diff --git a/build/unix/module.modulemap b/build/unix/module.modulemap
index da55cc3b54ed9..81b732e432bdb 100644
--- a/build/unix/module.modulemap
+++ b/build/unix/module.modulemap
@@ -8,6 +8,11 @@ module ROOT_Foundation_C  {
   export *
 }
 
+module ROOT_FwdNamespaces {
+  module "AutoGeneratedForwardDeclaredNamespacesForGMI" { header "AutoGeneratedForwardDeclaredNamespacesForGMI.h" export *}
+  export *
+}
+
 // This module contains header files from module Core which are used as
 // configuration for ROOT. They contain a lot of macro definitions which are
 // supposed to be textually expanded in each TU.
diff --git a/core/metacling/src/TCling.cxx b/core/metacling/src/TCling.cxx
index 5899e789b2021..84c909cc2db9e 100644
--- a/core/metacling/src/TCling.cxx
+++ b/core/metacling/src/TCling.cxx
@@ -121,6 +121,7 @@ clang/LLVM technology.
 #include "llvm/Support/DynamicLibrary.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Program.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Object/ELFObjectFile.h"
 #include "llvm/Object/ObjectFile.h"
@@ -1084,6 +1085,8 @@ static bool HasASTFileOnDisk(clang::Module *M, const clang::Preprocessor &PP, st
 }
 
 static bool HaveFullGlobalModuleIndex = false;
+static const std::string GeneratedFwdNamespaces = (TROOT::GetIncludeDir() + "AutoGeneratedForwardDeclaredNamespacesForGMI.h").Data();
+
 static GlobalModuleIndex *loadGlobalModuleIndex(cling::Interpreter &interp)
 {
    CompilerInstance &CI = *interp.getCI();
@@ -1124,6 +1127,8 @@ static GlobalModuleIndex *loadGlobalModuleIndex(cling::Interpreter &interp)
                TraverseDecl(TU);
             }
             bool VisitNamedDecl(NamedDecl *ND) {
+               if (ND->isInStdNamespace())
+                  return true;
                if (!ND->isFromASTFile())
                   return true;
                if (!ND->getIdentifier())
@@ -1135,17 +1140,22 @@ static GlobalModuleIndex *loadGlobalModuleIndex(cling::Interpreter &interp)
                if (TagDecl *TD = llvm::dyn_cast<TagDecl>(ND)) {
                   if (TD->isCompleteDefinition())
                      Register(TD);
-               } else if (NamespaceDecl *NSD = llvm::dyn_cast<NamespaceDecl>(ND)) {
-                  Register(NSD, /*AddSingleEntry=*/ false);
-               }
-               else if (TypedefNameDecl *TND = dyn_cast<TypedefNameDecl>(ND))
+               } else if (TypedefNameDecl *TND = dyn_cast<TypedefNameDecl>(ND))
                   Register(TND);
+               else if (TypeAliasDecl *TAD = dyn_cast<TypeAliasDecl>(ND))
+                  Register(TAD);
+               else if (isa<FunctionDecl>(ND) && !isa<CXXMethodDecl>(ND))
+                  Register(ND);
                // FIXME: Add the rest...
                return true; // continue decending
             }
+
          private:
             clang::GlobalModuleIndex::UserDefinedInterestingIDs &DefinitionIDs;
-            void Register(const NamedDecl* ND, bool AddSingleEntry = true) {
+            llvm::StringMap<llvm::SmallVector<const Decl *, 8>> Canonicals;
+
+            void Register(const NamedDecl *ND)
+            {
                assert(ND->isFromASTFile());
                // FIXME: All decls should have an owning module once rootcling
                // updates its generated decls from within the LookupHelper & co.
@@ -1163,17 +1173,74 @@ static GlobalModuleIndex *loadGlobalModuleIndex(cling::Interpreter &interp)
                Module *OwningModule = ND->getOwningModule()->getTopLevelModule();
                assert(OwningModule);
                assert(!ND->getName().empty() && "Empty name");
-               if (AddSingleEntry && DefinitionIDs.count(ND->getName()))
-                  return;
+               if (Canonicals.count(ND->getName())) {
+                  if (llvm::is_contained(Canonicals[ND->getName()], ND->getCanonicalDecl()))
+                     return;
+               }
+
                // FIXME: The FileEntry in not stable to serialize.
                // FIXME: We might end up with many times with the same module.
                // FIXME: We might end up two modules containing a definition.
                // FIXME: What do we do if no definition is found.
                DefinitionIDs[ND->getName()].push_back(OwningModule->getASTFile());
+               Canonicals[ND->getName()].push_back(ND->getCanonicalDecl());
             }
          };
          DefinitionFinder defFinder(IDs, CI.getASTContext().getTranslationUnitDecl());
 
