diff --git a/lib/Interpreter/DefinitionShadower.cpp b/lib/Interpreter/DefinitionShadower.cpp
index 04b9bb05..31331bcb 100644
--- a/lib/Interpreter/DefinitionShadower.cpp
+++ b/lib/Interpreter/DefinitionShadower.cpp
@@ -62,7 +62,11 @@ namespace cling {
 
   DefinitionShadower::DefinitionShadower(Sema& S, Interpreter& I)
           : ASTTransformer(&S), m_Context(S.getASTContext()), m_Interp(I),
-            m_TU(S.getASTContext().getTranslationUnitDecl())
+            m_TU(S.getASTContext().getTranslationUnitDecl()),
+            m_ShadowsDeclInStdDiagID(S.getDiagnostics().getCustomDiagID(
+                DiagnosticsEngine::Warning,
+                "'%0' shadows a declaration with the same name in the 'std' "
+                "namespace; consider using '::%0' to reference this declaration"))
   {}
 
   bool DefinitionShadower::isClingShadowNamespace(const DeclContext *DC) {
@@ -94,9 +98,12 @@ namespace cling {
   }
 
   void DefinitionShadower::invalidatePreviousDefinitions(NamedDecl *D) const {
+    // NotForRedeclaration: lookup anything visible; follows using directives
     LookupResult Previous(*m_Sema, D->getDeclName(), D->getLocation(),
-                   Sema::LookupOrdinaryName, Sema::ForVisibleRedeclaration);
+                   Sema::LookupOrdinaryName, Sema::NotForRedeclaration);
+    Previous.suppressDiagnostics();
     m_Sema->LookupQualifiedName(Previous, m_TU);
+    bool shadowsDeclInStd{};
 
     for (auto Prev : Previous) {
       if (Prev == D)
@@ -116,6 +123,7 @@ namespace cling {
                                 /*IsForUsingDecl=*/false))
         continue;
 
+      shadowsDeclInStd |= Prev->isInStdNamespace();
       hideDecl(Prev);
 
       // For unscoped enumerations, also invalidate all enumerators.
@@ -132,6 +140,18 @@ namespace cling {
     // : reference to 'xxx' is ambiguous" in `class C {}; class C; C foo;`.
     if (!Previous.empty() && !isDefinition(D))
       D->setInvalidDecl();
+
+    // Diagnose shadowing of decls in the `std` namespace (see ROOT-5971).
+    // For unnamed macros, the input is ingested in a single `Interpreter::process()`
+    // call. Do not emit the warning in that case, as all references are local
+    // to the wrapper function and this diagnostic might be misleading.
+    if (shadowsDeclInStd
+        && ((m_Interp.getInputFlags() & (Interpreter::kInputFromFile
+                                         | Interpreter::kIFFLineByLine))
+             != Interpreter::kInputFromFile)) {
+      m_Sema->Diag(D->getBeginLoc(), m_ShadowsDeclInStdDiagID)
+        << D->getQualifiedNameAsString();
+    }
   }
 
   void DefinitionShadower::invalidatePreviousDefinitions(FunctionDecl *D) const {
diff --git a/lib/Interpreter/DefinitionShadower.h b/lib/Interpreter/DefinitionShadower.h
index affffff5..b8b11301 100644
--- a/lib/Interpreter/DefinitionShadower.h
+++ b/lib/Interpreter/DefinitionShadower.h
@@ -53,6 +53,7 @@ namespace cling {
     clang::ASTContext          &m_Context;
     Interpreter                &m_Interp;
     clang::TranslationUnitDecl *m_TU;
+    unsigned                    m_ShadowsDeclInStdDiagID;
 
     unsigned long long m_UniqueNameCounter = 0;