Index: include/clang/AST/ASTContext.h
===================================================================
--- include/clang/AST/ASTContext.h
+++ include/clang/AST/ASTContext.h
@@ -185,6 +185,10 @@
                                      ASTContext&>
     SubstTemplateTemplateParmPacks;
 
+  /// Generation number for this external AST source. Must be increased
+  /// whenever we might have added new redeclarations for existing decls.
+  uint32_t CurrentGeneration = 0;
+
   /// \brief The set of nested name specifiers.
   ///
   /// This set is managed by the NestedNameSpecifier class.
@@ -607,6 +611,15 @@
 
   DynTypedNodeList getParents(const ast_type_traits::DynTypedNode &Node);
 
+  uint32_t getGeneration() const { return CurrentGeneration; }
+  uint32_t incrementGeneration() {
+    uint32_t OldGeneration = CurrentGeneration;
+    CurrentGeneration++;
+    assert(CurrentGeneration > OldGeneration &&
+           "Overflowed generation counter");
+    return OldGeneration;
+  }
+
   const clang::PrintingPolicy &getPrintingPolicy() const {
     return PrintingPolicy;
   }
@@ -2884,7 +2897,7 @@
   // include ASTContext.h. We explicitly instantiate it for all relevant types
   // in ASTContext.cpp.
   if (auto *Source = Ctx.getExternalSource())
-    return new (Ctx) LazyData(Source, Value);
+    return new (Ctx) LazyData(&Ctx, Source, Value);
   return Value;
 }
 
Index: include/clang/AST/ExternalASTSource.h
===================================================================
--- include/clang/AST/ExternalASTSource.h
+++ include/clang/AST/ExternalASTSource.h
@@ -42,9 +42,6 @@
 /// actual type and declaration nodes, and read parts of declaration
 /// contexts.
 class ExternalASTSource : public RefCountedBase<ExternalASTSource> {
-  /// Generation number for this external AST source. Must be increased
-  /// whenever we might have added new redeclarations for existing decls.
-  uint32_t CurrentGeneration;
 
   /// \brief Whether this AST source also provides information for
   /// semantic analysis.
@@ -53,7 +50,7 @@
   friend class ExternalSemaSource;
 
 public:
-  ExternalASTSource() : CurrentGeneration(0), SemaSource(false) { }
+  ExternalASTSource() : SemaSource(false) {}
 
   virtual ~ExternalASTSource();
 
@@ -74,7 +71,7 @@
   /// \brief Get the current generation of this AST source. This number
   /// is incremented each time the AST source lazily extends an existing
   /// entity.
-  uint32_t getGeneration() const { return CurrentGeneration; }
+  virtual uint32_t getGeneration(const ASTContext &C) const;
 
   /// \brief Resolve a declaration ID into a declaration, potentially
   /// building a new declaration.
@@ -392,8 +389,10 @@
   /// A cache of the value of this pointer, in the most recent generation in
   /// which we queried it.
   struct LazyData {
-    LazyData(ExternalASTSource *Source, T Value)
-        : ExternalSource(Source), LastGeneration(0), LastValue(Value) {}
+    LazyData(const ASTContext *Context, ExternalASTSource *Source, T Value)
+        : Context(Context), ExternalSource(Source), LastGeneration(0),
+          LastValue(Value) {}
+    const ASTContext *Context;
     ExternalASTSource *ExternalSource;
     uint32_t LastGeneration;
     T LastValue;
@@ -438,8 +437,10 @@
   /// Get the value of this pointer, updating its owner if necessary.
   T get(Owner O) {
     if (LazyData *LazyVal = Value.template dyn_cast<LazyData*>()) {
-      if (LazyVal->LastGeneration != LazyVal->ExternalSource->getGeneration()) {
-        LazyVal->LastGeneration = LazyVal->ExternalSource->getGeneration();
+      if (LazyVal->LastGeneration !=
+          LazyVal->ExternalSource->getGeneration(*LazyVal->Context)) {
+        LazyVal->LastGeneration =
+            LazyVal->ExternalSource->getGeneration(*LazyVal->Context);
         (LazyVal->ExternalSource->*Update)(O);
       }
       return LazyVal->LastValue;
Index: include/clang/Serialization/ASTReader.h
===================================================================
--- include/clang/Serialization/ASTReader.h
+++ include/clang/Serialization/ASTReader.h
@@ -1208,6 +1208,12 @@
       : Mod(Mod), ImportedBy(ImportedBy), ImportLoc(ImportLoc) { }
   };
 
