diff --git a/clang-tools-extra/clangd/unittests/TestTU.cpp b/clang-tools-extra/clangd/unittests/TestTU.cpp
index a36f2508cd313a..1c6e54774c0361 100644
--- a/clang-tools-extra/clangd/unittests/TestTU.cpp
+++ b/clang-tools-extra/clangd/unittests/TestTU.cpp
@@ -195,7 +195,7 @@ const NamedDecl &findDecl(ParsedAST &AST, llvm::StringRef QName) {
                            llvm::StringRef Name) -> const NamedDecl & {
     auto LookupRes = Scope.lookup(DeclarationName(&Ctx.Idents.get(Name)));
     assert(!LookupRes.empty() && "Lookup failed");
-    assert(LookupRes.size() == 1 && "Lookup returned multiple results");
+    assert(LookupRes.isSingleResult() && "Lookup returned multiple results");
     return *LookupRes.front();
   };
 
diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
index af2979d8743826..d0ce47b6bbd513 100644
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -604,6 +604,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
   std::unique_ptr<interp::Context> InterpContext;
   std::unique_ptr<ParentMapContext> ParentMapCtx;
 
+  /// Keeps track of the deallocated DeclListNodes for future reuse.
+  DeclListNode *ListNodeFreeList = nullptr;
+
 public:
   IdentifierTable &Idents;
   SelectorTable &Selectors;
@@ -655,6 +658,24 @@ class ASTContext : public RefCountedBase<ASTContext> {
   }
   void Deallocate(void *Ptr) const {}
 
+  /// Allocates a \c DeclListNode or returns one from the \c ListNodeFreeList
+  /// pool.
+  DeclListNode *AllocateDeclListNode(clang::NamedDecl *ND) {
+    if (DeclListNode *Alloc = ListNodeFreeList) {
+      ListNodeFreeList = Alloc->Rest.dyn_cast<DeclListNode*>();
+      Alloc->D = ND;
+      Alloc->Rest = nullptr;
+      return Alloc;
+    }
+    return new (*this) DeclListNode(ND);
+  }
+  /// Deallcates a \c DeclListNode by returning it to the \c ListNodeFreeList
+  /// pool.
+  void DeallocateDeclListNode(DeclListNode *N) {
+    N->Rest = ListNodeFreeList;
+    ListNodeFreeList = N;
+  }
+
   /// Return the total amount of physical memory allocated for representing
   /// AST nodes and type information.
   size_t getASTAllocatedMemory() const {
diff --git a/clang/include/clang/AST/CXXInheritance.h b/clang/include/clang/AST/CXXInheritance.h
index 709f08bff82aee..946b9e318baa27 100644
--- a/clang/include/clang/AST/CXXInheritance.h
+++ b/clang/include/clang/AST/CXXInheritance.h
@@ -76,9 +76,8 @@ class CXXBasePath : public SmallVector<CXXBasePathElement, 4> {
 
   CXXBasePath() = default;
 
-  /// The set of declarations found inside this base class
-  /// subobject.
-  DeclContext::lookup_result Decls;
+  /// The declarations found inside this base class subobject.
+  DeclContext::lookup_iterator Decls;
 
   void clear() {
     SmallVectorImpl<CXXBasePathElement>::clear();
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index 47c282f0a63dda..66dda5c9761ae2 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -579,6 +579,16 @@ class NamespaceDecl : public NamedDecl, public DeclContext,
     AnonOrFirstNamespaceAndInline.setInt(Inline);
   }
 
+  /// Returns true if the inline qualifier for \c Name is redundant.
+  bool isRedundantInlineQualifierFor(DeclarationName Name) const {
+    if (!isInline())
+      return false;
+    auto X = lookup(Name);
+    auto Y = getParent()->lookup(Name);
+    return std::distance(X.begin(), X.end()) ==
+      std::distance(Y.begin(), Y.end());
+  }
+
   /// Get the original (first) namespace declaration.
   NamespaceDecl *getOriginalNamespace();
 
diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
index 15eb29f725397f..084ecb5389cee8 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -1220,65 +1220,110 @@ class PrettyStackTraceDecl : public llvm::PrettyStackTraceEntry {
 
