diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -1031,6 +1031,39 @@
   return LangOpts.CPlusPlus ? Language::CXX : Language::C;
 }
 
+template <typename T>
+void SaveIteratorDebug(T &Container, SmallVectorImpl<void *> &Saver) {
+#if _ITERATOR_DEBUG_LEVEL == 2
+  std::_Container_proxy *C = Container._Myproxy();
+  if (!C)
+    return;
+  std::_Lockit _Lock(_LOCK_DEBUG);
+  for (std::_Iterator_base12 *P = C->_Myfirstiter; P != nullptr;
+       P = P->_Mynextiter) {
+    Saver.push_back(P);
+  }
+#endif
+}
+
+// NOTE: This doesn't try to clear out the nodes that weren't there before,
+// because they might have an address further down the (old) stack, and the OS
+// might have freed those virtual pages already, which would cause a GPF.
+template <typename T>
+void ApplyIteratorDebug(T &Container, const SmallVectorImpl<void *> &Saver) {
+#if _ITERATOR_DEBUG_LEVEL == 2
+  std::_Container_proxy *C = Container._Myproxy();
+  if (!C)
+    return;
+  std::_Lockit _Lock(_LOCK_DEBUG);
+  std::_Iterator_base12 **P = &C->_Myfirstiter;
+  for (auto S : Saver) {
+    *P = (std::_Iterator_base12 *)S;
+    P = &(*P)->_Mynextiter;
+  }
+  *P = nullptr;
+#endif
+}
+
 /// Compile a module file for the given module, using the options
 /// provided by the importing compiler instance. Returns true if the module
 /// was built without errors.
@@ -1139,6 +1172,9 @@
 
   PreBuildStep(Instance);
 
+  SmallVector<void*, 4> Iters;
+  SaveIteratorDebug(Instance.getFrontendOpts().Inputs, Iters);
+
   // Execute the action to actually build the module in-place. Use a separate
   // thread so that we get a stack large enough.
   llvm::CrashRecoveryContext CRC;
@@ -1149,6 +1185,8 @@
       },
       DesiredStackSize);
 
+  ApplyIteratorDebug(Instance.getFrontendOpts().Inputs, Iters);
+
   PostBuildStep(Instance);
 
   ImportingInstance.getDiagnostics().Report(ImportLoc,