diff --git a/clang-tools-extra/clangd/ClangdUnit.cpp b/clang-tools-extra/clangd/ClangdUnit.cpp
--- a/clang-tools-extra/clangd/ClangdUnit.cpp
+++ b/clang-tools-extra/clangd/ClangdUnit.cpp
@@ -304,6 +304,21 @@
     this->CTContext = CTContext;
   }
 
+  void HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
+                        const clang::Diagnostic &Info) override {
+    DiagnosticsEngine::Level Level = DiagLevel;
+
+    // Check for warning-as-error.
+    if (CTContext && DiagLevel == DiagnosticsEngine::Warning) {
+      std::string CheckName = CTContext->getCheckName(Info.getID());
+      if (!CheckName.empty() && CTContext->treatAsError(CheckName)) {
+        Level = DiagnosticsEngine::Error;
+      }
+    }
+
+    StoreDiags::HandleDiagnostic(Level, Info);
+  }
+
 private:
   tidy::ClangTidyContext *CTContext = nullptr;
 };