diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index b2b7b7c..b96dc70 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -538,6 +538,11 @@
                                   CodeCompleteConsumer *CompletionConsumer) {
   TheSema.reset(new Sema(getPreprocessor(), getASTContext(), getASTConsumer(),
                          TUKind, CompletionConsumer));
+  // Attach the external sema source if there is any.
+  if (ExternalSemaSrc) {
+    TheSema->addExternalSource(ExternalSemaSrc.get());
+    ExternalSemaSrc->InitializeSema(*TheSema);
+  }
 }
 
 // Output Files
@@ -1820,3 +1825,8 @@
   return false;
 }
 void CompilerInstance::resetAndLeakSema() { BuryPointer(takeSema()); }
+
+void CompilerInstance::setExternalSemaSource(
+    IntrusiveRefCntPtr<ExternalSemaSource> ESS) {
+  ExternalSemaSrc = std::move(ESS);
+}