If a declaration is loaded, and then a module import adds a redeclaration, then

ensure that querying the first declaration for its most recent declaration
checks for redeclarations from the imported module.

This works as follows:
 * The 'most recent' pointer on a canonical declaration grows a pointer to the
   external AST source and a generation number (space- and time-optimized for
   the case where there is no external source).
 * Each time the 'most recent' pointer is queried, if it has an external source,
   we check whether it's up to date, and update it if not.
 * The ancillary data stored on the canonical declaration is allocated lazily
   to avoid filling it in for declarations that end up being non-canonical.
   We'll still perform a redundant (ASTContext) allocation if someone asks for
   the most recent declaration from a decl before setPreviousDecl is called,
   but such cases are probably all bugs, and are now easy to find.

Some finessing is still in order here -- in particular, we use a very general
mechanism for handling the DefinitionData pointer on CXXRecordData, and a more
targeted approach would be more compact.

Also, the MayHaveOutOfDateDef mechanism should now be expunged, since it was
addressing only a corner of the full problem space here. That's not covered
by this patch.

Early performance benchmarks show that this makes no measurable difference to 
Clang performance without modules enabled (and fixes a major correctness issue
with modules enabled). I'll revert if a full performance comparison shows any
problems.

llvm-svn: 209046

14 files changed, 263 insertions, 171 deletions

diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 2f406166690..fe735dc92fc 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -8239,3 +8239,12 @@ ASTContext::ObjCMethodsAreEqual(const ObjCMethodDecl *MethodDecl,
   return (MethodDecl->isVariadic() == MethodImpl->isVariadic());
   
 }
+
+// Explicitly instantiate this in case a Redeclarable<T> is used from a TU that
+// doesn't include ASTContext.h
+template
+clang::LazyGenerationalUpdatePtr<
+    const Decl *, Decl *, &ExternalASTSource::CompleteRedeclChain>::ValueType
+clang::LazyGenerationalUpdatePtr<
+    const Decl *, Decl *, &ExternalASTSource::CompleteRedeclChain>::makeValue(
+        const clang::ASTContext &Ctx, Decl *Value);
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 101b1646248..fc15bdee010 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1616,10 +1616,12 @@ const char *VarDecl::getStorageClassSpecifierString(StorageClass SC) {
   llvm_unreachable("Invalid storage class");
 }
 
-VarDecl::VarDecl(Kind DK, DeclContext *DC, SourceLocation StartLoc,
-                 SourceLocation IdLoc, IdentifierInfo *Id, QualType T,
-                 TypeSourceInfo *TInfo, StorageClass SC)
-    : DeclaratorDecl(DK, DC, IdLoc, Id, T, TInfo, StartLoc), Init() {
+VarDecl::VarDecl(Kind DK, ASTContext &C, DeclContext *DC,
+                 SourceLocation StartLoc, SourceLocation IdLoc,
+                 IdentifierInfo *Id, QualType T, TypeSourceInfo *TInfo,
+                 StorageClass SC)
+    : DeclaratorDecl(DK, DC, IdLoc, Id, T, TInfo, StartLoc),
+      redeclarable_base(C), Init() {
   static_assert(sizeof(VarDeclBitfields) <= sizeof(unsigned),
                 "VarDeclBitfields too large!");
   static_assert(sizeof(ParmVarDeclBitfields) <= sizeof(unsigned),
@@ -1633,12 +1635,13 @@ VarDecl *VarDecl::Create(ASTContext &C, DeclContext *DC,
                          SourceLocation StartL, SourceLocation IdL,
                          IdentifierInfo *Id, QualType T, TypeSourceInfo *TInfo,
                          StorageClass S) {
-  return new (C, DC) VarDecl(Var, DC, StartL, IdL, Id, T, TInfo, S);
+  return new (C, DC) VarDecl(Var, C, DC, StartL, IdL, Id, T, TInfo, S);
 }
 
 VarDecl *VarDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) VarDecl(Var, nullptr, SourceLocation(), SourceLocation(),
-                             nullptr, QualType(), nullptr, SC_None);
+  return new (C, ID)
+      VarDecl(Var, C, nullptr, SourceLocation(), SourceLocation(), nullptr,
+              QualType(), nullptr, SC_None);
 }
 
 void VarDecl::setStorageClass(StorageClass SC) {
@@ -2114,7 +2117,7 @@ ParmVarDecl *ParmVarDecl::Create(ASTContext &C, DeclContext *DC,
                                  SourceLocation IdLoc, IdentifierInfo *Id,
                                  QualType T, TypeSourceInfo *TInfo,
                                  StorageClass S, Expr *DefArg) {
-  return new (C, DC) ParmVarDecl(ParmVar, DC, StartLoc, IdLoc, Id, T, TInfo,
+  return new (C, DC) ParmVarDecl(ParmVar, C, DC, StartLoc, IdLoc, Id, T, TInfo,
                                  S, DefArg);
 }
 
@@ -2127,9 +2130,9 @@ QualType ParmVarDecl::getOriginalType() const {
 }
 
 ParmVarDecl *ParmVarDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) ParmVarDecl(ParmVar, nullptr, SourceLocation(),
-                                 SourceLocation(), nullptr, QualType(),
-                                 nullptr, SC_None, nullptr);
+  return new (C, ID)
+      ParmVarDecl(ParmVar, C, nullptr, SourceLocation(), SourceLocation(),
+                  nullptr, QualType(), nullptr, SC_None, nullptr);
 }
 
 SourceRange ParmVarDecl::getSourceRange() const {
@@ -3234,10 +3237,10 @@ void TagDecl::startDefinition() {
   IsBeingDefined = true;
 
   if (CXXRecordDecl *D = dyn_cast<CXXRecordDecl>(this)) {
-    struct CXXRecordDecl::DefinitionData *Data = 
+    struct CXXRecordDecl::DefinitionData *Data =
       new (getASTContext()) struct CXXRecordDecl::DefinitionData(D);
     for (auto I : redecls())
-      cast<CXXRecordDecl>(I)->DefinitionData = Data;
+      cast<CXXRecordDecl>(I)->DefinitionData.setNotUpdated(Data);
   }
 }
 
@@ -3319,7 +3322,7 @@ EnumDecl *EnumDecl::Create(ASTContext &C, DeclContext *DC,
                            IdentifierInfo *Id,
                            EnumDecl *PrevDecl, bool IsScoped,
                            bool IsScopedUsingClassTag, bool IsFixed) {
-  EnumDecl *Enum = new (C, DC) EnumDecl(DC, StartLoc, IdLoc, Id, PrevDecl,
+  EnumDecl *Enum = new (C, DC) EnumDecl(C, DC, StartLoc, IdLoc, Id, PrevDecl,
                                         IsScoped, IsScopedUsingClassTag,
                                         IsFixed);
   Enum->MayHaveOutOfDateDef = C.getLangOpts().Modules;
@@ -3328,9 +3331,9 @@ EnumDecl *EnumDecl::Create(ASTContext &C, DeclContext *DC,
 }
 
 EnumDecl *EnumDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  EnumDecl *Enum = new (C, ID) EnumDecl(nullptr, SourceLocation(),
-                                        SourceLocation(), nullptr, nullptr,
-                                        false, false, false);
+  EnumDecl *Enum =
+      new (C, ID) EnumDecl(C, nullptr, SourceLocation(), SourceLocation(),
+                           nullptr, nullptr, false, false, false);
   Enum->MayHaveOutOfDateDef = C.getLangOpts().Modules;
   return Enum;
 }
@@ -3389,10 +3392,11 @@ void EnumDecl::setInstantiationOfMemberEnum(ASTContext &C, EnumDecl *ED,
 // RecordDecl Implementation
 //===----------------------------------------------------------------------===//
 
