diff --git a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
--- a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
+++ b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
@@ -46,21 +46,25 @@
   DependencyConsumer &C;
 };
 
+using PrebuiltModuleFilesT = decltype(HeaderSearchOptions::PrebuiltModuleFiles);
+
 /// A listener that collects the imported modules and optionally the input
 /// files.
 class PrebuiltModuleListener : public ASTReaderListener {
 public:
-  PrebuiltModuleListener(llvm::StringMap<std::string> &PrebuiltModuleFiles,
-                         llvm::StringSet<> &InputFiles, bool VisitInputFiles)
+  PrebuiltModuleListener(PrebuiltModuleFilesT &PrebuiltModuleFiles,
+                         llvm::StringSet<> &InputFiles, bool VisitInputFiles,
+                         llvm::SmallVector<std::string> &NewModuleFiles)
       : PrebuiltModuleFiles(PrebuiltModuleFiles), InputFiles(InputFiles),
-        VisitInputFiles(VisitInputFiles) {}
+        VisitInputFiles(VisitInputFiles), NewModuleFiles(NewModuleFiles) {}
 
   bool needsImportVisitation() const override { return true; }
   bool needsInputFileVisitation() override { return VisitInputFiles; }
   bool needsSystemInputFileVisitation() override { return VisitInputFiles; }
 
   void visitImport(StringRef ModuleName, StringRef Filename) override {
-    PrebuiltModuleFiles.insert({ModuleName, Filename.str()});
+    if (PrebuiltModuleFiles.insert({ModuleName.str(), Filename.str()}).second)
+      NewModuleFiles.push_back(Filename.str());
   }
 
   bool visitInputFile(StringRef Filename, bool isSystem, bool isOverridden,
@@ -70,13 +74,12 @@
   }
 
 private:
-  llvm::StringMap<std::string> &PrebuiltModuleFiles;
+  PrebuiltModuleFilesT &PrebuiltModuleFiles;
   llvm::StringSet<> &InputFiles;
   bool VisitInputFiles;
+  llvm::SmallVector<std::string> &NewModuleFiles;
 };
 
-using PrebuiltModuleFilesT = decltype(HeaderSearchOptions::PrebuiltModuleFiles);
-
 /// Visit the given prebuilt module and collect all of the modules it
 /// transitively imports and contributing input files.
 static void visitPrebuiltModule(StringRef PrebuiltModuleFilename,
@@ -84,33 +87,17 @@
                                 PrebuiltModuleFilesT &ModuleFiles,
                                 llvm::StringSet<> &InputFiles,
                                 bool VisitInputFiles) {
-  // Maps the names of modules that weren't yet visited to their PCM path.
-  llvm::StringMap<std::string> ModuleFilesWorklist;
-  // Contains PCM paths of all visited modules.
-  llvm::StringSet<> VisitedModuleFiles;
-
-  PrebuiltModuleListener Listener(ModuleFilesWorklist, InputFiles,
-                                  VisitInputFiles);
+  // List of module files to be processed.
+  llvm::SmallVector<std::string> Worklist{PrebuiltModuleFilename.str()};
+  PrebuiltModuleListener Listener(ModuleFiles, InputFiles, VisitInputFiles,
+                                  Worklist);
 
-  auto GatherModuleFileInfo = [&](StringRef ASTFile) {
+  while (!Worklist.empty())
     ASTReader::readASTFileControlBlock(
-        ASTFile, CI.getFileManager(), CI.getPCHContainerReader(),
+        Worklist.pop_back_val(), CI.getFileManager(),
+        CI.getPCHContainerReader(),
         /*FindModuleFileExtensions=*/false, Listener,
         /*ValidateDiagnosticOptions=*/false);
-  };
-
-  GatherModuleFileInfo(PrebuiltModuleFilename);
-  while (!ModuleFilesWorklist.empty()) {
-    auto WorklistItemIt = ModuleFilesWorklist.begin();
-
-    if (!VisitedModuleFiles.contains(WorklistItemIt->getValue())) {
-      VisitedModuleFiles.insert(WorklistItemIt->getValue());
-      GatherModuleFileInfo(WorklistItemIt->getValue());
-      ModuleFiles[WorklistItemIt->getKey().str()] = WorklistItemIt->getValue();
-    }
-
-    ModuleFilesWorklist.erase(WorklistItemIt);
-  }
 }
 
