Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -4252,8 +4252,8 @@
                                 = NotForRedeclaration);
   bool LookupBuiltin(LookupResult &R);
   void LookupNecessaryTypesForBuiltin(Scope *S, unsigned ID);
-  bool LookupName(LookupResult &R, Scope *S,
-                  bool AllowBuiltinCreation = false);
+  bool LookupName(LookupResult &R, Scope *S, bool AllowBuiltinCreation = false,
+                  bool ForceNoCPlusPlus = false);
   bool LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
                            bool InUnqualifiedLookup = false);
   bool LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
Index: clang/lib/Sema/SemaLookup.cpp
===================================================================
--- clang/lib/Sema/SemaLookup.cpp
+++ clang/lib/Sema/SemaLookup.cpp
@@ -1931,13 +1931,14 @@
 /// used to diagnose ambiguities.
 ///
 /// @returns \c true if lookup succeeded and false otherwise.
-bool Sema::LookupName(LookupResult &R, Scope *S, bool AllowBuiltinCreation) {
+bool Sema::LookupName(LookupResult &R, Scope *S, bool AllowBuiltinCreation,
+                      bool ForceNoCPlusPlus) {
   DeclarationName Name = R.getLookupName();
   if (!Name) return false;
 
   LookupNameKind NameKind = R.getLookupKind();
 
-  if (!getLangOpts().CPlusPlus) {
+  if (!getLangOpts().CPlusPlus || ForceNoCPlusPlus) {
     // Unqualified name lookup in C/Objective-C is purely lexical, so
     // search in the declarations attached to the name.
     if (NameKind == Sema::LookupRedeclarationWithLinkage) {
Index: clang/unittests/Sema/CMakeLists.txt
===================================================================
--- clang/unittests/Sema/CMakeLists.txt
+++ clang/unittests/Sema/CMakeLists.txt
@@ -7,6 +7,7 @@
   ExternalSemaSourceTest.cpp
   CodeCompleteTest.cpp
   GslOwnerPointerInference.cpp
+  SemaLookupTest.cpp
   )
 
 clang_target_link_libraries(SemaTests
Index: clang/unittests/Sema/SemaLookupTest.cpp
===================================================================
--- /dev/null
+++ clang/unittests/Sema/SemaLookupTest.cpp
@@ -0,0 +1,60 @@
+#include "clang/AST/DeclarationName.h"
+#include "clang/Frontend/CompilerInstance.h"
+#include "clang/Frontend/FrontendAction.h"
+#include "clang/Parse/ParseAST.h"
+#include "clang/Sema/Lookup.h"
+#include "clang/Sema/Sema.h"
+#include "clang/Tooling/Tooling.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+using namespace clang;
+using namespace clang::tooling;
+
+namespace {
+
+class LookupAction : public ASTFrontendAction {
+  std::unique_ptr<ASTConsumer>
+  CreateASTConsumer(CompilerInstance &CI, StringRef /*Unused*/) override {
+    return std::make_unique<clang::ASTConsumer>();
+  }
+
+  void ExecuteAction() override {
+    CompilerInstance &CI = getCompilerInstance();
+    ASSERT_FALSE(CI.hasSema());
+    CI.createSema(getTranslationUnitKind(), nullptr);
+    ASSERT_TRUE(CI.hasSema());
+    Sema &S = CI.getSema();
+    ParseAST(S);
+
+    ASTContext &Ctx = S.getASTContext();
+    auto Name = &Ctx.Idents.get("Foo");
+    LookupResult R_cpp(S, Name, SourceLocation(), Sema::LookupOrdinaryName);
+    S.LookupName(R_cpp, S.TUScope, /*AllowBuiltinCreation=*/false,
+                 /*ForceNoCPlusPlus=*/false);
+    // By this point, parsing is done and S.TUScope is nullptr
+    // CppLookupName will perform an early return with no results if the Scope
+    // we pass in is nullptr. We expect to find nothing.
+    ASSERT_TRUE(R_cpp.empty());
+
+    // On the other hand, the non-C++ path doesn't care if the Scope passed in
+    // is nullptr. We'll force the non-C++ path with a flag.
+    LookupResult R_nocpp(S, Name, SourceLocation(), Sema::LookupOrdinaryName);
+    S.LookupName(R_nocpp, S.TUScope, /*AllowBuiltinCreation=*/false,
+                 /*ForceNoCPlusPlus=*/true);
+    ASSERT_TRUE(!R_nocpp.empty());
+  }
+};
+
+TEST(SemaLookupTest, ForceNoCPlusPlusPath) {
+  const char *file_contents = R"objcxx(
+@protocol Foo
+@end
+@interface Foo <Foo>
+@end
+  )objcxx";
+  ASSERT_TRUE(runToolOnCodeWithArgs(std::make_unique<LookupAction>(),
+                                    file_contents, {"-x", "objective-c++"},
+                                    "test.mm"));
+}
+} // namespace