-RecordDecl::RecordDecl(Kind DK, TagKind TK, DeclContext *DC,
-                       SourceLocation StartLoc, SourceLocation IdLoc,
-                       IdentifierInfo *Id, RecordDecl *PrevDecl)
-  : TagDecl(DK, TK, DC, IdLoc, Id, PrevDecl, StartLoc) {
+RecordDecl::RecordDecl(Kind DK, TagKind TK, const ASTContext &C,
+                       DeclContext *DC, SourceLocation StartLoc,
+                       SourceLocation IdLoc, IdentifierInfo *Id,
+                       RecordDecl *PrevDecl)
+    : TagDecl(DK, TK, C, DC, IdLoc, Id, PrevDecl, StartLoc) {
   HasFlexibleArrayMember = false;
   AnonymousStructOrUnion = false;
   HasObjectMember = false;
@@ -3404,8 +3408,8 @@ RecordDecl::RecordDecl(Kind DK, TagKind TK, DeclContext *DC,
 RecordDecl *RecordDecl::Create(const ASTContext &C, TagKind TK, DeclContext *DC,
                                SourceLocation StartLoc, SourceLocation IdLoc,
                                IdentifierInfo *Id, RecordDecl* PrevDecl) {
-  RecordDecl* R = new (C, DC) RecordDecl(Record, TK, DC, StartLoc, IdLoc, Id,
-                                         PrevDecl);
+  RecordDecl *R = new (C, DC) RecordDecl(Record, TK, C, DC,
+                                         StartLoc, IdLoc, Id, PrevDecl);
   R->MayHaveOutOfDateDef = C.getLangOpts().Modules;
 
   C.getTypeDeclType(R, PrevDecl);
@@ -3413,9 +3417,9 @@ RecordDecl *RecordDecl::Create(const ASTContext &C, TagKind TK, DeclContext *DC,
 }
 
 RecordDecl *RecordDecl::CreateDeserialized(const ASTContext &C, unsigned ID) {
-  RecordDecl *R = new (C, ID) RecordDecl(Record, TTK_Struct, nullptr,
-                                         SourceLocation(), SourceLocation(),
-                                         nullptr, nullptr);
+  RecordDecl *R =
+      new (C, ID) RecordDecl(Record, TTK_Struct, C, nullptr, SourceLocation(),
+                             SourceLocation(), nullptr, nullptr);
   R->MayHaveOutOfDateDef = C.getLangOpts().Modules;
   return R;
 }
@@ -3577,12 +3581,12 @@ ImplicitParamDecl *ImplicitParamDecl::Create(ASTContext &C, DeclContext *DC,
                                              SourceLocation IdLoc,
                                              IdentifierInfo *Id,
                                              QualType Type) {
-  return new (C, DC) ImplicitParamDecl(DC, IdLoc, Id, Type);
+  return new (C, DC) ImplicitParamDecl(C, DC, IdLoc, Id, Type);
 }
 
 ImplicitParamDecl *ImplicitParamDecl::CreateDeserialized(ASTContext &C,
                                                          unsigned ID) {
-  return new (C, ID) ImplicitParamDecl(nullptr, SourceLocation(), nullptr,
+  return new (C, ID) ImplicitParamDecl(C, nullptr, SourceLocation(), nullptr,
                                        QualType());
 }
 
@@ -3595,14 +3599,14 @@ FunctionDecl *FunctionDecl::Create(ASTContext &C, DeclContext *DC,
                                    bool hasWrittenPrototype,
                                    bool isConstexprSpecified) {
   FunctionDecl *New =
-      new (C, DC) FunctionDecl(Function, DC, StartLoc, NameInfo, T, TInfo, SC,
-                               isInlineSpecified, isConstexprSpecified);
+      new (C, DC) FunctionDecl(Function, C, DC, StartLoc, NameInfo, T, TInfo,
+                               SC, isInlineSpecified, isConstexprSpecified);
   New->HasWrittenPrototype = hasWrittenPrototype;
   return New;
 }
 
 FunctionDecl *FunctionDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) FunctionDecl(Function, nullptr, SourceLocation(),
+  return new (C, ID) FunctionDecl(Function, C, nullptr, SourceLocation(),
                                   DeclarationNameInfo(), QualType(), nullptr,
                                   SC_None, false, false);
 }
@@ -3668,13 +3672,13 @@ void TypeDecl::anchor() { }
 TypedefDecl *TypedefDecl::Create(ASTContext &C, DeclContext *DC,
                                  SourceLocation StartLoc, SourceLocation IdLoc,
                                  IdentifierInfo *Id, TypeSourceInfo *TInfo) {
-  return new (C, DC) TypedefDecl(DC, StartLoc, IdLoc, Id, TInfo);
+  return new (C, DC) TypedefDecl(C, DC, StartLoc, IdLoc, Id, TInfo);
 }
 
 void TypedefNameDecl::anchor() { }
 
 TypedefDecl *TypedefDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) TypedefDecl(nullptr, SourceLocation(), SourceLocation(),
+  return new (C, ID) TypedefDecl(C, nullptr, SourceLocation(), SourceLocation(),
                                  nullptr, nullptr);
 }
 
@@ -3682,12 +3686,12 @@ TypeAliasDecl *TypeAliasDecl::Create(ASTContext &C, DeclContext *DC,
                                      SourceLocation StartLoc,
                                      SourceLocation IdLoc, IdentifierInfo *Id,
                                      TypeSourceInfo *TInfo) {
-  return new (C, DC) TypeAliasDecl(DC, StartLoc, IdLoc, Id, TInfo);
+  return new (C, DC) TypeAliasDecl(C, DC, StartLoc, IdLoc, Id, TInfo);
 }
 
 TypeAliasDecl *TypeAliasDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) TypeAliasDecl(nullptr, SourceLocation(), SourceLocation(),
-                                   nullptr, nullptr);
+  return new (C, ID) TypeAliasDecl(C, nullptr, SourceLocation(),
+                                   SourceLocation(), nullptr, nullptr);
 }
 
 SourceRange TypedefDecl::getSourceRange() const {
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index 4cd35fc462b..1c416282759 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -82,19 +82,21 @@ CXXBaseSpecifier *CXXRecordDecl::DefinitionData::getVBasesSlowCase() const {
   return VBases.get(Definition->getASTContext().getExternalSource());
 }
 
-CXXRecordDecl::CXXRecordDecl(Kind K, TagKind TK, DeclContext *DC,
-                             SourceLocation StartLoc, SourceLocation IdLoc,
-                             IdentifierInfo *Id, CXXRecordDecl *PrevDecl)
-  : RecordDecl(K, TK, DC, StartLoc, IdLoc, Id, PrevDecl),
-    DefinitionData(PrevDecl ? PrevDecl->DefinitionData : nullptr),
-    TemplateOrInstantiation() { }
+CXXRecordDecl::CXXRecordDecl(Kind K, TagKind TK, const ASTContext &C,
+                             DeclContext *DC, SourceLocation StartLoc,
+                             SourceLocation IdLoc, IdentifierInfo *Id,
+                             CXXRecordDecl *PrevDecl)
+    : RecordDecl(K, TK, C, DC, StartLoc, IdLoc, Id, PrevDecl),
+      DefinitionData(PrevDecl ? PrevDecl->DefinitionData
+                              : DefinitionDataPtr(C)),
+      TemplateOrInstantiation() {}
 
 CXXRecordDecl *CXXRecordDecl::Create(const ASTContext &C, TagKind TK,
                                      DeclContext *DC, SourceLocation StartLoc,
                                      SourceLocation IdLoc, IdentifierInfo *Id,
                                      CXXRecordDecl* PrevDecl,
                                      bool DelayTypeCreation) {
-  CXXRecordDecl *R = new (C, DC) CXXRecordDecl(CXXRecord, TK, DC, StartLoc,
+  CXXRecordDecl *R = new (C, DC) CXXRecordDecl(CXXRecord, TK, C, DC, StartLoc,
                                                IdLoc, Id, PrevDecl);
   R->MayHaveOutOfDateDef = C.getLangOpts().Modules;
 
