[TClingUtils] Fix normalization of variadic template classes

template_arguments() has the types, but getTemplateArgs() has the
pack argument, so their counts may be different. Add test for failing
CMS build with LLVM 20.

Co-authored-by: Devajith Valaparambil Sreeramaswamy <devajith.valaparambil.sreeramaswamy@cern.ch>

Author: hahnjo

core/clingutils/src/TClingUtils.cxx

@@ -3105,7 +3105,12 @@ clang::QualType ROOT::TMetaUtils::AddDefaultParameters(clang::QualType instanceT
       llvm::SmallVector<clang::TemplateArgument, 4> canonArgs;
       llvm::ArrayRef<clang::TemplateArgument> template_arguments = TST->template_arguments();
       unsigned int Idecl = 0, Edecl = TSTdecl->getTemplateArgs().size();
-      unsigned int maxAddArg = TSTdecl->getTemplateArgs().size() - dropDefault;
+      // If we have more arguments than the TSTdecl, it is a variadic template
+      // and we want all template arguments.
+      if (template_arguments.size() > Edecl) {
+         Edecl = template_arguments.size();
+      }
+      unsigned int maxAddArg = Edecl - dropDefault;
       for (const clang::TemplateArgument *I = template_arguments.begin(), *E = template_arguments.end(); Idecl != Edecl;
            I != E ? ++I : nullptr, ++Idecl, ++Param) {
 

roottest/root/meta/rootcling/CMakeLists.txt

@@ -48,3 +48,7 @@ ROOTTEST_ADD_TEST(streamerInfoStdFunction
 ROOTTEST_ADD_TEST(selectTemplateInvalidArg
                   COMMAND ${ROOT_rootcling_CMD} -f selectTemplateInvalidArg.Dict.cc ${CMAKE_CURRENT_SOURCE_DIR}/selectTemplateInvalidArg.h ${CMAKE_CURRENT_SOURCE_DIR}/selectTemplateInvalidArg.LinkDef.h
                   ERRREF selectTemplateInvalidArg.ref)
+
+# Test reproducing a failure seen in CMS builds with LLVM 20
+# ROOT get confused normalizing variadic template arguments
+ROOTTEST_GENERATE_DICTIONARY(VariadicTemplateConflict variadicTemplate.h LINKDEF variadicTemplate.xml)

roottest/root/meta/rootcling/variadicTemplate.h

@@ -0,0 +1,14 @@
+// The problem appears when generating dictionaries for variadic templates with 2 vs. 3 arguments
+// Mostly copied and adapted from CMSSW
+template <int A = 64, bool B = false>
+struct TestSoALayout {};
+
+template <typename T0, typename... Args>
+struct PortableHostMultiCollection {};
+
+using TestSoA = TestSoALayout<>;
+
+namespace portabletest {
+using TestHostMultiCollection2 = PortableHostMultiCollection<TestSoA, TestSoA>;
+using TestHostMultiCollection3 = PortableHostMultiCollection<TestSoA, TestSoA, TestSoA>;
+} // namespace portabletest

roottest/root/meta/rootcling/variadicTemplate.xml

@@ -0,0 +1,4 @@
+<lcgdict>
+  <class name="portabletest::TestHostMultiCollection2"/>
+  <class name="portabletest::TestHostMultiCollection3"/>
+</lcgdict>