diff --git a/clang/lib/Basic/VirtualFileSystem.cpp b/clang/lib/Basic/VirtualFileSystem.cpp
index dd41ed2..be7a637 100644
--- a/clang/lib/Basic/VirtualFileSystem.cpp
+++ b/clang/lib/Basic/VirtualFileSystem.cpp
@@ -1504,8 +1504,9 @@
 
 public:
   JSONWriter(llvm::raw_ostream &OS) : OS(OS) {}
-  void write(ArrayRef<YAMLVFSEntry> Entries, Optional<bool> IsCaseSensitive,
-             Optional<bool> IsOverlayRelative, StringRef OverlayDir);
+  void write(ArrayRef<YAMLVFSEntry> Entries, Optional<bool> UseExternalNames,
+             Optional<bool> IsCaseSensitive, Optional<bool> IsOverlayRelative,
+             StringRef OverlayDir);
 };
 }
 
@@ -1558,6 +1559,7 @@
 }
 
 void JSONWriter::write(ArrayRef<YAMLVFSEntry> Entries,
+                       Optional<bool> UseExternalNames,
                        Optional<bool> IsCaseSensitive,
                        Optional<bool> IsOverlayRelative,
                        StringRef OverlayDir) {
@@ -1568,6 +1570,9 @@
   if (IsCaseSensitive.hasValue())
     OS << "  'case-sensitive': '"
        << (IsCaseSensitive.getValue() ? "true" : "false") << "',\n";
+  if (UseExternalNames.hasValue())
+    OS << "  'use-external-names': '"
+       << (UseExternalNames.getValue() ? "true" : "false") << "',\n";
   bool UseOverlayRelative = false;
   if (IsOverlayRelative.hasValue()) {
     UseOverlayRelative = IsOverlayRelative.getValue();
@@ -1629,8 +1634,8 @@
     return LHS.VPath < RHS.VPath;
   });
 
-  JSONWriter(OS).write(Mappings, IsCaseSensitive, IsOverlayRelative,
-                       OverlayDir);
+  JSONWriter(OS).write(Mappings, UseExternalNames, IsCaseSensitive,
+                       IsOverlayRelative, OverlayDir);
 }
 
 VFSFromYamlDirIterImpl::VFSFromYamlDirIterImpl(