@@ -104,18 +106,18 @@ CXXRecordDecl *CXXRecordDecl::Create(const ASTContext &C, TagKind TK,
   return R;
 }
 
-CXXRecordDecl *CXXRecordDecl::CreateLambda(const ASTContext &C, DeclContext *DC,
-                                           TypeSourceInfo *Info, SourceLocation Loc,
-                                           bool Dependent, bool IsGeneric, 
-                                           LambdaCaptureDefault CaptureDefault) {
+CXXRecordDecl *
+CXXRecordDecl::CreateLambda(const ASTContext &C, DeclContext *DC,
+                            TypeSourceInfo *Info, SourceLocation Loc,
+                            bool Dependent, bool IsGeneric,
+                            LambdaCaptureDefault CaptureDefault) {
   CXXRecordDecl *R =
-      new (C, DC) CXXRecordDecl(CXXRecord, TTK_Class, DC, Loc, Loc, nullptr,
-                                nullptr);
+      new (C, DC) CXXRecordDecl(CXXRecord, TTK_Class, C, DC, Loc, Loc,
+                                nullptr, nullptr);
   R->IsBeingDefined = true;
-  R->DefinitionData = new (C) struct LambdaDefinitionData(R, Info, 
-                                                          Dependent, 
-                                                          IsGeneric, 
-                                                          CaptureDefault);
+  R->DefinitionData.setNotUpdated(
+      new (C) struct LambdaDefinitionData(R, Info, Dependent, IsGeneric,
+                                          CaptureDefault));
   R->MayHaveOutOfDateDef = false;
   R->setImplicit(true);
   C.getTypeDeclType(R, /*PrevDecl=*/nullptr);
@@ -125,7 +127,7 @@ CXXRecordDecl *CXXRecordDecl::CreateLambda(const ASTContext &C, DeclContext *DC,
 CXXRecordDecl *
 CXXRecordDecl::CreateDeserialized(const ASTContext &C, unsigned ID) {
   CXXRecordDecl *R = new (C, ID) CXXRecordDecl(
-      CXXRecord, TTK_Struct, nullptr, SourceLocation(), SourceLocation(),
+      CXXRecord, TTK_Struct, C, nullptr, SourceLocation(), SourceLocation(),
       nullptr, nullptr);
   R->MayHaveOutOfDateDef = false;
   return R;
@@ -1410,12 +1412,13 @@ CXXMethodDecl::Create(ASTContext &C, CXXRecordDecl *RD,
                       QualType T, TypeSourceInfo *TInfo,
                       StorageClass SC, bool isInline,
                       bool isConstexpr, SourceLocation EndLocation) {
-  return new (C, RD) CXXMethodDecl(CXXMethod, RD, StartLoc, NameInfo, T, TInfo,
-                                   SC, isInline, isConstexpr, EndLocation);
+  return new (C, RD) CXXMethodDecl(CXXMethod, C, RD, StartLoc, NameInfo,
+                                   T, TInfo, SC, isInline, isConstexpr,
+                                   EndLocation);
 }
 
 CXXMethodDecl *CXXMethodDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) CXXMethodDecl(CXXMethod, nullptr, SourceLocation(),
+  return new (C, ID) CXXMethodDecl(CXXMethod, C, nullptr, SourceLocation(),
                                    DeclarationNameInfo(), QualType(), nullptr,
                                    SC_None, false, false, SourceLocation());
 }
@@ -1680,7 +1683,7 @@ void CXXConstructorDecl::anchor() { }
 
 CXXConstructorDecl *
 CXXConstructorDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) CXXConstructorDecl(nullptr, SourceLocation(),
+  return new (C, ID) CXXConstructorDecl(C, nullptr, SourceLocation(),
                                         DeclarationNameInfo(), QualType(),
                                         nullptr, false, false, false, false);
 }
@@ -1695,7 +1698,7 @@ CXXConstructorDecl::Create(ASTContext &C, CXXRecordDecl *RD,
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXConstructorName &&
          "Name must refer to a constructor");
-  return new (C, RD) CXXConstructorDecl(RD, StartLoc, NameInfo, T, TInfo,
+  return new (C, RD) CXXConstructorDecl(C, RD, StartLoc, NameInfo, T, TInfo,
                                         isExplicit, isInline,
                                         isImplicitlyDeclared, isConstexpr);
 }
@@ -1830,9 +1833,9 @@ void CXXDestructorDecl::anchor() { }
 
 CXXDestructorDecl *
 CXXDestructorDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) CXXDestructorDecl(
-      nullptr, SourceLocation(), DeclarationNameInfo(), QualType(), nullptr,
-      false, false);
+  return new (C, ID)
+      CXXDestructorDecl(C, nullptr, SourceLocation(), DeclarationNameInfo(),
+                        QualType(), nullptr, false, false);
 }
 
 CXXDestructorDecl *
@@ -1844,7 +1847,7 @@ CXXDestructorDecl::Create(ASTContext &C, CXXRecordDecl *RD,
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXDestructorName &&
          "Name must refer to a destructor");
-  return new (C, RD) CXXDestructorDecl(RD, StartLoc, NameInfo, T, TInfo,
+  return new (C, RD) CXXDestructorDecl(C, RD, StartLoc, NameInfo, T, TInfo,
                                        isInline, isImplicitlyDeclared);
 }
 
@@ -1852,7 +1855,7 @@ void CXXConversionDecl::anchor() { }
 
 CXXConversionDecl *
 CXXConversionDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) CXXConversionDecl(nullptr, SourceLocation(),
+  return new (C, ID) CXXConversionDecl(C, nullptr, SourceLocation(),
                                        DeclarationNameInfo(), QualType(),
                                        nullptr, false, false, false,
                                        SourceLocation());
@@ -1868,7 +1871,7 @@ CXXConversionDecl::Create(ASTContext &C, CXXRecordDecl *RD,
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXConversionFunctionName &&
          "Name must refer to a conversion function");
-  return new (C, RD) CXXConversionDecl(RD, StartLoc, NameInfo, T, TInfo,
+  return new (C, RD) CXXConversionDecl(C, RD, StartLoc, NameInfo, T, TInfo,
                                        isInline, isExplicit, isConstexpr,
                                        EndLocation);
 }
@@ -1925,15 +1928,14 @@ NamespaceDecl *UsingDirectiveDecl::getNominatedNamespace() {
   return cast_or_null<NamespaceDecl>(NominatedNamespace);
 }
 
-NamespaceDecl::NamespaceDecl(DeclContext *DC, bool Inline, 
-                             SourceLocation StartLoc,
-                             SourceLocation IdLoc, IdentifierInfo *Id,
-                             NamespaceDecl *PrevDecl)
-  : NamedDecl(Namespace, DC, IdLoc, Id), DeclContext(Namespace),
-    LocStart(StartLoc), RBraceLoc(),
-    AnonOrFirstNamespaceAndInline(nullptr, Inline) {
+NamespaceDecl::NamespaceDecl(ASTContext &C, DeclContext *DC, bool Inline,
+                             SourceLocation StartLoc, SourceLocation IdLoc,
+                             IdentifierInfo *Id, NamespaceDecl *PrevDecl)
+    : NamedDecl(Namespace, DC, IdLoc, Id), DeclContext(Namespace),
+      redeclarable_base(C), LocStart(StartLoc), RBraceLoc(),
+      AnonOrFirstNamespaceAndInline(nullptr, Inline) {
   setPreviousDecl(PrevDecl);
-  
+
   if (PrevDecl)
     AnonOrFirstNamespaceAndInline.setPointer(PrevDecl->getOriginalNamespace());
 }
@@ -1942,11 +1944,12 @@ NamespaceDecl *NamespaceDecl::Create(ASTContext &C, DeclContext *DC,
                                      bool Inline, SourceLocation StartLoc,
                                      SourceLocation IdLoc, IdentifierInfo *Id,
                                      NamespaceDecl *PrevDecl) {
-  return new (C, DC) NamespaceDecl(DC, Inline, StartLoc, IdLoc, Id, PrevDecl);
+  return new (C, DC) NamespaceDecl(C, DC, Inline, StartLoc, IdLoc, Id,
+                                   PrevDecl);
 }
 