   void print(raw_ostream &OS) const override;
 };
+} // namespace clang
 
-/// The results of name lookup within a DeclContext. This is either a
-/// single result (with no stable storage) or a collection of results (with
-/// stable storage provided by the lookup table).
-class DeclContextLookupResult {
-  using ResultTy = ArrayRef<NamedDecl *>;
-
-  ResultTy Result;
-
-  // If there is only one lookup result, it would be invalidated by
-  // reallocations of the name table, so store it separately.
-  NamedDecl *Single = nullptr;
-
-  static NamedDecl *const SingleElementDummyList;
+// Required to determine the layout of the PointerUnion<NamedDecl*> before
+// seeing the NamedDecl definition being first used in DeclListNode::operator*.
+namespace llvm {
+  template <> struct PointerLikeTypeTraits<::clang::NamedDecl *> {
+    static inline void *getAsVoidPointer(::clang::NamedDecl *P) { return P; }
+    static inline ::clang::NamedDecl *getFromVoidPointer(void *P) {
+      return static_cast<::clang::NamedDecl *>(P);
+    }
+    static constexpr int NumLowBitsAvailable = 3;
+  };
+}
 
+namespace clang {
+/// A list storing NamedDecls in the lookup tables.
+class DeclListNode {
+  friend class ASTContext; // allocate, deallocate nodes.
+  friend class StoredDeclsList;
 public:
-  DeclContextLookupResult() = default;
-  DeclContextLookupResult(ArrayRef<NamedDecl *> Result)
-      : Result(Result) {}
-  DeclContextLookupResult(NamedDecl *Single)
-      : Result(SingleElementDummyList), Single(Single) {}
-
-  class iterator;
-
-  using IteratorBase =
-      llvm::iterator_adaptor_base<iterator, ResultTy::iterator,
-                                  std::random_access_iterator_tag, NamedDecl *>;
-
-  class iterator : public IteratorBase {
-    value_type SingleElement;
+  using Decls = llvm::PointerUnion<NamedDecl*, DeclListNode*>;
+  class iterator {
+    friend class DeclContextLookupResult;
+    friend class StoredDeclsList;
 
+    Decls Ptr;
+    iterator(Decls Node) : Ptr(Node) { }
   public:
-    explicit iterator(pointer Pos, value_type Single = nullptr)
-        : IteratorBase(Pos), SingleElement(Single) {}
+    using difference_type = ptrdiff_t;
+    using value_type = NamedDecl*;
+    using pointer = void;
+    using reference = value_type;
+    using iterator_category = std::forward_iterator_tag;
+
+    iterator() = default;
 
     reference operator*() const {
-      return SingleElement ? SingleElement : IteratorBase::operator*();
+      assert(Ptr && "dereferencing end() iterator");
+      if (DeclListNode *CurNode = Ptr.dyn_cast<DeclListNode*>())
+        return CurNode->D;
+      return Ptr.get<NamedDecl*>();
     }
+    void operator->() const { } // Unsupported.
+    bool operator==(const iterator &X) const { return Ptr == X.Ptr; }
+    bool operator!=(const iterator &X) const { return Ptr != X.Ptr; }
+    inline iterator &operator++() { // ++It
+      assert(!Ptr.isNull() && "Advancing empty iterator");
+
+      if (DeclListNode *CurNode = Ptr.dyn_cast<DeclListNode*>())
+        Ptr = CurNode->Rest;
+      else
+        Ptr = nullptr;
+      return *this;
+    }
+    iterator operator++(int) { // It++
+      iterator temp = *this;
+      ++(*this);
+      return temp;
+    }
+    // Enables the pattern for (iterator I =..., E = I.end(); I != E; ++I)
+    iterator end() { return iterator(); }
   };
+private:
+  NamedDecl *D = nullptr;
+  Decls Rest = nullptr;
+  DeclListNode(NamedDecl *ND) : D(ND) {}
+};
+
+/// The results of name lookup within a DeclContext.
+class DeclContextLookupResult {
+  using Decls = DeclListNode::Decls;
 
+  /// When in collection form, this is what the Data pointer points to.
+  Decls Result;
+
+public:
+  DeclContextLookupResult() = default;
+  DeclContextLookupResult(Decls Result) : Result(Result) {}
+
+  using iterator = DeclListNode::iterator;
   using const_iterator = iterator;
-  using pointer = iterator::pointer;
   using reference = iterator::reference;
 
