif()
 
 ## list(APPEND _clad_patches_list "patch1.patch" "patch2.patch")
-#list(APPEND _clad_patches_list "")
-#set(_clad_patch_command
-#      ${CMAKE_COMMAND} -E copy_directory
-#      ${CMAKE_SOURCE_DIR}/interpreter/cling/tools/plugins/clad/patches <SOURCE_DIR>
-#      && git checkout -q <SOURCE_DIR>
-#      && git apply --ignore-space-change --ignore-whitespace ${_clad_patches_list}
-#      )
+list(APPEND _clad_patches_list "0001-Revert-DSO-self-resolution-for-backend-pass-registra.patch")
+set(_clad_patch_command
+      ${CMAKE_COMMAND} -E copy_directory
+      ${CMAKE_SOURCE_DIR}/interpreter/cling/tools/plugins/clad/patches <SOURCE_DIR>
+      && git checkout -q <SOURCE_DIR>
+      && git apply --ignore-space-change --ignore-whitespace ${_clad_patches_list}
+      )
 
 ExternalProject_Add(
   clad

interpreter/cling/tools/plugins/clad/patches/0001-Revert-DSO-self-resolution-for-backend-pass-registra.patch

@@ -0,0 +1,58 @@
+From ccb2dbe67648d8a24306c82136c5457817fcf562 Mon Sep 17 00:00:00 2001
+From: Devajith Valaparambil Sreeramaswamy
+ <devajith.valaparambil.sreeramaswamy@cern.ch>
+Date: Thu, 25 Jun 2026 09:27:15 +0200
+Subject: [PATCH] Revert DSO self-resolution for backend pass registration
+
+When clad is built via CLAD_BUILD_STATIC_ONLY=ON, no clad.so is generated
+dladdr is resolved to libCling.so and then registered as an LLVM pass plugin.
+
+In LLVM20 this is ignored. In LLVM22 this errors out with:
+
+```
+  error: unable to load plugin 'libCling.so':
+  'Plugin entry point not found in 'libCling.so'. Is this a legacy plugin?'
+```
+
+causing all rootcling dictionary generation steps to fail.
+
+Temporarily revert this.
+---
+ tools/ClangPlugin.cpp | 21 ---------------------
+ 1 file changed, 21 deletions(-)
+
+diff --git a/tools/ClangPlugin.cpp b/tools/ClangPlugin.cpp
+index 52d171fd..6302cafc 100644
+--- a/tools/ClangPlugin.cpp
++++ b/tools/ClangPlugin.cpp
+@@ -139,27 +139,6 @@ void InitTimers();
+       if (WantTiming || getenv("CLAD_ENABLE_TIMING"))
+         InitTimers();
+ 
+-        // Register clad as a backend pass via the path of clad.so itself,
+-        // resolved from any symbol we own. Cleaner than iterating
+-        // CI.getFrontendOpts().Plugins (which depends on how clang was
+-        // invoked) and keeps the lookup inside this DSO.
+-#ifdef _WIN32
+-      HMODULE hm = nullptr;
+-      if (GetModuleHandleExA(GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS |
+-                                 GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT,
+-                             reinterpret_cast<LPCSTR>(&InitTimers), &hm) &&
+-          hm) {
+-        char buf[MAX_PATH];
+-        if (DWORD n = GetModuleFileNameA(hm, buf, MAX_PATH);
+-            n > 0 && n < MAX_PATH)
+-          CGOpts.PassPlugins.emplace_back(buf);
+-      }
+-#else
+-      if (Dl_info info;
+-          dladdr(reinterpret_cast<void*>(&InitTimers), &info) && info.dli_fname)
+-        CGOpts.PassPlugins.emplace_back(info.dli_fname);
+-#endif
+-
+       // Add define for __CLAD__, so that CladFunction::CladFunction()
+       // doesn't throw an error.
+       auto predefines = m_CI.getPreprocessor().getPredefines();
+-- 
+2.43.0
+