Index: clangd/ClangdLSPServer.cpp
===================================================================
--- clangd/ClangdLSPServer.cpp
+++ clangd/ClangdLSPServer.cpp
@@ -308,7 +308,7 @@
   if (UseDirBasedCDB)
     BaseCDB = llvm::make_unique<DirectoryBasedGlobalCompilationDatabase>(
         CompileCommandsDir);
-  CDB.emplace(BaseCDB.get());
+  CDB.emplace(BaseCDB.get(), Params.initializationOptions.fallbackFlags);
   Server.emplace(*CDB, FSProvider, static_cast<DiagnosticsConsumer &>(*this),
                  ClangdServerOpts);
   applyConfiguration(Params.initializationOptions.ConfigSettings);
@@ -664,9 +664,10 @@
         tooling::CompileCommand(std::move(Entry.second.workingDirectory), File,
                                 std::move(Entry.second.compilationCommand),
                                 /*Output=*/"");
-    if (Old != New)
+    if (Old != New) {
       CDB->setCompileCommand(File, std::move(New));
-    ShouldReparseOpenFiles = true;
+      ShouldReparseOpenFiles = true;
+    }
   }
   if (ShouldReparseOpenFiles)
     reparseOpenedFiles();
Index: clangd/Protocol.h
===================================================================
--- clangd/Protocol.h
+++ clangd/Protocol.h
@@ -368,6 +368,10 @@
   ConfigurationSettings ConfigSettings;
 
   llvm::Optional<std::string> compilationDatabasePath;
+  // Additional flags to be included in the "fallback command" used when
+  // the compilation database doesn't describe an opened file.
+  // The command used will be approximately `clang $FILE $fallbackFlags`.
+  std::vector<std::string> fallbackFlags;
 };
 bool fromJSON(const llvm::json::Value &, InitializationOptions &);
 
Index: clangd/Protocol.cpp
===================================================================
--- clangd/Protocol.cpp
+++ clangd/Protocol.cpp
@@ -669,6 +669,7 @@
 
   fromJSON(Params, Opts.ConfigSettings);
   O.map("compilationDatabasePath", Opts.compilationDatabasePath);
+  O.map("fallbackFlags", Opts.fallbackFlags);
   return true;
 }