+  uint32_t getGenerationOrNull() const {
+    if (ContextObj)
+      return getGeneration(*ContextObj);
+    return 0u;
+  }
+
   ASTReadResult ReadASTCore(StringRef FileName, ModuleKind Type,
                             SourceLocation ImportLoc, ModuleFile *ImportedBy,
                             SmallVectorImpl<ImportedModule> &Loaded,
Index: lib/AST/ExternalASTSource.cpp
===================================================================
--- lib/AST/ExternalASTSource.cpp
+++ lib/AST/ExternalASTSource.cpp
@@ -23,6 +23,10 @@
 
 ExternalASTSource::~ExternalASTSource() { }
 
+uint32_t ExternalASTSource::getGeneration(const ASTContext &C) const {
+  return C.getGeneration();
+}
+
 llvm::Optional<ExternalASTSource::ASTSourceDescriptor>
 ExternalASTSource::getSourceDescriptor(unsigned ID) {
   return None;
@@ -117,19 +121,5 @@
 void ExternalASTSource::getMemoryBufferSizes(MemoryBufferSizes &sizes) const {}
 
 uint32_t ExternalASTSource::incrementGeneration(ASTContext &C) {
-  uint32_t OldGeneration = CurrentGeneration;
-
-  // Make sure the generation of the topmost external source for the context is
-  // incremented. That might not be us.
-  auto *P = C.getExternalSource();
-  if (P && P != this)
-    CurrentGeneration = P->incrementGeneration(C);
-  else {
-    // FIXME: Only bump the generation counter if the current generation number
-    // has been observed?
-    if (!++CurrentGeneration)
-      llvm::report_fatal_error("generation counter overflowed", false);
-  }
-
-  return OldGeneration;
+  return C.incrementGeneration();
 }
Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -1893,7 +1893,7 @@
 
   // Update the generation for this identifier.
   if (getContext().getLangOpts().Modules)
-    IdentifierGeneration[II] = getGeneration();
+    IdentifierGeneration[II] = getGenerationOrNull();
 }
 
 void ASTReader::resolvePendingMacro(IdentifierInfo *II,
@@ -4057,11 +4057,10 @@
                        unsigned ClientLoadCapabilities) {
   ModuleFile *M;
   std::string ErrorStr;
-  ModuleManager::AddModuleResult AddResult
-    = ModuleMgr.addModule(FileName, Type, ImportLoc, ImportedBy,
-                          getGeneration(), ExpectedSize, ExpectedModTime,
-                          ExpectedSignature, readASTFileSignature,
-                          M, ErrorStr);
+  ModuleManager::AddModuleResult AddResult =
+      ModuleMgr.addModule(FileName, Type, ImportLoc, ImportedBy,
+                          getGenerationOrNull(), ExpectedSize, ExpectedModTime,
+                          ExpectedSignature, readASTFileSignature, M, ErrorStr);
 
   switch (AddResult) {
   case ModuleManager::AlreadyLoaded:
@@ -7884,7 +7883,7 @@
   // Get the selector generation and update it to the current generation.
   unsigned &Generation = SelectorGeneration[Sel];
   unsigned PriorGeneration = Generation;
-  Generation = getGeneration();
+  Generation = getGenerationOrNull();
   SelectorOutOfDate[Sel] = false;
 
   // Search for methods defined with this selector.