+         struct NamespaceDeclarer : public RecursiveASTVisitor<NamespaceDeclarer> {
+            std::string Fwds;
+            std::unordered_set<const NamespaceDecl *> Namespaces;
+
+            NamespaceDeclarer(clang::TranslationUnitDecl *TU) { TraverseDecl(TU); }
+            std::string PrintNamespaceOpen(NamespaceDecl *NSD)
+            {
+               std::string result;
+               if (NSD->isInline())
+                  result += "inline ";
+               result += "namespace " + NSD->getNameAsString() + "{";
+               return result;
+            }
+            bool VisitNamespaceDecl(NamespaceDecl *NSD)
+            {
+               if (NSD->isAnonymousNamespace() || NSD->isInline() || NSD->isInStdNamespace())
+                  return true;
+               if (Namespaces.count(NSD->getCanonicalDecl()))
+                  return true;
+
+               std::vector<NamespaceDecl *> Parents;
+               // NS1::NS2::NS3-> namespace NS1 { namespace NS2 {namespace NS3{}}}
+               for (DeclContext *DC = NSD; DC && !DC->isTranslationUnit(); DC = DC->getParent()) {
+                  if (auto D = dyn_cast<NamespaceDecl>(DC)) {
+                     if (D->isAnonymousNamespace() || D->isInline() || D->isInStdNamespace())
+                        continue;
+                     Namespaces.insert(D);
+                     Parents.push_back(D);
+                  }
+               }
+               std::reverse(Parents.begin(), Parents.end());
+
+               for (auto *D : Parents)
+                  Fwds += PrintNamespaceOpen(D);
+               Fwds += std::string(Parents.size(), '}');
+               Fwds += "\n";
+
+               Namespaces.insert(NSD->getCanonicalDecl());
+               for (Decl *D : NSD->decls()) {
+                  if (!isa<NamespaceDecl>(D))
+                     continue;
+                  TraverseDecl(D);
+               }
+               return true;
+            }
+         };
+
+         if (!llvm::sys::fs::exists(GeneratedFwdNamespaces)) {
+            NamespaceDeclarer NSDeclarer(CI.getASTContext().getTranslationUnitDecl());
+
+            llvm::sys::writeFileWithEncoding(GeneratedFwdNamespaces, NSDeclarer.Fwds);
+         }
+
          llvm::cantFail(GlobalModuleIndex::writeIndex(CI.getFileManager(),
                                                       CI.getPCHContainerReader(),
                                                       ModuleIndexPath,
@@ -1196,6 +1263,8 @@ static void RegisterCxxModules(cling::Interpreter &clingInterp)
    cling::Interpreter::PushTransactionRAII deserRAII(&clingInterp);
 
    // Setup core C++ modules if we have any to setup.
+   if (llvm::sys::fs::exists(GeneratedFwdNamespaces))
+      LoadModule("ROOT_FwdNamespaces", clingInterp);
 
    // Load libc and stl first.
    // Load vcruntime module for windows
@@ -1233,7 +1302,9 @@ static void RegisterCxxModules(cling::Interpreter &clingInterp)
       // These modules should not be preloaded but they fix issues.
       // FIXME: Hist is not a core module but is very entangled to MathCore and
       // causes issues.
-      std::vector<std::string> FIXMEModules = {"Hist"};
+      // ROOTDataFrame is not a core module as well, but without it Clang reports
+      // some ODR related issues.
+      std::vector<std::string> FIXMEModules = {"Hist", "ROOTDataFrame"};
       clang::CompilerInstance &CI = *clingInterp.getCI();
       clang::Preprocessor &PP = CI.getPreprocessor();
       ModuleMap &MMap = PP.getHeaderSearchInfo().getModuleMap();