 NamespaceDecl *NamespaceDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) NamespaceDecl(nullptr, false, SourceLocation(),
+  return new (C, ID) NamespaceDecl(C, nullptr, false, SourceLocation(),
                                    SourceLocation(), nullptr, nullptr);
 }
 
@@ -1987,8 +1990,8 @@ void UsingShadowDecl::anchor() { }
 
 UsingShadowDecl *
 UsingShadowDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) UsingShadowDecl(nullptr, SourceLocation(), nullptr,
-                                     nullptr);
+  return new (C, ID) UsingShadowDecl(C, nullptr, SourceLocation(),
+                                     nullptr, nullptr);
 }
 
 UsingDecl *UsingShadowDecl::getUsingDecl() const {
diff --git a/clang/lib/AST/DeclObjC.cpp b/clang/lib/AST/DeclObjC.cpp
index 42462a1b140..186a7417b65 100644
--- a/clang/lib/AST/DeclObjC.cpp
+++ b/clang/lib/AST/DeclObjC.cpp
@@ -1134,15 +1134,15 @@ ObjCInterfaceDecl *ObjCInterfaceDecl::Create(const ASTContext &C,
                                              SourceLocation ClassLoc,
                                              bool isInternal){
   ObjCInterfaceDecl *Result = new (C, DC)
-      ObjCInterfaceDecl(DC, atLoc, Id, ClassLoc, PrevDecl, isInternal);
+      ObjCInterfaceDecl(C, DC, atLoc, Id, ClassLoc, PrevDecl, isInternal);
   Result->Data.setInt(!C.getLangOpts().Modules);
   C.getObjCInterfaceType(Result, PrevDecl);
   return Result;
 }
 
-ObjCInterfaceDecl *ObjCInterfaceDecl::CreateDeserialized(ASTContext &C,
+ObjCInterfaceDecl *ObjCInterfaceDecl::CreateDeserialized(const ASTContext &C,
                                                          unsigned ID) {
-  ObjCInterfaceDecl *Result = new (C, ID) ObjCInterfaceDecl(nullptr,
+  ObjCInterfaceDecl *Result = new (C, ID) ObjCInterfaceDecl(C, nullptr,
                                                             SourceLocation(),
                                                             nullptr,
                                                             SourceLocation(),
@@ -1151,20 +1151,20 @@ ObjCInterfaceDecl *ObjCInterfaceDecl::CreateDeserialized(ASTContext &C,
   return Result;
 }
 
-ObjCInterfaceDecl::
-ObjCInterfaceDecl(DeclContext *DC, SourceLocation atLoc, IdentifierInfo *Id,
-                  SourceLocation CLoc, ObjCInterfaceDecl *PrevDecl,
-                  bool isInternal)
-  : ObjCContainerDecl(ObjCInterface, DC, Id, CLoc, atLoc),
-    TypeForDecl(nullptr), Data()
-{
+ObjCInterfaceDecl::ObjCInterfaceDecl(const ASTContext &C, DeclContext *DC,
+                                     SourceLocation AtLoc, IdentifierInfo *Id,
+                                     SourceLocation CLoc,
+                                     ObjCInterfaceDecl *PrevDecl,
+                                     bool IsInternal)
+    : ObjCContainerDecl(ObjCInterface, DC, Id, CLoc, AtLoc),
+      redeclarable_base(C), TypeForDecl(nullptr), Data() {
   setPreviousDecl(PrevDecl);
   
   // Copy the 'data' pointer over.
   if (PrevDecl)
     Data = PrevDecl->Data;
   
-  setImplicit(isInternal);
+  setImplicit(IsInternal);
 }
 
 void ObjCInterfaceDecl::LoadExternalDefinition() const {
@@ -1487,12 +1487,12 @@ ObjCAtDefsFieldDecl *ObjCAtDefsFieldDecl::CreateDeserialized(ASTContext &C,
 
 void ObjCProtocolDecl::anchor() { }
 
-ObjCProtocolDecl::ObjCProtocolDecl(DeclContext *DC, IdentifierInfo *Id,
-                                   SourceLocation nameLoc, 
+ObjCProtocolDecl::ObjCProtocolDecl(ASTContext &C, DeclContext *DC,
+                                   IdentifierInfo *Id, SourceLocation nameLoc,
                                    SourceLocation atStartLoc,
                                    ObjCProtocolDecl *PrevDecl)
-  : ObjCContainerDecl(ObjCProtocol, DC, Id, nameLoc, atStartLoc), Data()
-{
+    : ObjCContainerDecl(ObjCProtocol, DC, Id, nameLoc, atStartLoc),
+      redeclarable_base(C), Data() {
   setPreviousDecl(PrevDecl);
   if (PrevDecl)
     Data = PrevDecl->Data;
@@ -1504,7 +1504,7 @@ ObjCProtocolDecl *ObjCProtocolDecl::Create(ASTContext &C, DeclContext *DC,
                                            SourceLocation atStartLoc,
                                            ObjCProtocolDecl *PrevDecl) {
   ObjCProtocolDecl *Result =
-      new (C, DC) ObjCProtocolDecl(DC, Id, nameLoc, atStartLoc, PrevDecl);
+      new (C, DC) ObjCProtocolDecl(C, DC, Id, nameLoc, atStartLoc, PrevDecl);
   Result->Data.setInt(!C.getLangOpts().Modules);
   return Result;
 }
@@ -1512,7 +1512,7 @@ ObjCProtocolDecl *ObjCProtocolDecl::Create(ASTContext &C, DeclContext *DC,
 ObjCProtocolDecl *ObjCProtocolDecl::CreateDeserialized(ASTContext &C,
                                                        unsigned ID) {
   ObjCProtocolDecl *Result =
-      new (C, ID) ObjCProtocolDecl(nullptr, nullptr, SourceLocation(),
+      new (C, ID) ObjCProtocolDecl(C, nullptr, nullptr, SourceLocation(),
                                    SourceLocation(), nullptr);
   Result->Data.setInt(!C.getLangOpts().Modules);
   return Result;
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index 4a934747e94..a69146b5b88 100644
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -229,12 +229,12 @@ FunctionTemplateDecl *FunctionTemplateDecl::Create(ASTContext &C,
                                                TemplateParameterList *Params,
                                                    NamedDecl *Decl) {
   AdoptTemplateParameterList(Params, cast<DeclContext>(Decl));
-  return new (C, DC) FunctionTemplateDecl(DC, L, Name, Params, Decl);
+  return new (C, DC) FunctionTemplateDecl(C, DC, L, Name, Params, Decl);
 }
 
 FunctionTemplateDecl *FunctionTemplateDecl::CreateDeserialized(ASTContext &C,
                                                                unsigned ID) {
-  return new (C, ID) FunctionTemplateDecl(nullptr, SourceLocation(),
+  return new (C, ID) FunctionTemplateDecl(C, nullptr, SourceLocation(),
                                           DeclarationName(), nullptr, nullptr);
 }
 
