Index: include/clang/Basic/VirtualFileSystem.h
===================================================================
--- include/clang/Basic/VirtualFileSystem.h
+++ include/clang/Basic/VirtualFileSystem.h
@@ -340,6 +340,7 @@
   Optional<bool> IsCaseSensitive;
   Optional<bool> IsOverlayRelative;
   Optional<bool> UseExternalNames;
+  Optional<bool> IgnoreNonExistentContents;
   std::string OverlayDir;
 
 public:
@@ -351,6 +352,9 @@
   void setUseExternalNames(bool UseExtNames) {
     UseExternalNames = UseExtNames;
   }
+  void setIgnoreNonExistentContents(bool IgnoreContents) {
+    IgnoreNonExistentContents = IgnoreContents;
+  }
   void setOverlayDir(StringRef OverlayDirectory) {
     IsOverlayRelative = true;
     OverlayDir.assign(OverlayDirectory.str());
Index: lib/Basic/VirtualFileSystem.cpp
===================================================================
--- lib/Basic/VirtualFileSystem.cpp
+++ lib/Basic/VirtualFileSystem.cpp
@@ -801,6 +801,7 @@
 ///   'case-sensitive': <boolean, default=true>
 ///   'use-external-names': <boolean, default=true>
 ///   'overlay-relative': <boolean, default=false>
+///   'ignore-non-existent-contents': <boolean, default=true>
 ///
 /// Virtual directories are represented as
 /// \verbatim
@@ -860,6 +861,14 @@
   /// \brief Whether to use to use the value of 'external-contents' for the
   /// names of files.  This global value is overridable on a per-file basis.
   bool UseExternalNames = true;
+
+  /// \brief Whether an invalid path obtained via 'external-contents' should
+  /// cause iteration on the VFS to stop. If 'true', the VFS should ignore
+  /// the entry and continue with the next. Allows YAML files to be shared
+  /// across multiple compiler invocations regardless of prior existent
+  /// paths in 'external-contents'. This global value is overridable on a
+  /// per-file basis.
+  bool IgnoreNonExistentContents = true;
   /// @}
 
   /// Virtual file paths and external files could be canonicalized without "..",
@@ -1301,6 +1310,7 @@
       KeyStatusPair("case-sensitive", false),
       KeyStatusPair("use-external-names", false),
       KeyStatusPair("overlay-relative", false),
+      KeyStatusPair("ignore-non-existent-contents", false),
       KeyStatusPair("roots", true),
     };
 
@@ -1359,6 +1369,9 @@
       } else if (Key == "use-external-names") {
         if (!parseScalarBool(I->getValue(), FS->UseExternalNames))
           return false;
+      } else if (Key == "ignore-non-existent-contents") {
+        if (!parseScalarBool(I->getValue(), FS->IgnoreNonExistentContents))
+          return false;
       } else {
         llvm_unreachable("key missing from Keys");
       }
@@ -1619,7 +1632,7 @@
   JSONWriter(llvm::raw_ostream &OS) : OS(OS) {}
   void write(ArrayRef<YAMLVFSEntry> Entries, Optional<bool> UseExternalNames,
              Optional<bool> IsCaseSensitive, Optional<bool> IsOverlayRelative,
-             StringRef OverlayDir);
+             Optional<bool> IgnoreNonExistentContents, StringRef OverlayDir);
 };
 }
 
@@ -1675,6 +1688,7 @@
                        Optional<bool> UseExternalNames,
                        Optional<bool> IsCaseSensitive,
                        Optional<bool> IsOverlayRelative,
+                       Optional<bool> IgnoreNonExistentContents,
                        StringRef OverlayDir) {
   using namespace llvm::sys;
 
@@ -1692,6 +1706,9 @@
     OS << "  'overlay-relative': '"
        << (UseOverlayRelative ? "true" : "false") << "',\n";
   }
+  if (IgnoreNonExistentContents.hasValue())
+    OS << "  'ignore-non-existent-contents': '"
+       << (IgnoreNonExistentContents.getValue() ? "true" : "false") << "',\n";
   OS << "  'roots': [\n";
 
   if (!Entries.empty()) {
@@ -1748,7 +1765,8 @@
   });
 
   JSONWriter(OS).write(Mappings, UseExternalNames, IsCaseSensitive,
-                       IsOverlayRelative, OverlayDir);
+                       IsOverlayRelative, IgnoreNonExistentContents,
+                       OverlayDir);
 }
 
 VFSFromYamlDirIterImpl::VFSFromYamlDirIterImpl(
Index: test/VFS/Inputs/vfsoverlay2.yaml
===================================================================
--- test/VFS/Inputs/vfsoverlay2.yaml
+++ test/VFS/Inputs/vfsoverlay2.yaml
@@ -1,5 +1,6 @@
 {
   'version': 0,
+  'ignore-non-existent-contents': false,
   'roots': [
     { 'name': 'OUT_DIR', 'type': 'directory',
       'contents': [