-  iterator begin() const { return iterator(Result.begin(), Single); }
-  iterator end() const { return iterator(Result.end(), Single); }
-
-  bool empty() const { return Result.empty(); }
-  pointer data() const { return Single ? &Single : Result.data(); }
-  size_t size() const { return Single ? 1 : Result.size(); }
-  reference front() const { return Single ? Single : Result.front(); }
-  reference back() const { return Single ? Single : Result.back(); }
-  reference operator[](size_t N) const { return Single ? Single : Result[N]; }
-
-  // FIXME: Remove this from the interface
-  DeclContextLookupResult slice(size_t N) const {
-    DeclContextLookupResult Sliced = Result.slice(N);
-    Sliced.Single = Single;
-    return Sliced;
+  iterator begin() { return iterator(Result); }
+  iterator end() { return iterator(); }
+  const_iterator begin() const {
+    return const_cast<DeclContextLookupResult*>(this)->begin();
+  }
+  const_iterator end() const { return iterator(); }
+
+  bool empty() const { return Result.isNull();  }
+  bool isSingleResult() const { return Result.dyn_cast<NamedDecl*>(); }
+  reference front() const { return *begin(); }
+
+  // Find the first declaration of the given type in the list. Note that this
+  // is not in general the earliest-declared declaration, and should only be
+  // used when it's not possible for there to be more than one match or where
+  // it doesn't matter which one is found.
+  template<class T> T *find_first() const {
+    for (auto *D : *this)
+      if (T *Decl = dyn_cast<T>(D))
+        return Decl;
+
+    return nullptr;
   }
 };
 
diff --git a/clang/include/clang/AST/DeclContextInternals.h b/clang/include/clang/AST/DeclContextInternals.h
index e6a4cd4381e4aa..3556044098c466 100644
--- a/clang/include/clang/AST/DeclContextInternals.h
+++ b/clang/include/clang/AST/DeclContextInternals.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_AST_DECLCONTEXTINTERNALS_H
 #define LLVM_CLANG_AST_DECLCONTEXTINTERNALS_H
 
+#include "clang/AST/ASTContext.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
@@ -21,7 +22,6 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/PointerUnion.h"
-#include "llvm/ADT/SmallVector.h"
 #include <algorithm>
 #include <cassert>
 
@@ -31,231 +31,287 @@ class DependentDiagnostic;
 
 /// An array of decls optimized for the common case of only containing
 /// one entry.
-struct StoredDeclsList {
-  /// When in vector form, this is what the Data pointer points to.
-  using DeclsTy = SmallVector<NamedDecl *, 4>;
+class StoredDeclsList {
+  using Decls = DeclListNode::Decls;
 
   /// A collection of declarations, with a flag to indicate if we have
   /// further external declarations.
-  using DeclsAndHasExternalTy = llvm::PointerIntPair<DeclsTy *, 1, bool>;
+  using DeclsAndHasExternalTy = llvm::PointerIntPair<Decls, 1, bool>;
 
   /// The stored data, which will be either a pointer to a NamedDecl,
-  /// or a pointer to a vector with a flag to indicate if there are further
+  /// or a pointer to a list with a flag to indicate if there are further
   /// external declarations.
-  llvm::PointerUnion<NamedDecl *, DeclsAndHasExternalTy> Data;
+  DeclsAndHasExternalTy Data;
+
+  template<typename Fn>
+  void erase_if(Fn ShouldErase) {
+    Decls List = Data.getPointer();
+    if (!List)
+      return;
+    ASTContext &C = getASTContext();
+    DeclListNode::Decls NewHead = nullptr;
+    DeclListNode::Decls *NewLast = nullptr;
+    DeclListNode::Decls *NewTail = &NewHead;
+    while (true) {
+      if (!ShouldErase(*DeclListNode::iterator(List))) {
+        NewLast = NewTail;
+        *NewTail = List;
+        if (auto *Node = List.dyn_cast<DeclListNode*>()) {
+          NewTail = &Node->Rest;
+          List = Node->Rest;
+        } else {
+          break;
+        }
+      } else if (DeclListNode *N = List.dyn_cast<DeclListNode*>()) {
+        List = N->Rest;
+        C.DeallocateDeclListNode(N);
+      } else {
+        // We're discarding the last declaration in the list. The last node we
+        // want to keep (if any) will be of the form DeclListNode(D, <rest>);
+        // replace it with just D.
+        if (NewLast) {
+          DeclListNode *Node = NewLast->get<DeclListNode*>();
+          *NewLast = Node->D;
+          C.DeallocateDeclListNode(Node);
+        }
+        break;
+      }
+    }
+    Data.setPointer(NewHead);
+
+    assert(llvm::find_if(getLookupResult(), ShouldErase) ==
+           getLookupResult().end() && "Still exists!");
+  }
+
+  void erase(NamedDecl *ND) {
+    erase_if([ND](NamedDecl *D) { return D == ND; });
+  }
 
 public:
   StoredDeclsList() = default;
 