@@ -307,15 +307,16 @@ ClassTemplateDecl *ClassTemplateDecl::Create(ASTContext &C,
                                              NamedDecl *Decl,
                                              ClassTemplateDecl *PrevDecl) {
   AdoptTemplateParameterList(Params, cast<DeclContext>(Decl));
-  ClassTemplateDecl *New =
-      new (C, DC) ClassTemplateDecl(DC, L, Name, Params, Decl);
+  ClassTemplateDecl *New = new (C, DC) ClassTemplateDecl(C, DC, L, Name,
+                                                         Params, Decl);
   New->setPreviousDecl(PrevDecl);
   return New;
 }
 
-ClassTemplateDecl *ClassTemplateDecl::CreateDeserialized(ASTContext &C, 
+ClassTemplateDecl *ClassTemplateDecl::CreateDeserialized(ASTContext &C,
                                                          unsigned ID) {
-  return new (C, ID) ClassTemplateDecl(EmptyShell());
+  return new (C, ID) ClassTemplateDecl(C, nullptr, SourceLocation(),
+                                       DeclarationName(), nullptr, nullptr);
 }
 
 void ClassTemplateDecl::LoadLazySpecializations() const {
@@ -695,7 +696,7 @@ ClassTemplateSpecializationDecl(ASTContext &Context, Kind DK, TagKind TK,
                                 const TemplateArgument *Args,
                                 unsigned NumArgs,
                                 ClassTemplateSpecializationDecl *PrevDecl)
-  : CXXRecordDecl(DK, TK, DC, StartLoc, IdLoc,
+  : CXXRecordDecl(DK, TK, Context, DC, StartLoc, IdLoc,
                   SpecializedTemplate->getIdentifier(),
                   PrevDecl),
     SpecializedTemplate(SpecializedTemplate),
@@ -704,12 +705,11 @@ ClassTemplateSpecializationDecl(ASTContext &Context, Kind DK, TagKind TK,
     SpecializationKind(TSK_Undeclared) {
 }
 
-ClassTemplateSpecializationDecl::ClassTemplateSpecializationDecl(Kind DK)
-  : CXXRecordDecl(DK, TTK_Struct, nullptr, SourceLocation(), SourceLocation(),
-                  nullptr, nullptr),
-    ExplicitInfo(nullptr),
-    SpecializationKind(TSK_Undeclared) {
-}
+ClassTemplateSpecializationDecl::ClassTemplateSpecializationDecl(ASTContext &C,
+                                                                 Kind DK)
+    : CXXRecordDecl(DK, TTK_Struct, C, nullptr, SourceLocation(),
+                    SourceLocation(), nullptr, nullptr),
+      ExplicitInfo(nullptr), SpecializationKind(TSK_Undeclared) {}
 
 ClassTemplateSpecializationDecl *
 ClassTemplateSpecializationDecl::Create(ASTContext &Context, TagKind TK,
@@ -734,7 +734,7 @@ ClassTemplateSpecializationDecl *
 ClassTemplateSpecializationDecl::CreateDeserialized(ASTContext &C,
                                                     unsigned ID) {
   ClassTemplateSpecializationDecl *Result =
-    new (C, ID) ClassTemplateSpecializationDecl(ClassTemplateSpecialization);
+    new (C, ID) ClassTemplateSpecializationDecl(C, ClassTemplateSpecialization);
   Result->MayHaveOutOfDateDef = false;
   return Result;
 }
@@ -851,7 +851,7 @@ ClassTemplatePartialSpecializationDecl *
 ClassTemplatePartialSpecializationDecl::CreateDeserialized(ASTContext &C,
                                                            unsigned ID) {
   ClassTemplatePartialSpecializationDecl *Result =
-      new (C, ID) ClassTemplatePartialSpecializationDecl();
+      new (C, ID) ClassTemplatePartialSpecializationDecl(C);
   Result->MayHaveOutOfDateDef = false;
   return Result;
 }
@@ -889,12 +889,12 @@ TypeAliasTemplateDecl *TypeAliasTemplateDecl::Create(ASTContext &C,
                                                   TemplateParameterList *Params,
                                                      NamedDecl *Decl) {
   AdoptTemplateParameterList(Params, DC);
-  return new (C, DC) TypeAliasTemplateDecl(DC, L, Name, Params, Decl);
+  return new (C, DC) TypeAliasTemplateDecl(C, DC, L, Name, Params, Decl);
 }
 
 TypeAliasTemplateDecl *TypeAliasTemplateDecl::CreateDeserialized(ASTContext &C,
                                                                  unsigned ID) {
-  return new (C, ID) TypeAliasTemplateDecl(nullptr, SourceLocation(),
+  return new (C, ID) TypeAliasTemplateDecl(C, nullptr, SourceLocation(),
                                            DeclarationName(), nullptr, nullptr);
 }
 
@@ -943,12 +943,13 @@ VarTemplateDecl *VarTemplateDecl::Create(ASTContext &C, DeclContext *DC,
                                          SourceLocation L, DeclarationName Name,
                                          TemplateParameterList *Params,
                                          VarDecl *Decl) {
-  return new (C, DC) VarTemplateDecl(DC, L, Name, Params, Decl);
+  return new (C, DC) VarTemplateDecl(C, DC, L, Name, Params, Decl);
 }
 
 VarTemplateDecl *VarTemplateDecl::CreateDeserialized(ASTContext &C,
                                                      unsigned ID) {
-  return new (C, ID) VarTemplateDecl(EmptyShell());
+  return new (C, ID) VarTemplateDecl(C, nullptr, SourceLocation(),
+                                     DeclarationName(), nullptr, nullptr);
 }
 
 // TODO: Unify across class, function and variable templates?
@@ -1057,18 +1058,19 @@ VarTemplateDecl::findPartialSpecInstantiatedFromMember(
 // VarTemplateSpecializationDecl Implementation
 //===----------------------------------------------------------------------===//
 VarTemplateSpecializationDecl::VarTemplateSpecializationDecl(
-    ASTContext &Context, Kind DK, DeclContext *DC, SourceLocation StartLoc,
+    Kind DK, ASTContext &Context, DeclContext *DC, SourceLocation StartLoc,
     SourceLocation IdLoc, VarTemplateDecl *SpecializedTemplate, QualType T,
     TypeSourceInfo *TInfo, StorageClass S, const TemplateArgument *Args,
     unsigned NumArgs)
-    : VarDecl(DK, DC, StartLoc, IdLoc, SpecializedTemplate->getIdentifier(), T,
-              TInfo, S),
+    : VarDecl(DK, Context, DC, StartLoc, IdLoc,
+              SpecializedTemplate->getIdentifier(), T, TInfo, S),
       SpecializedTemplate(SpecializedTemplate), ExplicitInfo(nullptr),
       TemplateArgs(TemplateArgumentList::CreateCopy(Context, Args, NumArgs)),
       SpecializationKind(TSK_Undeclared) {}
 
-VarTemplateSpecializationDecl::VarTemplateSpecializationDecl(Kind DK)
-    : VarDecl(DK, nullptr, SourceLocation(), SourceLocation(), nullptr,
+VarTemplateSpecializationDecl::VarTemplateSpecializationDecl(Kind DK,
+                                                             ASTContext &C)
+    : VarDecl(DK, C, nullptr, SourceLocation(), SourceLocation(), nullptr,
               QualType(), nullptr, SC_None),
       ExplicitInfo(nullptr), SpecializationKind(TSK_Undeclared) {}
 
@@ -1078,13 +1080,14 @@ VarTemplateSpecializationDecl *VarTemplateSpecializationDecl::Create(
     TypeSourceInfo *TInfo, StorageClass S, const TemplateArgument *Args,
     unsigned NumArgs) {
   return new (Context, DC) VarTemplateSpecializationDecl(
-      Context, VarTemplateSpecialization, DC, StartLoc, IdLoc,
+      VarTemplateSpecialization, Context, DC, StartLoc, IdLoc,
       SpecializedTemplate, T, TInfo, S, Args, NumArgs);
 }
 