 /// Transform arbitrary file name into an object-like file name.
diff --git a/clang/test/ClangScanDeps/modules-pch-dangling.c b/clang/test/ClangScanDeps/modules-pch-dangling.c
new file mode 100644
--- /dev/null
+++ b/clang/test/ClangScanDeps/modules-pch-dangling.c
@@ -0,0 +1,139 @@
+// Unsupported on AIX because we don't support the requisite "__clangast"
+// section in XCOFF yet.
+// UNSUPPORTED: aix
+
+// This test checks that the dependency scanner can handle larger amount of
+// explicitly built modules retrieved from the PCH.
+// (Previously, there was a bug dangling iterator bug that manifested only with
+// 16 and more retrieved modules.)
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+//--- mod_00.h
+//--- mod_01.h
+//--- mod_02.h
+//--- mod_03.h
+//--- mod_04.h
+//--- mod_05.h
+//--- mod_06.h
+//--- mod_07.h
+//--- mod_08.h
+//--- mod_09.h
+//--- mod_10.h
+//--- mod_11.h
+//--- mod_12.h
+//--- mod_13.h
+//--- mod_14.h
+//--- mod_15.h
+//--- mod_16.h
+//--- mod.h
+#include "mod_00.h"
+#include "mod_01.h"
+#include "mod_02.h"
+#include "mod_03.h"
+#include "mod_04.h"
+#include "mod_05.h"
+#include "mod_06.h"
+#include "mod_07.h"
+#include "mod_08.h"
+#include "mod_09.h"
+#include "mod_10.h"
+#include "mod_11.h"
+#include "mod_12.h"
+#include "mod_13.h"
+#include "mod_14.h"
+#include "mod_15.h"
+#include "mod_16.h"
+//--- module.modulemap
+module mod_00 { header "mod_00.h" }
+module mod_01 { header "mod_01.h" }
+module mod_02 { header "mod_02.h" }
+module mod_03 { header "mod_03.h" }
+module mod_04 { header "mod_04.h" }
+module mod_05 { header "mod_05.h" }
+module mod_06 { header "mod_06.h" }
+module mod_07 { header "mod_07.h" }
+module mod_08 { header "mod_08.h" }
+module mod_09 { header "mod_09.h" }
+module mod_10 { header "mod_10.h" }
+module mod_11 { header "mod_11.h" }
+module mod_12 { header "mod_12.h" }
+module mod_13 { header "mod_13.h" }
+module mod_14 { header "mod_14.h" }
+module mod_15 { header "mod_15.h" }
+module mod_16 { header "mod_16.h" }
+module mod    { header "mod.h"    }
+
+//--- pch.h
+#include "mod.h"
+
+//--- tu.c
+
+//--- cdb_pch.json.template
+[{
+  "file": "DIR/pch.h",
+  "directory": "DIR",
+  "command": "clang -x c-header DIR/pch.h -fmodules -gmodules -fimplicit-module-maps -fmodules-cache-path=DIR/cache -o DIR/pch.h.gch"
+}]
+
+//--- cdb_tu.json.template
+[{
+  "file": "DIR/tu.c",
+  "directory": "DIR",
+  "command": "clang -fsyntax-only DIR/tu.c -fmodules -gmodules -fimplicit-module-maps -fmodules-cache-path=DIR/cache -include DIR/pch.h -o DIR/tu.o"
+}]
+
+// Scan dependencies of the PCH:
+//
+// RUN: sed "s|DIR|%/t|g" %t/cdb_pch.json.template > %t/cdb_pch.json
+// RUN: clang-scan-deps -compilation-database %t/cdb_pch.json -format experimental-full \
+// RUN:   -generate-modules-path-args -module-files-dir %t/build > %t/result_pch.json
+
+// Explicitly build the PCH:
+//
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_00 > %t/mod_00.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_01 > %t/mod_01.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_02 > %t/mod_02.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_03 > %t/mod_03.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_04 > %t/mod_04.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_05 > %t/mod_05.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_06 > %t/mod_06.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_07 > %t/mod_07.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_08 > %t/mod_08.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_09 > %t/mod_09.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_10 > %t/mod_10.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_11 > %t/mod_11.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_12 > %t/mod_12.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_13 > %t/mod_13.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_14 > %t/mod_14.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_15 > %t/mod_15.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod_16 > %t/mod_16.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --module-name=mod    > %t/mod.cc1.rsp
+// RUN: %python %S/../../utils/module-deps-to-rsp.py %t/result_pch.json --tu-index=0 > %t/pch.rsp
+//
+// RUN: %clang @%t/mod_00.cc1.rsp
+// RUN: %clang @%t/mod_01.cc1.rsp
+// RUN: %clang @%t/mod_02.cc1.rsp
+// RUN: %clang @%t/mod_03.cc1.rsp
+// RUN: %clang @%t/mod_04.cc1.rsp
+// RUN: %clang @%t/mod_05.cc1.rsp
+// RUN: %clang @%t/mod_06.cc1.rsp
+// RUN: %clang @%t/mod_07.cc1.rsp
+// RUN: %clang @%t/mod_08.cc1.rsp
+// RUN: %clang @%t/mod_09.cc1.rsp
+// RUN: %clang @%t/mod_10.cc1.rsp
+// RUN: %clang @%t/mod_11.cc1.rsp
+// RUN: %clang @%t/mod_12.cc1.rsp
+// RUN: %clang @%t/mod_13.cc1.rsp
+// RUN: %clang @%t/mod_14.cc1.rsp
+// RUN: %clang @%t/mod_15.cc1.rsp
+// RUN: %clang @%t/mod_16.cc1.rsp
+// RUN: %clang @%t/mod.cc1.rsp
+// RUN: %clang @%t/pch.rsp
+
+// Scan dependencies of the TU, checking it doesn't crash:
+//
+// RUN: sed "s|DIR|%/t|g" %t/cdb_tu.json.template > %t/cdb_tu.json
+// RUN: clang-scan-deps -compilation-database %t/cdb_tu.json -format experimental-full \
+// RUN:   -generate-modules-path-args -module-files-dir %t/build