2 files changed, 21 insertions, 29 deletions

diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 0b2995b8eed..7bc3010fcad 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -1078,28 +1078,29 @@ bool ASTUnit::Parse(std::shared_ptr<PCHContainerOperations> PCHContainerOps,
   if (!Invocation)
     return true;
 
+  if (VFS && FileMgr)
+    assert(VFS == FileMgr->getVirtualFileSystem() &&
+           "VFS passed to Parse and VFS in FileMgr are different");
+
   auto CCInvocation = std::make_shared<CompilerInvocation>(*Invocation);
   if (OverrideMainBuffer) {
     assert(Preamble &&
            "No preamble was built, but OverrideMainBuffer is not null");
-    IntrusiveRefCntPtr<llvm::vfs::FileSystem> OldVFS = VFS;
     Preamble->AddImplicitPreamble(*CCInvocation, VFS, OverrideMainBuffer.get());
-    if (OldVFS != VFS && FileMgr) {
-      assert(OldVFS == FileMgr->getVirtualFileSystem() &&
-             "VFS passed to Parse and VFS in FileMgr are different");
-      FileMgr = new FileManager(FileMgr->getFileSystemOpts(), VFS);
-    }
+    // VFS may have changed...
   }
 
   // Create the compiler instance to use for building the AST.
   std::unique_ptr<CompilerInstance> Clang(
       new CompilerInstance(std::move(PCHContainerOps)));
-  if (FileMgr && VFS) {
-    assert(VFS == FileMgr->getVirtualFileSystem() &&
-           "VFS passed to Parse and VFS in FileMgr are different");
-  } else if (VFS) {
-    Clang->setVirtualFileSystem(VFS);
-  }
+
+  // Ensure that Clang has a FileManager with the right VFS, which may have
+  // changed above in AddImplicitPreamble.  If VFS is nullptr, rely on
+  // createFileManager to create one.
+  if (VFS && FileMgr && FileMgr->getVirtualFileSystem() == VFS)
+    Clang->setFileManager(&*FileMgr);
+  else
+    FileMgr = Clang->createFileManager(std::move(VFS));
 
   // Recover resources if we crash before exiting this method.
   llvm::CrashRecoveryContextCleanupRegistrar<CompilerInstance>
@@ -1136,10 +1137,6 @@ bool ASTUnit::Parse(std::shared_ptr<PCHContainerOperations> PCHContainerOps,
   // Configure the various subsystems.
   LangOpts = Clang->getInvocation().LangOpts;
   FileSystemOpts = Clang->getFileSystemOpts();
-  if (!FileMgr) {
-    Clang->createFileManager();
-    FileMgr = &Clang->getFileManager();
-  }
 
   ResetForParse();
 
diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index e7385962994..6c82a6e9532 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -89,10 +89,6 @@ void CompilerInstance::setAuxTarget(TargetInfo *Value) { AuxTarget = Value; }
 
 void CompilerInstance::setFileManager(FileManager *Value) {
   FileMgr = Value;
-  if (Value)
-    VirtualFileSystem = Value->getVirtualFileSystem();
-  else
-    VirtualFileSystem.reset();
 }
 
 void CompilerInstance::setSourceManager(SourceManager *Value) {
@@ -297,13 +293,14 @@ CompilerInstance::createDiagnostics(DiagnosticOptions *Opts,
 
 // File Manager
 
-FileManager *CompilerInstance::createFileManager() {
-  if (!hasVirtualFileSystem()) {
-    IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
-        createVFSFromCompilerInvocation(getInvocation(), getDiagnostics());
-    setVirtualFileSystem(VFS);
-  }
-  FileMgr = new FileManager(getFileSystemOpts(), VirtualFileSystem);
+FileManager *CompilerInstance::createFileManager(
+    IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS) {
+  if (!VFS)
+    VFS = FileMgr ? FileMgr->getVirtualFileSystem()
+                  : createVFSFromCompilerInvocation(getInvocation(),
+                                                    getDiagnostics());
+  assert(VFS && "FileManager has no VFS?");
+  FileMgr = new FileManager(getFileSystemOpts(), std::move(VFS));
   return FileMgr.get();
 }
 
@@ -1101,8 +1098,6 @@ compileModuleImpl(CompilerInstance &ImportingInstance, SourceLocation ImportLoc,
                                    ImportingInstance.getDiagnosticClient()),
                              /*ShouldOwnClient=*/true);
 
-  Instance.setVirtualFileSystem(&ImportingInstance.getVirtualFileSystem());
-
   // Note that this module is part of the module build stack, so that we
   // can detect cycles in the module graph.
   Instance.setFileManager(&ImportingInstance.getFileManager());