 VarTemplateSpecializationDecl *
 VarTemplateSpecializationDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) VarTemplateSpecializationDecl(VarTemplateSpecialization);
+  return new (C, ID)
+      VarTemplateSpecializationDecl(VarTemplateSpecialization, C);
 }
 
 void VarTemplateSpecializationDecl::getNameForDiagnostic(
@@ -1123,7 +1126,7 @@ VarTemplatePartialSpecializationDecl::VarTemplatePartialSpecializationDecl(
     VarTemplateDecl *SpecializedTemplate, QualType T, TypeSourceInfo *TInfo,
     StorageClass S, const TemplateArgument *Args, unsigned NumArgs,
     const ASTTemplateArgumentListInfo *ArgInfos)
-    : VarTemplateSpecializationDecl(Context, VarTemplatePartialSpecialization,
+    : VarTemplateSpecializationDecl(VarTemplatePartialSpecialization, Context,
                                     DC, StartLoc, IdLoc, SpecializedTemplate, T,
                                     TInfo, S, Args, NumArgs),
       TemplateParams(Params), ArgsAsWritten(ArgInfos),
@@ -1153,5 +1156,5 @@ VarTemplatePartialSpecializationDecl::Create(
 VarTemplatePartialSpecializationDecl *
 VarTemplatePartialSpecializationDecl::CreateDeserialized(ASTContext &C,
                                                          unsigned ID) {
-  return new (C, ID) VarTemplatePartialSpecializationDecl();
+  return new (C, ID) VarTemplatePartialSpecializationDecl(C);
 }
diff --git a/clang/lib/AST/ExternalASTSource.cpp b/clang/lib/AST/ExternalASTSource.cpp
index 05a17db6bf0..ae8297cecd1 100644
--- a/clang/lib/AST/ExternalASTSource.cpp
+++ b/clang/lib/AST/ExternalASTSource.cpp
@@ -14,7 +14,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/ExternalASTSource.h"
+#include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclarationName.h"
+#include "llvm/Support/ErrorHandling.h"
 
 using namespace clang;
 
@@ -60,3 +62,21 @@ ExternalASTSource::FindExternalLexicalDecls(const DeclContext *DC,
 }
 
 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;
+}
diff --git a/clang/lib/CodeGen/CGBlocks.cpp b/clang/lib/CodeGen/CGBlocks.cpp
index 941410a7416..56c8a07140b 100644
--- a/clang/lib/CodeGen/CGBlocks.cpp
+++ b/clang/lib/CodeGen/CGBlocks.cpp
@@ -841,8 +841,8 @@ llvm::Value *CodeGenFunction::EmitBlockLiteral(const CGBlockInfo &blockInfo) {
     } else {
       // Fake up a new variable so that EmitScalarInit doesn't think
       // we're referring to the variable in its own initializer.
-      ImplicitParamDecl blockFieldPseudoVar(/*DC*/ 0, SourceLocation(),
-                                            /*name*/ 0, type);
+      ImplicitParamDecl blockFieldPseudoVar(getContext(), /*DC*/ 0,
+                                            SourceLocation(), /*name*/ 0, type);
 
       // We use one of these or the other depending on whether the
       // reference is nested.
@@ -1108,7 +1108,7 @@ CodeGenFunction::GenerateBlockFunction(GlobalDecl GD,
   QualType selfTy = getContext().VoidPtrTy;
   IdentifierInfo *II = &CGM.getContext().Idents.get(".block_descriptor");
 
-  ImplicitParamDecl selfDecl(const_cast<BlockDecl*>(blockDecl),
+  ImplicitParamDecl selfDecl(getContext(), const_cast<BlockDecl*>(blockDecl),
                              SourceLocation(), II, selfTy);
   args.push_back(&selfDecl);
 
@@ -1279,9 +1279,9 @@ CodeGenFunction::GenerateCopyHelperFunction(const CGBlockInfo &blockInfo) {
   ASTContext &C = getContext();
 
   FunctionArgList args;
-  ImplicitParamDecl dstDecl(0, SourceLocation(), 0, C.VoidPtrTy);
+  ImplicitParamDecl dstDecl(getContext(), 0, SourceLocation(), 0, C.VoidPtrTy);
   args.push_back(&dstDecl);
-  ImplicitParamDecl srcDecl(0, SourceLocation(), 0, C.VoidPtrTy);
+  ImplicitParamDecl srcDecl(getContext(), 0, SourceLocation(), 0, C.VoidPtrTy);
   args.push_back(&srcDecl);
 
   const CGFunctionInfo &FI = CGM.getTypes().arrangeFreeFunctionDeclaration(
@@ -1453,7 +1453,7 @@ CodeGenFunction::GenerateDestroyHelperFunction(const CGBlockInfo &blockInfo) {
   ASTContext &C = getContext();
 
   FunctionArgList args;
-  ImplicitParamDecl srcDecl(0, SourceLocation(), 0, C.VoidPtrTy);
+  ImplicitParamDecl srcDecl(getContext(), 0, SourceLocation(), 0, C.VoidPtrTy);
   args.push_back(&srcDecl);
 
   const CGFunctionInfo &FI = CGM.getTypes().arrangeFreeFunctionDeclaration(
@@ -1738,10 +1738,12 @@ generateByrefCopyHelper(CodeGenFunction &CGF,
   QualType R = Context.VoidTy;
 
   FunctionArgList args;
-  ImplicitParamDecl dst(0, SourceLocation(), 0, Context.VoidPtrTy);
+  ImplicitParamDecl dst(CGF.getContext(), 0, SourceLocation(), 0,
+                        Context.VoidPtrTy);
   args.push_back(&dst);
 
-  ImplicitParamDecl src(0, SourceLocation(), 0, Context.VoidPtrTy);
+  ImplicitParamDecl src(CGF.getContext(), 0, SourceLocation(), 0,
+                        Context.VoidPtrTy);
   args.push_back(&src);
 
   const CGFunctionInfo &FI = CGF.CGM.getTypes().arrangeFreeFunctionDeclaration(
@@ -1810,7 +1812,8 @@ generateByrefDisposeHelper(CodeGenFunction &CGF,
   QualType R = Context.VoidTy;
 
   FunctionArgList args;
-  ImplicitParamDecl src(0, SourceLocation(), 0, Context.VoidPtrTy);
+  ImplicitParamDecl src(CGF.getContext(), 0, SourceLocation(), 0,
+                        Context.VoidPtrTy);
   args.push_back(&src);
 
   const CGFunctionInfo &FI = CGF.CGM.getTypes().arrangeFreeFunctionDeclaration(
diff --git a/clang/lib/CodeGen/CGDeclCXX.cpp b/clang/lib/CodeGen/CGDeclCXX.cpp
index 59129f6aeea..0cc9330a866 100644
--- a/clang/lib/CodeGen/CGDeclCXX.cpp
+++ b/clang/lib/CodeGen/CGDeclCXX.cpp
@@ -522,7 +522,8 @@ llvm::Function *CodeGenFunction::generateDestroyHelper(
     llvm::Constant *addr, QualType type, Destroyer *destroyer,
     bool useEHCleanupForArray, const VarDecl *VD) {
   FunctionArgList args;
-  ImplicitParamDecl dst(0, SourceLocation(), 0, getContext().VoidPtrTy);
+  ImplicitParamDecl dst(getContext(), 0, SourceLocation(), 0,
+                        getContext().VoidPtrTy);
   args.push_back(&dst);
 
   const CGFunctionInfo &FI = CGM.getTypes().arrangeFreeFunctionDeclaration(
diff --git a/clang/lib/CodeGen/CGObjC.cpp b/clang/lib/CodeGen/CGObjC.cpp
index 47a096d5404..d1cfddce9f1 100644
--- a/clang/lib/CodeGen/CGObjC.cpp
+++ b/clang/lib/CodeGen/CGObjC.cpp
@@ -2901,9 +2901,9 @@ CodeGenFunction::GenerateObjCAtomicSetterCopyHelperFunction(
   SrcTy = C.getPointerType(SrcTy);
   
