2 files changed, 38 insertions, 2 deletions
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 3827891..c5c5bb7 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -87,6 +87,7 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/FileUtilities.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/Timer.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
@@ -524,6 +525,7 @@ class ASTInfoCollector : public ASTReaderListener {
   IntrusiveRefCntPtr<TargetInfo> &Target;
   unsigned &Counter;
   bool InitializedLanguage = false;
+  bool InitializedHeaderSearchPaths = false;
 
 public:
   ASTInfoCollector(Preprocessor &PP, ASTContext *Context,
@@ -550,7 +552,34 @@ public:
   bool ReadHeaderSearchOptions(const HeaderSearchOptions &HSOpts,
                                StringRef SpecificModuleCachePath,
                                bool Complain) override {
+    // Preserve previously set header search paths.
+    llvm::SaveAndRestore X(this->HSOpts.UserEntries);
+    llvm::SaveAndRestore Y(this->HSOpts.SystemHeaderPrefixes);
+    llvm::SaveAndRestore Z(this->HSOpts.VFSOverlayFiles);
+
     this->HSOpts = HSOpts;
+
+    return false;
+  }
+
+  bool ReadHeaderSearchPaths(const HeaderSearchOptions &HSOpts,
+                             bool Complain) override {
+    if (InitializedHeaderSearchPaths)
+      return false;
+
+    this->HSOpts.UserEntries = HSOpts.UserEntries;
+    this->HSOpts.SystemHeaderPrefixes = HSOpts.SystemHeaderPrefixes;
+    this->HSOpts.VFSOverlayFiles = HSOpts.VFSOverlayFiles;
+
+    // Initialize the FileManager. We can't do this in update(), since that
+    // performs the initialization too late (once both target and language
+    // options are read).
+    PP.getFileManager().setVirtualFileSystem(createVFSFromOverlayFiles(
+        HSOpts.VFSOverlayFiles, PP.getDiagnostics(),
+        PP.getFileManager().getVirtualFileSystemPtr()));
+
+    InitializedHeaderSearchPaths = true;
+
     return false;
   }
 
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 9536319..a637b92 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -4737,12 +4737,19 @@ IntrusiveRefCntPtr<llvm::vfs::FileSystem>
 clang::createVFSFromCompilerInvocation(
     const CompilerInvocation &CI, DiagnosticsEngine &Diags,
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS) {
-  if (CI.getHeaderSearchOpts().VFSOverlayFiles.empty())
+  return createVFSFromOverlayFiles(CI.getHeaderSearchOpts().VFSOverlayFiles,
+                                   Diags, std::move(BaseFS));
+}
+
+IntrusiveRefCntPtr<llvm::vfs::FileSystem> clang::createVFSFromOverlayFiles(
+    ArrayRef<std::string> VFSOverlayFiles, DiagnosticsEngine &Diags,
+    IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS) {
+  if (VFSOverlayFiles.empty())
     return BaseFS;
 
   IntrusiveRefCntPtr<llvm::vfs::FileSystem> Result = BaseFS;
   // earlier vfs files are on the bottom
-  for (const auto &File : CI.getHeaderSearchOpts().VFSOverlayFiles) {
+  for (const auto &File : VFSOverlayFiles) {
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Buffer =
         Result->getBufferForFile(File);
     if (!Buffer) {