diff --git a/clang/include/clang/CrossTU/CrossTranslationUnit.h b/clang/include/clang/CrossTU/CrossTranslationUnit.h
--- a/clang/include/clang/CrossTU/CrossTranslationUnit.h
+++ b/clang/include/clang/CrossTU/CrossTranslationUnit.h
@@ -164,6 +164,7 @@
 
 private:
   bool checkThresholdReached() const;
+  llvm::Error lazyInitCTUIndex(StringRef CrossTUDir, StringRef IndexName);
   void lazyInitImporterSharedSt(TranslationUnitDecl *ToTU);
   ASTImporter &getOrCreateASTImporter(ASTContext &From);
   template <typename T>
diff --git a/clang/lib/CrossTU/CrossTranslationUnit.cpp b/clang/lib/CrossTU/CrossTranslationUnit.cpp
--- a/clang/lib/CrossTU/CrossTranslationUnit.cpp
+++ b/clang/lib/CrossTU/CrossTranslationUnit.cpp
@@ -348,6 +348,29 @@
   return false;
 }
 
+llvm::Error CrossTranslationUnitContext::lazyInitCTUIndex(StringRef CrossTUDir,
+                                                          StringRef IndexName) {
+  // Dont initialize if the map is filled.
+  if (!NameFileMap.empty())
+    return llvm::Error::success();
+
+  // Get the absolute path to the index file.
+  SmallString<256> IndexFile = CrossTUDir;
+  if (llvm::sys::path::is_absolute(IndexName))
+    IndexFile = IndexName;
+  else
+    llvm::sys::path::append(IndexFile, IndexName);
+
+  if (auto IndexMapping = parseCrossTUIndex(IndexFile, CrossTUDir)) {
+    // Initialize member map.
+    NameFileMap = *IndexMapping;
+    return llvm::Error::success();
+  } else {
+    // Error while parsing CrossTU index file.
+    return IndexMapping.takeError();
+  };
+}
+
 llvm::Expected<ASTUnit *> CrossTranslationUnitContext::loadExternalAST(
     StringRef LookupName, StringRef CrossTUDir, StringRef IndexName,
     bool DisplayCTUProgress) {
@@ -364,19 +387,9 @@
   ASTUnit *Unit = nullptr;
   auto NameUnitCacheEntry = NameASTUnitMap.find(LookupName);
   if (NameUnitCacheEntry == NameASTUnitMap.end()) {
-    if (NameFileMap.empty()) {
-      SmallString<256> IndexFile = CrossTUDir;
-      if (llvm::sys::path::is_absolute(IndexName))
-        IndexFile = IndexName;
-      else
-        llvm::sys::path::append(IndexFile, IndexName);
-      llvm::Expected<llvm::StringMap<std::string>> IndexOrErr =
-          parseCrossTUIndex(IndexFile, CrossTUDir);
-      if (IndexOrErr)
-        NameFileMap = *IndexOrErr;
-      else
-        return IndexOrErr.takeError();
-    }
+    // Lazily initialize the mapping from function names to AST files.
+    if (llvm::Error InitFailed = lazyInitCTUIndex(CrossTUDir, IndexName))
+      return std::move(InitFailed);
 
     auto It = NameFileMap.find(LookupName);
     if (It == NameFileMap.end()) {