   FunctionArgList args;
-  ImplicitParamDecl dstDecl(FD, SourceLocation(), 0, DestTy);
+  ImplicitParamDecl dstDecl(getContext(), FD, SourceLocation(), 0, DestTy);
   args.push_back(&dstDecl);
-  ImplicitParamDecl srcDecl(FD, SourceLocation(), 0, SrcTy);
+  ImplicitParamDecl srcDecl(getContext(), FD, SourceLocation(), 0, SrcTy);
   args.push_back(&srcDecl);
 
   const CGFunctionInfo &FI = CGM.getTypes().arrangeFreeFunctionDeclaration(
@@ -2980,9 +2980,9 @@ CodeGenFunction::GenerateObjCAtomicGetterCopyHelperFunction(
   SrcTy = C.getPointerType(SrcTy);
   
   FunctionArgList args;
-  ImplicitParamDecl dstDecl(FD, SourceLocation(), 0, DestTy);
+  ImplicitParamDecl dstDecl(getContext(), FD, SourceLocation(), 0, DestTy);
   args.push_back(&dstDecl);
-  ImplicitParamDecl srcDecl(FD, SourceLocation(), 0, SrcTy);
+  ImplicitParamDecl srcDecl(getContext(), FD, SourceLocation(), 0, SrcTy);
   args.push_back(&srcDecl);
 
   const CGFunctionInfo &FI = CGM.getTypes().arrangeFreeFunctionDeclaration(
diff --git a/clang/lib/Frontend/FrontendAction.cpp b/clang/lib/Frontend/FrontendAction.cpp
index 77eb1a14434..ade23b87d17 100644
--- a/clang/lib/Frontend/FrontendAction.cpp
+++ b/clang/lib/Frontend/FrontendAction.cpp
@@ -358,8 +358,21 @@ bool FrontendAction::BeginSourceFile(CompilerInstance &CI,
   // source.
   if (!CI.hasASTContext() || !CI.getASTContext().getExternalSource()) {
     Preprocessor &PP = CI.getPreprocessor();
+
+    // If modules are enabled, create the module manager before creating
+    // any builtins, so that all declarations know that they might be
+    // extended by an external source.
+    if (CI.getLangOpts().Modules)
+      CI.createModuleManager();
+
     PP.getBuiltinInfo().InitializeBuiltins(PP.getIdentifierTable(),
                                            PP.getLangOpts());
+  } else {
+    // FIXME: If this is a problem, recover from it by creating a multiplex
+    // source.
+    assert((!CI.getLangOpts().Modules || CI.getModuleManager()) &&
+           "modules enabled but created an external source that "
+           "doesn't support modules");
   }
 
   // If there is a layout overrides file, attach an external AST source that
@@ -371,7 +384,7 @@ bool FrontendAction::BeginSourceFile(CompilerInstance &CI,
                      CI.getFrontendOpts().OverrideRecordLayoutsFile));
     CI.getASTContext().setExternalSource(Override);
   }
-  
+
   return true;
 
   // If we failed, reset state since the client will not end up calling the
diff --git a/clang/lib/Sema/MultiplexExternalSemaSource.cpp b/clang/lib/Sema/MultiplexExternalSemaSource.cpp
index ad7627a4571..35072a5faca 100644
--- a/clang/lib/Sema/MultiplexExternalSemaSource.cpp
+++ b/clang/lib/Sema/MultiplexExternalSemaSource.cpp
@@ -49,6 +49,11 @@ Decl *MultiplexExternalSemaSource::GetExternalDecl(uint32_t ID) {
   return 0;
 }
 
+void MultiplexExternalSemaSource::CompleteRedeclChain(const Decl *D) {
+  for (size_t i = 0; i < Sources.size(); ++i)
+    Sources[i]->CompleteRedeclChain(D);
+}
+
 Selector MultiplexExternalSemaSource::GetExternalSelector(uint32_t ID) {
   Selector Sel;
   for(size_t i = 0; i < Sources.size(); ++i) {
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index d61372cc1bd..eb151ef16f6 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -1707,7 +1707,7 @@ void ASTReader::markIdentifierUpToDate(IdentifierInfo *II) {
 
   // Update the generation for this identifier.
   if (getContext().getLangOpts().Modules)
-    IdentifierGeneration[II] = CurrentGeneration;
+    IdentifierGeneration[II] = getGeneration();
 }
 
 struct ASTReader::ModuleMacroInfo {
@@ -3435,7 +3435,7 @@ ASTReader::ASTReadResult ASTReader::ReadAST(const std::string &FileName,
   Deserializing AnASTFile(this);
 
   // Bump the generation number.
-  unsigned PreviousGeneration = CurrentGeneration++;
+  unsigned PreviousGeneration = incrementGeneration(Context);
 
   unsigned NumModules = ModuleMgr.size();
   SmallVector<ImportedModule, 4> Loaded;
@@ -3615,7 +3615,7 @@ ASTReader::ReadASTCore(StringRef FileName,
   std::string ErrorStr;
   ModuleManager::AddModuleResult AddResult
     = ModuleMgr.addModule(FileName, Type, ImportLoc, ImportedBy,
-                          CurrentGeneration, ExpectedSize, ExpectedModTime,
+                          getGeneration(), ExpectedSize, ExpectedModTime,
                           M, ErrorStr);
 
   switch (AddResult) {
@@ -5939,6 +5939,37 @@ Decl *ASTReader::GetExternalDecl(uint32_t ID) {
   return GetDecl(ID);
 }
 
+void ASTReader::CompleteRedeclChain(const Decl *D) {
+  const DeclContext *DC = D->getDeclContext()->getRedeclContext();
+
+  // Recursively ensure that the decl context itself is complete
+  // (in particular, this matters if the decl context is a namespace).
+  //
+  // FIXME: This should be performed by lookup instead of here.
+  cast<Decl>(DC)->getMostRecentDecl();
+
+  // If this is a named declaration, complete it by looking it up
+  // within its context.
+  //
+  // FIXME: We don't currently handle the cases where we can't do this;
+  // merging a class definition that contains unnamed entities should merge
+  // those entities. Likewise, merging a function definition should merge
+  // all mergeable entities within it.
+  if (isa<TranslationUnitDecl>(DC) || isa<NamespaceDecl>(DC) ||
+      isa<CXXRecordDecl>(DC) || isa<EnumDecl>(DC)) {
+    if (DeclarationName Name = cast<NamedDecl>(D)->getDeclName()) {
+      auto *II = Name.getAsIdentifierInfo();
+      if (isa<TranslationUnitDecl>(DC) && II) {
+        // Outside of C++, we don't have a lookup table for the TU, so update
+        // the identifier instead. In C++, either way should work fine.
+        if (II->isOutOfDate())
+          updateOutOfDateIdentifier(*II);
+      } else
+        DC->lookup(Name);
+    }
+  }
+}
+
 uint64_t ASTReader::readCXXBaseSpecifiers(ModuleFile &M,
                                           const RecordData &Record,
                                           unsigned &Idx) {
@@ -6926,7 +6957,7 @@ void ASTReader::ReadMethodPool(Selector Sel) {
   // Get the selector generation and update it to the current generation.
   unsigned &Generation = SelectorGeneration[Sel];
   unsigned PriorGeneration = Generation;
-  Generation = CurrentGeneration;
+  Generation = getGeneration();
   
