Index: clangd/GlobalCompilationDatabase.h
===================================================================
--- clangd/GlobalCompilationDatabase.h
+++ clangd/GlobalCompilationDatabase.h
@@ -11,6 +11,8 @@
 
 #include "Function.h"
 #include "Path.h"
+#include "clang/Tooling/ArgumentsAdjusters.h"
+#include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringMap.h"
 #include <memory>
@@ -110,6 +112,9 @@
   setCompileCommand(PathRef File,
                     llvm::Optional<tooling::CompileCommand> CompilationCommand);
 
+  /// Adjusts given compile command for clangd.
+  tooling::CompileCommand adjustArguments(tooling::CompileCommand Cmd) const;
+
 private:
   mutable std::mutex Mutex;
   llvm::StringMap<tooling::CompileCommand> Commands; /* GUARDED_BY(Mut) */
Index: clangd/GlobalCompilationDatabase.cpp
===================================================================
--- clangd/GlobalCompilationDatabase.cpp
+++ clangd/GlobalCompilationDatabase.cpp
@@ -19,25 +19,6 @@
 namespace clangd {
 namespace {
 
-void adjustArguments(tooling::CompileCommand &Cmd,
-                     llvm::StringRef ResourceDir) {
-  // Clangd does not generate dependency file.
-  Cmd.CommandLine = tooling::getClangStripDependencyFileAdjuster()(
-      Cmd.CommandLine, Cmd.Filename);
-  // Strip plugin related command line arguments. Clangd does
-  // not support plugins currently. Therefore it breaks if
-  // compiler tries to load plugins.
-  Cmd.CommandLine =
-      tooling::getStripPluginsAdjuster()(Cmd.CommandLine, Cmd.Filename);
-
-  Cmd.CommandLine =
-      tooling::getClangSyntaxOnlyAdjuster()(Cmd.CommandLine, Cmd.Filename);
-  // Inject the resource dir.
-  // FIXME: Don't overwrite it if it's already there.
-  if (!ResourceDir.empty())
-    Cmd.CommandLine.push_back(("-resource-dir=" + ResourceDir).str());
-}
-
 std::string getStandardResourceDir() {
   static int Dummy; // Just an address in this process.
   return CompilerInvocation::GetResourcesPath("clangd", (void *)&Dummy);
@@ -162,8 +143,7 @@
     Cmd = Base->getCompileCommand(File, Project);
   if (!Cmd)
     return llvm::None;
-  adjustArguments(*Cmd, ResourceDir);
-  return Cmd;
+  return adjustArguments(*Cmd);
 }
 
 tooling::CompileCommand OverlayCDB::getFallbackCommand(PathRef File) const {
@@ -187,5 +167,25 @@
   OnCommandChanged.broadcast({File});
 }
 
+tooling::CompileCommand
+OverlayCDB::adjustArguments(tooling::CompileCommand Cmd) const {
+  // Clangd does not generate dependency file.
+  Cmd.CommandLine = tooling::getClangStripDependencyFileAdjuster()(
+      Cmd.CommandLine, Cmd.Filename);
+  // Strip plugin related command line arguments. Clangd does
+  // not support plugins currently. Therefore it breaks if
+  // compiler tries to load plugins.
+  Cmd.CommandLine =
+      tooling::getStripPluginsAdjuster()(Cmd.CommandLine, Cmd.Filename);
+
+  Cmd.CommandLine =
+      tooling::getClangSyntaxOnlyAdjuster()(Cmd.CommandLine, Cmd.Filename);
+  // Inject the resource dir.
+  // FIXME: Don't overwrite it if it's already there.
+  if (!ResourceDir.empty())
+    Cmd.CommandLine.push_back("-resource-dir=" + ResourceDir);
+  return std::move(Cmd);
+}
+
 } // namespace clangd
 } // namespace clang
Index: unittests/clangd/GlobalCompilationDatabaseTests.cpp
===================================================================
--- unittests/clangd/GlobalCompilationDatabaseTests.cpp
+++ unittests/clangd/GlobalCompilationDatabaseTests.cpp
@@ -23,8 +23,8 @@
   DirectoryBasedGlobalCompilationDatabase DB(None);
   auto Cmd = DB.getFallbackCommand(testPath("foo/bar.cc"));
   EXPECT_EQ(Cmd.Directory, testPath("foo"));
-  EXPECT_THAT(Cmd.CommandLine, ElementsAre(
-    EndsWith("clang"), testPath("foo/bar.cc")));
+  EXPECT_THAT(Cmd.CommandLine,
+              ElementsAre(EndsWith("clang"), testPath("foo/bar.cc")));
   EXPECT_EQ(Cmd.Output, "");
 
   // .h files have unknown language, so they are parsed liberally as obj-c++.
@@ -66,15 +66,17 @@
 TEST_F(OverlayCDBTest, GetCompileCommand) {
   OverlayCDB CDB(Base.get(), {}, std::string(""));
   EXPECT_EQ(CDB.getCompileCommand(testPath("foo.cc")),
-            Base->getCompileCommand(testPath("foo.cc")));
+            CDB.adjustArguments(*Base->getCompileCommand(testPath("foo.cc"))));
   EXPECT_EQ(CDB.getCompileCommand(testPath("missing.cc")), llvm::None);
 
   auto Override = cmd(testPath("foo.cc"), "-DA=3");
   CDB.setCompileCommand(testPath("foo.cc"), Override);
-  EXPECT_EQ(CDB.getCompileCommand(testPath("foo.cc")), Override);
+  EXPECT_EQ(CDB.getCompileCommand(testPath("foo.cc")),
+            CDB.adjustArguments(Override));
   EXPECT_EQ(CDB.getCompileCommand(testPath("missing.cc")), llvm::None);
   CDB.setCompileCommand(testPath("missing.cc"), Override);
-  EXPECT_EQ(CDB.getCompileCommand(testPath("missing.cc")), Override);
+  EXPECT_EQ(CDB.getCompileCommand(testPath("missing.cc")),
+            CDB.adjustArguments(Override));
 }
 
 TEST_F(OverlayCDBTest, GetFallbackCommand) {
@@ -88,6 +90,7 @@
   EXPECT_EQ(CDB.getCompileCommand(testPath("bar.cc")), None);
   auto Override = cmd(testPath("bar.cc"), "-DA=5");
   CDB.setCompileCommand(testPath("bar.cc"), Override);
+  Override = CDB.adjustArguments(Override);
   EXPECT_EQ(CDB.getCompileCommand(testPath("bar.cc")), Override);
 
   EXPECT_THAT(CDB.getFallbackCommand(testPath("foo.cc")).CommandLine,