diff --git a/src/indexer.cc b/src/indexer.cc
index 2a0ce922..281d1d09 100644
--- a/src/indexer.cc
+++ b/src/indexer.cc
@@ -1098,7 +1098,7 @@ public:
                           StringRef included, bool isAngled,
                           CharSourceRange filenameRange,
 #if LLVM_VERSION_MAJOR >= 16 // llvmorg-16-init-14884-g8f0df9f3bbc6
-                          std::optional<FileEntryRef> fileRef,
+                          llvm::Optional<FileEntryRef> fileRef,
 #elif LLVM_VERSION_MAJOR >= 15 // llvmorg-15-init-7692-gd79ad2f1dbc2
                           llvm::Optional<FileEntryRef> fileRef,
 #else
diff --git a/src/project.cc b/src/project.cc
index b1b7f557..f7003ce1 100644
--- a/src/project.cc
+++ b/src/project.cc
@@ -392,7 +392,7 @@ void Project::loadDirectory(const std::string &root, Project::Folder &folder) {
       fclose(fout);
     }
 #if LLVM_VERSION_MAJOR >= 16 // llvmorg-16-init-12589-ge748db0f7f09
-    std::array<std::optional<StringRef>, 3>
+    std::array<Optional<StringRef>, 3>
 #else
     std::array<Optional<StringRef>, 3>
 #endif
diff --git a/src/sema_manager.cc b/src/sema_manager.cc
index 5bcd9ef4..396e004a 100644
--- a/src/sema_manager.cc
+++ b/src/sema_manager.cc
@@ -180,7 +180,7 @@ public:
                           StringRef fileName, bool isAngled,
                           CharSourceRange filenameRange,
 #if LLVM_VERSION_MAJOR >= 16 // llvmorg-16-init-14884-g8f0df9f3bbc6
-                          std::optional<FileEntryRef> fileRef,
+                          llvm::Optional<FileEntryRef> fileRef,
 #elif LLVM_VERSION_MAJOR >= 15 // llvmorg-15-init-7692-gd79ad2f1dbc2
                           llvm::Optional<FileEntryRef> fileRef,
 #else