   StoredDeclsList(StoredDeclsList &&RHS) : Data(RHS.Data) {
-    RHS.Data = (NamedDecl *)nullptr;
+    RHS.Data.setPointer(nullptr);
+    RHS.Data.setInt(0);
+  }
+
+  void MaybeDeallocList() {
+    if (isNull())
+      return;
+    // If this is a list-form, free the list.
+    ASTContext &C = getASTContext();
+    Decls List = Data.getPointer();
+    while (DeclListNode *ToDealloc = List.dyn_cast<DeclListNode *>()) {
+      List = ToDealloc->Rest;
+      C.DeallocateDeclListNode(ToDealloc);
+    }
   }
 
   ~StoredDeclsList() {
-    // If this is a vector-form, free the vector.
-    if (DeclsTy *Vector = getAsVector())
-      delete Vector;
+    MaybeDeallocList();
   }
 
   StoredDeclsList &operator=(StoredDeclsList &&RHS) {
-    if (DeclsTy *Vector = getAsVector())
-      delete Vector;
+    MaybeDeallocList();
+
     Data = RHS.Data;
-    RHS.Data = (NamedDecl *)nullptr;
+    RHS.Data.setPointer(nullptr);
+    RHS.Data.setInt(0);
     return *this;
   }
 
-  bool isNull() const { return Data.isNull(); }
+  bool isNull() const { return Data.getPointer().isNull(); }
 
-  NamedDecl *getAsDecl() const {
-    return Data.dyn_cast<NamedDecl *>();
+  ASTContext &getASTContext() {
+    assert(!isNull() && "No ASTContext.");
+    if (NamedDecl *ND = getAsDecl())
+      return ND->getASTContext();
+    return getAsList()->D->getASTContext();
   }
 
-  DeclsAndHasExternalTy getAsVectorAndHasExternal() const {
-    return Data.dyn_cast<DeclsAndHasExternalTy>();
+  DeclsAndHasExternalTy getAsListAndHasExternal() const { return Data; }
+
+  NamedDecl *getAsDecl() const {
+    return getAsListAndHasExternal().getPointer().dyn_cast<NamedDecl *>();
   }
 
-  DeclsTy *getAsVector() const {
-    return getAsVectorAndHasExternal().getPointer();
+  DeclListNode *getAsList() const {
+    return getAsListAndHasExternal().getPointer().dyn_cast<DeclListNode*>();
   }
 
   bool hasExternalDecls() const {
-    return getAsVectorAndHasExternal().getInt();
+    return getAsListAndHasExternal().getInt();
   }
 
   void setHasExternalDecls() {
-    if (DeclsTy *Vec = getAsVector())
-      Data = DeclsAndHasExternalTy(Vec, true);
-    else {
-      DeclsTy *VT = new DeclsTy();
-      if (NamedDecl *OldD = getAsDecl())
-        VT->push_back(OldD);
-      Data = DeclsAndHasExternalTy(VT, true);
-    }
-  }
-
-  void setOnlyValue(NamedDecl *ND) {
-    assert(!getAsVector() && "Not inline");
-    Data = ND;
-    // Make sure that Data is a plain NamedDecl* so we can use its address
-    // at getLookupResult.
-    assert(*(NamedDecl **)&Data == ND &&
-           "PointerUnion mangles the NamedDecl pointer!");
+    Data.setInt(1);
   }
 
   void remove(NamedDecl *D) {
     assert(!isNull() && "removing from empty list");
-    if (NamedDecl *Singleton = getAsDecl()) {
-      assert(Singleton == D && "list is different singleton");
-      (void)Singleton;
-      Data = (NamedDecl *)nullptr;
-      return;
-    }
-
-    DeclsTy &Vec = *getAsVector();
-    DeclsTy::iterator I = llvm::find(Vec, D);
-    assert(I != Vec.end() && "list does not contain decl");
-    Vec.erase(I);
-
-    assert(llvm::find(Vec, D) == Vec.end() && "list still contains decl");
+    erase(D);
   }
 
