Index: include/clang/Frontend/PrecompiledPreamble.h
===================================================================
--- include/clang/Frontend/PrecompiledPreamble.h
+++ include/clang/Frontend/PrecompiledPreamble.h
@@ -289,7 +289,6 @@
   PreambleIsEmpty = 1,
   CouldntCreateTempFile,
   CouldntCreateTargetInfo,
-  CouldntCreateVFSOverlay,
   BeginSourceFileFailed,
   CouldntEmitPCH
 };
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -1307,7 +1307,6 @@
       case BuildPreambleError::CouldntCreateTargetInfo:
       case BuildPreambleError::BeginSourceFileFailed:
       case BuildPreambleError::CouldntEmitPCH:
-      case BuildPreambleError::CouldntCreateVFSOverlay:
         // These erros are more likely to repeat, retry after some period.
         PreambleRebuildCounter = DefaultPreambleRebuildInterval;
         return nullptr;
@@ -1409,8 +1408,6 @@
   ConfigureDiags(Diags, *AST, CaptureDiagnostics);
   IntrusiveRefCntPtr<vfs::FileSystem> VFS =
       createVFSFromCompilerInvocation(*CI, *Diags);
-  if (!VFS)
-    return nullptr;
   AST->Diagnostics = Diags;
   AST->FileSystemOpts = CI->getFileSystemOpts();
   AST->Invocation = std::move(CI);
@@ -1689,14 +1686,14 @@
   // Create the AST unit.
   std::unique_ptr<ASTUnit> AST;
   AST.reset(new ASTUnit(false));
+  AST->NumStoredDiagnosticsFromDriver = StoredDiagnostics.size();
+  AST->StoredDiagnostics.swap(StoredDiagnostics);
   ConfigureDiags(Diags, *AST, CaptureDiagnostics);
   AST->Diagnostics = Diags;
   AST->FileSystemOpts = CI->getFileSystemOpts();
   if (!VFS)
     VFS = vfs::getRealFileSystem();
   VFS = createVFSFromCompilerInvocation(*CI, *Diags, VFS);
-  if (!VFS)
-    return nullptr;
   AST->FileMgr = new FileManager(AST->FileSystemOpts, VFS);
   AST->PCMCache = new MemoryBufferCache;
   AST->OnlyLocalDecls = OnlyLocalDecls;
@@ -1706,8 +1703,6 @@
   AST->IncludeBriefCommentsInCodeCompletion
     = IncludeBriefCommentsInCodeCompletion;
   AST->UserFilesAreVolatile = UserFilesAreVolatile;
-  AST->NumStoredDiagnosticsFromDriver = StoredDiagnostics.size();
-  AST->StoredDiagnostics.swap(StoredDiagnostics);
   AST->Invocation = CI;
   if (ForSerialization)
     AST->WriterData.reset(new ASTWriterData(*AST->PCMCache));
Index: lib/Frontend/CompilerInstance.cpp
===================================================================
--- lib/Frontend/CompilerInstance.cpp
+++ lib/Frontend/CompilerInstance.cpp
@@ -302,11 +302,9 @@
 
 FileManager *CompilerInstance::createFileManager() {
   if (!hasVirtualFileSystem()) {
-    if (IntrusiveRefCntPtr<vfs::FileSystem> VFS =
-            createVFSFromCompilerInvocation(getInvocation(), getDiagnostics()))
-      setVirtualFileSystem(VFS);
-    else
-      return nullptr;
+    IntrusiveRefCntPtr<vfs::FileSystem> VFS =
+        createVFSFromCompilerInvocation(getInvocation(), getDiagnostics());
+    setVirtualFileSystem(VFS);
   }
   FileMgr = new FileManager(getFileSystemOpts(), VirtualFileSystem);
   return FileMgr.get();
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -3072,16 +3072,16 @@
         BaseFS->getBufferForFile(File);
     if (!Buffer) {
       Diags.Report(diag::err_missing_vfs_overlay_file) << File;
-      return IntrusiveRefCntPtr<vfs::FileSystem>();
+      continue;
     }
 
