[cling] Unload all redecls of ClassTemplateDecl

interpreter/cling/lib/Interpreter/DeclUnloader.cpp

@@ -1042,7 +1042,15 @@ namespace cling {
       Successful &=
           VisitClassTemplateSpecializationDecl(*I, /*RemoveSpec=*/false);
 
-    Successful &= VisitRedeclarableTemplateDecl(CTD);
+    // Visit all redeclarations of this template to ensure the full
+    // redecl chain is unloaded properly.
+    // Not doing this will lead to a corrupted decl chain and can cause
+    // assertions later down the line.
+    // We also need to make a copy of `CTD->redecls()` to not mess up the chain
+    llvm::SmallVector<RedeclarableTemplateDecl*> redecls(CTD->redecls());
+    for (auto* Redecl : redecls)
+      Successful &= VisitRedeclarableTemplateDecl(Redecl);
+
     Successful &= Visit(CTD->getTemplatedDecl());
     return Successful;
   }

roottest/root/meta/redeclUnload/CMakeLists.txt

@@ -0,0 +1,12 @@
+include_directories(${CMAKE_CURRENT_SOURCE_DIR}/inc)
+
+ROOTTEST_GENERATE_REFLEX_DICTIONARY(TemplateRedecl
+                                    inc/RedeclTrigger.h
+                                    SELECTION inc/redecl_selection.xml
+                                    FIXTURES_SETUP root-meta-redeclUnload-TemplateRedecl-fixture)
+
+ROOTTEST_ADD_TEST(templateRedeclUnload
+                  MACRO testTemplateRedeclUnload.C
+                  ROOTEXE_OPTS -e "gInterpreter->AddIncludePath(\"-I${CMAKE_CURRENT_SOURCE_DIR}\")"
+                  OUTREF TemplateRedeclUnload.ref
+                  FIXTURES_REQUIRED root-meta-redeclUnload-TemplateRedecl-fixture)

roottest/root/meta/redeclUnload/TemplateRedeclUnload.ref

@@ -0,0 +1 @@
+

roottest/root/meta/redeclUnload/inc/RedeclTrigger.h

@@ -0,0 +1,8 @@
+// Two wrappers are needed to reproduce the assertion with broken redecl chain.
+template <typename T>
+class Wrapper1 {};
+template <typename T>
+class Wrapper2 {};
+
+class RandomClass {};
+Wrapper1<Wrapper2<RandomClass>> var;

roottest/root/meta/redeclUnload/inc/redecl_selection.xml

@@ -0,0 +1,4 @@
+<lcgdict>
+   <class name="RandomClass" splitLevel="0"/>
+   <class name="Wrapper1<Wrapper2<RandomClass>>" splitLevel="0"/>
+</lcgdict>

roottest/root/meta/redeclUnload/testTemplateRedeclUnload.C

@@ -0,0 +1,12 @@
+void testTemplateRedeclUnload()
+{
+   gROOT->ProcessLine("gSystem->Load(\"libTemplateRedecl_dictrflx\");");
+   gROOT->ProcessLine("gInterpreter->AutoParse(\"RandomClass\");");
+
+   // Undo the last 2 commands, which unloads the templates.
+   gROOT->ProcessLine(".undo 2");
+
+   // Before the fix: cling crashes here with:
+   //   "Passed first decl twice, invalid redecl chain!"
+   gROOT->ProcessLine("RandomClass obj;");
+}