   // Search for methods defined with this selector.
   ++NumMethodPoolLookups;
@@ -8114,7 +8145,6 @@ void ASTReader::finishPendingActions() {
       if (auto RD = dyn_cast<CXXRecordDecl>(*D)) {
         for (auto R : RD->redecls())
           cast<CXXRecordDecl>(R)->DefinitionData = RD->DefinitionData;
-        
       }
 
       continue;
@@ -8250,7 +8280,7 @@ ASTReader::ASTReader(Preprocessor &PP, ASTContext &Context, StringRef isysroot,
       AllowConfigurationMismatch(AllowConfigurationMismatch),
       ValidateSystemInputs(ValidateSystemInputs),
       UseGlobalIndex(UseGlobalIndex), TriedLoadingGlobalIndex(false),
-      CurrentGeneration(0), CurrSwitchCaseStmts(&SwitchCaseStmts),
+      CurrSwitchCaseStmts(&SwitchCaseStmts),
       NumSLocEntriesRead(0), TotalNumSLocEntries(0), NumStatementsRead(0),
       TotalNumStatements(0), NumMacrosRead(0), TotalNumMacros(0),
       NumIdentifierLookups(0), NumIdentifierLookupHits(0), NumSelectorsRead(0),
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index d2219d49812..719d56edf2c 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1289,8 +1289,9 @@ void ASTDeclReader::ReadCXXDefinitionData(
 
 void ASTDeclReader::MergeDefinitionData(
     CXXRecordDecl *D, struct CXXRecordDecl::DefinitionData &MergeDD) {
-  assert(D->DefinitionData && "merging class definition into non-definition");
-  auto &DD = *D->DefinitionData;
+  assert(D->DefinitionData.getNotUpdated() &&
+         "merging class definition into non-definition");
+  auto &DD = *D->DefinitionData.getNotUpdated();
 
   // If the new definition has new special members, let the name lookup
   // code know that it needs to look in the new definition too.
@@ -1390,7 +1391,7 @@ void ASTDeclReader::ReadCXXRecordDefinition(CXXRecordDecl *D) {
 
   // If we're reading an update record, we might already have a definition for
   // this record. If so, just merge into it.
-  if (D->DefinitionData) {
+  if (D->DefinitionData.getNotUpdated()) {
     MergeDefinitionData(D, *DD);
     return;
   }
@@ -1399,25 +1400,26 @@ void ASTDeclReader::ReadCXXRecordDefinition(CXXRecordDecl *D) {
   // that all other deserialized declarations will see it.
   CXXRecordDecl *Canon = D->getCanonicalDecl();
   if (Canon == D) {
-    D->DefinitionData = DD;
+    D->DefinitionData.setNotUpdated(DD);
     D->IsCompleteDefinition = true;
-  } else if (!Canon->DefinitionData) {
-    Canon->DefinitionData = D->DefinitionData = DD;
-    D->IsCompleteDefinition = true;
-
-    // Note that we have deserialized a definition. Any declarations
-    // deserialized before this one will be be given the DefinitionData
-    // pointer at the end.
-    Reader.PendingDefinitions.insert(D);
-  } else {
+  } else if (auto *CanonDD = Canon->DefinitionData.getNotUpdated()) {
     // We have already deserialized a definition of this record. This
     // definition is no longer really a definition. Note that the pre-existing
     // definition is the *real* definition.
     Reader.MergedDeclContexts.insert(
-        std::make_pair(D, Canon->DefinitionData->Definition));
-    D->DefinitionData = D->getCanonicalDecl()->DefinitionData;
+        std::make_pair(D, CanonDD->Definition));
+    D->DefinitionData = Canon->DefinitionData;
     D->IsCompleteDefinition = false;
     MergeDefinitionData(D, *DD);
+  } else {
+    Canon->DefinitionData.setNotUpdated(DD);
+    D->DefinitionData = Canon->DefinitionData;
+    D->IsCompleteDefinition = true;
+
+    // Note that we have deserialized a definition. Any declarations
+    // deserialized before this one will be be given the DefinitionData
+    // pointer at the end.
+    Reader.PendingDefinitions.insert(D);
   }
 }
 
@@ -1741,16 +1743,16 @@ ASTDeclReader::VisitClassTemplateSpecializationDeclImpl(
 
         // This declaration might be a definition. Merge with any existing
         // definition.
-        if (D->DefinitionData) {
-          if (!CanonSpec->DefinitionData) {
-            CanonSpec->DefinitionData = D->DefinitionData;
-          } else {
-            MergeDefinitionData(CanonSpec, *D->DefinitionData);
+        if (auto *DDD = D->DefinitionData.getNotUpdated()) {
+          if (auto *CanonDD = CanonSpec->DefinitionData.getNotUpdated()) {
+            MergeDefinitionData(CanonSpec, *DDD);
             Reader.PendingDefinitions.erase(D);
             Reader.MergedDeclContexts.insert(
-                std::make_pair(D, CanonSpec->DefinitionData->Definition));
+                std::make_pair(D, CanonDD->Definition));
             D->IsCompleteDefinition = false;
             D->DefinitionData = CanonSpec->DefinitionData;
+          } else {
+            CanonSpec->DefinitionData = D->DefinitionData;
           }
         }
       }
@@ -2459,7 +2461,7 @@ ASTDeclReader::FindExistingResult ASTDeclReader::findExisting(NamedDecl *D) {
 template<typename DeclT>
 void ASTDeclReader::attachPreviousDeclImpl(Redeclarable<DeclT> *D,
                                            Decl *Previous) {
-  D->RedeclLink.setNext(cast<DeclT>(Previous));
+  D->RedeclLink.setPrevious(cast<DeclT>(Previous));
 }
 void ASTDeclReader::attachPreviousDeclImpl(...) {
   llvm_unreachable("attachPreviousDecl on non-redeclarable declaration");
@@ -2489,7 +2491,7 @@ void ASTDeclReader::attachPreviousDecl(Decl *D, Decl *Previous) {
 
 template<typename DeclT>
 void ASTDeclReader::attachLatestDeclImpl(Redeclarable<DeclT> *D, Decl *Latest) {
-  D->RedeclLink = Redeclarable<DeclT>::LatestDeclLink(cast<DeclT>(Latest));
+  D->RedeclLink.setLatest(cast<DeclT>(Latest));
 }
 void ASTDeclReader::attachLatestDeclImpl(...) {
   llvm_unreachable("attachLatestDecl on non-redeclarable declaration");
@@ -2500,8 +2502,8 @@ void ASTDeclReader::attachLatestDecl(Decl *D, Decl *Latest) {
 
   switch (D->getKind()) {
 #define ABSTRACT_DECL(TYPE)
-#define DECL(TYPE, BASE)                               \
-  case Decl::TYPE:                                     \
+#define DECL(TYPE, BASE)                                  \
+  case Decl::TYPE:                                        \
     attachLatestDeclImpl(cast<TYPE##Decl>(D), Latest); \
     break;
 #include "clang/AST/DeclNodes.inc"
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index d9bef82130a..d88d1ca99f1 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -5301,8 +5301,7 @@ void ASTWriter::AddCXXCtorInitializers(
 }
 
 void ASTWriter::AddCXXDefinitionData(const CXXRecordDecl *D, RecordDataImpl &Record) {
-  assert(D->DefinitionData);
-  struct CXXRecordDecl::DefinitionData &Data = *D->DefinitionData;
+  auto &Data = D->data();
   Record.push_back(Data.IsLambda);
   Record.push_back(Data.UserDeclaredConstructor);
   Record.push_back(Data.UserDeclaredSpecialMembers);
@@ -5361,7 +5360,7 @@ void ASTWriter::AddCXXDefinitionData(const CXXRecordDecl *D, RecordDataImpl &Rec
   
   // Add lambda-specific data.
   if (Data.IsLambda) {
-    CXXRecordDecl::LambdaDefinitionData &Lambda = D->getLambdaData();
+    auto &Lambda = D->getLambdaData();
     Record.push_back(Lambda.Dependent);
     Record.push_back(Lambda.IsGenericLambda);
     Record.push_back(Lambda.CaptureDefault);