     IntrusiveRefCntPtr<vfs::FileSystem> FS = vfs::getVFSFromYAML(
         std::move(Buffer.get()), /*DiagHandler*/ nullptr, File);
-    if (!FS.get()) {
+    if (FS) {
+      Overlay->pushOverlay(FS);
+    } else {
       Diags.Report(diag::err_invalid_vfs_overlay) << File;
-      return IntrusiveRefCntPtr<vfs::FileSystem>();
     }
-    Overlay->pushOverlay(FS);
   }
   return Overlay;
 }
Index: lib/Frontend/PrecompiledPreamble.cpp
===================================================================
--- lib/Frontend/PrecompiledPreamble.cpp
+++ lib/Frontend/PrecompiledPreamble.cpp
@@ -303,8 +303,6 @@
 
   VFS =
       createVFSFromCompilerInvocation(Clang->getInvocation(), Diagnostics, VFS);
-  if (!VFS)
-    return BuildPreambleError::CouldntCreateVFSOverlay;
 
   // Create a file manager object to provide access to and cache the filesystem.
   Clang->setFileManager(new FileManager(Clang->getFileSystemOpts(), VFS));
@@ -756,8 +754,6 @@
     return "Could not create temporary file for PCH";
   case BuildPreambleError::CouldntCreateTargetInfo:
     return "CreateTargetInfo() return null";
-  case BuildPreambleError::CouldntCreateVFSOverlay:
-    return "Could not create VFS Overlay";
   case BuildPreambleError::BeginSourceFileFailed:
     return "BeginSourceFile() return an error";
   case BuildPreambleError::CouldntEmitPCH:
Index: test/Index/missing_vfs.c
===================================================================
--- /dev/null
+++ test/Index/missing_vfs.c
@@ -0,0 +1,6 @@
+// RUN: c-index-test -test-load-source local %s -ivfsoverlay %t/does-not-exist.yaml &> %t.out
+// RUN: FileCheck -check-prefix=STDERR %s < %t.out
+// STDERR: fatal error: virtual filesystem overlay file '{{.*}}' not found
+// RUN: FileCheck %s < %t.out
+// CHECK: missing_vfs.c:[[@LINE+1]]:6: FunctionDecl=foo:[[@LINE+1]]:6
+void foo(void);
Index: test/VFS/Inputs/MissingVFS/a.h
===================================================================
--- /dev/null
+++ test/VFS/Inputs/MissingVFS/a.h
@@ -0,0 +1 @@
+// void funcA(void);
Index: test/VFS/Inputs/MissingVFS/module.modulemap
===================================================================
--- /dev/null
+++ test/VFS/Inputs/MissingVFS/module.modulemap
@@ -0,0 +1,3 @@
+module A {
+  header "a.h"
+}
Index: test/VFS/Inputs/MissingVFS/vfsoverlay.yaml
===================================================================
--- /dev/null
+++ test/VFS/Inputs/MissingVFS/vfsoverlay.yaml
@@ -0,0 +1,13 @@
+{
+  'version': 0,
+  'ignore-non-existent-contents': false,
+  'roots': [
+    { 'name': 'INPUT_DIR', 'type': 'directory',
+      'contents': [
+        { 'name': 'a.h', 'type': 'file',
+          'external-contents': 'OUT_DIR/a.h'
+        }
+      ]
+    }
+  ]
+}
Index: test/VFS/module_missing_vfs.m
===================================================================
--- /dev/null
+++ test/VFS/module_missing_vfs.m
@@ -0,0 +1,16 @@
+// REQUIRES: shell
+// RUN: rm -rf %t && mkdir -p %t
+// RUN: echo "void funcA(void);" >> %t/a.h
+
+// RUN: not %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/mcp -I %S/Inputs/MissingVFS %s -fsyntax-only -ivfsoverlay %t/vfs.yaml 2>&1 | FileCheck %s -check-prefix=ERROR
+// ERROR: virtual filesystem overlay file '{{.*}}' not found
+// RUN: find %t/mcp -name "A-*.pcm" | count 1
+
+// RUN: sed -e "s:INPUT_DIR:%S/Inputs:g" -e "s:OUT_DIR:%t:g" %S/Inputs/MissingVFS/vfsoverlay.yaml > %t/vfs.yaml
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/mcp -I %S/Inputs/MissingVFS %s -fsyntax-only -ivfsoverlay %t/vfs.yaml
+// RUN: find %t/mcp -name "A-*.pcm" | count 1
+
+@import A;
+void test(void) {
+  funcA();
+}