diff --git a/clang-tools-extra/clang-tidy/llvm/HeaderGuardCheck.cpp b/clang-tools-extra/clang-tidy/llvm/HeaderGuardCheck.cpp
--- a/clang-tools-extra/clang-tidy/llvm/HeaderGuardCheck.cpp
+++ b/clang-tools-extra/clang-tidy/llvm/HeaderGuardCheck.cpp
@@ -33,6 +33,13 @@
   if (PosToolsClang != StringRef::npos)
     Guard = Guard.substr(PosToolsClang + std::strlen("tools/"));
 
+  // Unlike LLVM svn, LLVM git monorepo is named llvm-project, so we replace
+  // "/llvm-project/" with the cannonical "/llvm/".
+  const static StringRef LLVMProject = "/llvm-project/";
+  size_t PosLLVMProject = Guard.rfind(LLVMProject);
+  if (PosLLVMProject != StringRef::npos)
+    Guard = Guard.replace(PosLLVMProject, LLVMProject.size(), "/llvm/");
+
   // The remainder is LLVM_FULL_PATH_TO_HEADER_H
   size_t PosLLVM = Guard.rfind("llvm/");
   if (PosLLVM != StringRef::npos)
diff --git a/clang-tools-extra/clangd/FindTarget.h b/clang-tools-extra/clangd/FindTarget.h
--- a/clang-tools-extra/clangd/FindTarget.h
+++ b/clang-tools-extra/clangd/FindTarget.h
@@ -19,6 +19,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_CLANG_TOOLS_EXTRA_UNITTESTS_CLANGD_FINDTARGET_H
+#define LLVM_CLANG_TOOLS_EXTRA_UNITTESTS_CLANGD_FINDTARGET_H
+
 #include "clang/AST/ASTTypeTraits.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/Stmt.h"
@@ -176,3 +179,5 @@
 
 } // namespace clangd
 } // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_UNITTESTS_CLANGD_FINDTARGET_H
diff --git a/clang-tools-extra/clangd/HeaderSourceSwitch.h b/clang-tools-extra/clangd/HeaderSourceSwitch.h
--- a/clang-tools-extra/clangd/HeaderSourceSwitch.h
+++ b/clang-tools-extra/clangd/HeaderSourceSwitch.h
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_CLANG_TOOLS_EXTRA_UNITTESTS_CLANGD_HEADERSOURCESWITCH_H
+#define LLVM_CLANG_TOOLS_EXTRA_UNITTESTS_CLANGD_HEADERSOURCESWITCH_H
+
 #include "ParsedAST.h"
 #include "llvm/ADT/Optional.h"
 
@@ -20,3 +23,5 @@
 
 } // namespace clangd
 } // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_UNITTESTS_CLANGD_HEADERSOURCESWITCH_H
diff --git a/clang-tools-extra/unittests/clang-tidy/LLVMModuleTest.cpp b/clang-tools-extra/unittests/clang-tidy/LLVMModuleTest.cpp
--- a/clang-tools-extra/unittests/clang-tidy/LLVMModuleTest.cpp
+++ b/clang-tools-extra/unittests/clang-tidy/LLVMModuleTest.cpp
@@ -211,6 +211,15 @@
                                          "#endif  /* LLVM_ADT_FOO_H\\ \n"
                                          " FOO */",
                                          "include/llvm/ADT/foo.h", None));
+
+  EXPECT_EQ("#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_FOO_H\n"
+            "#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_FOO_H\n"
+            "\n"
+            "\n"
+            "#endif\n",
+            runHeaderGuardCheck(
+                "", "/llvm-project/clang-tools-extra/clangd/foo.h",
+                StringRef("header is missing header guard")));
 }
 #endif