-  /// Remove any declarations which were imported from an external
-  /// AST source.
+  /// Remove any declarations which were imported from an external AST source.
   void removeExternalDecls() {
-    if (isNull()) {
-      // Nothing to do.
-    } else if (NamedDecl *Singleton = getAsDecl()) {
-      if (Singleton->isFromASTFile())
-        *this = StoredDeclsList();
-    } else {
-      DeclsTy &Vec = *getAsVector();
-      Vec.erase(std::remove_if(Vec.begin(), Vec.end(),
-                               [](Decl *D) { return D->isFromASTFile(); }),
-                Vec.end());
-      // Don't have any external decls any more.
-      Data = DeclsAndHasExternalTy(&Vec, false);
-    }
+    erase_if([](NamedDecl *ND) { return ND->isFromASTFile(); });
+
+    // Don't have any pending external decls any more.
+    Data.setInt(0);
   }
 
-  /// getLookupResult - Return an array of all the decls that this list
-  /// represents.
-  DeclContext::lookup_result getLookupResult() {
-    if (isNull())
-      return DeclContext::lookup_result();
+  void replaceExternalDecls(ArrayRef<NamedDecl*> Decls) {
+    // Remove all declarations that are either external or are replaced with
+    // external declarations.
+    erase_if([Decls](NamedDecl *ND) {
+      if (ND->isFromASTFile())
+        return true;
+      for (NamedDecl *D : Decls)
+        if (D->declarationReplaces(ND, /*IsKnownNewer=*/false))
+          return true;
+      return false;
+    });
 
-    // If we have a single NamedDecl, return it.
-    if (NamedDecl *ND = getAsDecl()) {
-      assert(!isNull() && "Empty list isn't allowed");
+    // Don't have any pending external decls any more.
+    Data.setInt(0);
+
+    if (Decls.empty())
+      return;
+
+    // Convert Decls into a list, in order.
+    ASTContext &C = Decls.front()->getASTContext();
+    DeclListNode::Decls DeclsAsList = Decls.back();
+    for (size_t I = Decls.size() - 1; I != 0; --I) {
+      DeclListNode *Node = C.AllocateDeclListNode(Decls[I - 1]);
+      Node->Rest = DeclsAsList;
+      DeclsAsList = Node;
+    }
 
-      // Data is a raw pointer to a NamedDecl*, return it.
-      return DeclContext::lookup_result(ND);
+    DeclListNode::Decls Head = Data.getPointer();
+    if (Head.isNull()) {
+      Data.setPointer(DeclsAsList);
+      return;
     }
 
-    assert(getAsVector() && "Must have a vector at this point");
-    DeclsTy &Vector = *getAsVector();
+    // Find the end of the existing list.
+    // FIXME: It would be possible to preserve information from erase_if to
+    // avoid this rescan looking for the end of the list.
+    DeclListNode::Decls *Tail = &Head;
+    while (DeclListNode *Node = Tail->dyn_cast<DeclListNode *>())
+      Tail = &Node->Rest;
+
+    // Append the Decls.
+    DeclListNode *Node = C.AllocateDeclListNode(Tail->get<NamedDecl *>());
+    Node->Rest = DeclsAsList;
+    *Tail = Node;
+    Data.setPointer(Head);
+  }
 
-    // Otherwise, we have a range result.
-    return DeclContext::lookup_result(Vector);
+  /// Return an array of all the decls that this list represents.
+  DeclContext::lookup_result getLookupResult() const {
+    return DeclContext::lookup_result(Data.getPointer());
   }
 
-  /// HandleRedeclaration - If this is a redeclaration of an existing decl,
-  /// replace the old one with D and return true.  Otherwise return false.
-  bool HandleRedeclaration(NamedDecl *D, bool IsKnownNewer) {
+  /// If this is a redeclaration of an existing decl, replace the old one with
+  /// D. Otherwise, append D.
+  void addOrReplaceDecl(NamedDecl *D) {
+    const bool IsKnownNewer = true;
+
+    if (isNull()) {
+      Data.setPointer(D);
+      return;
+    }
+
     // Most decls only have one entry in their list, special case it.
     if (NamedDecl *OldD = getAsDecl()) {
-      if (!D->declarationReplaces(OldD, IsKnownNewer))
-        return false;
-      setOnlyValue(D);
-      return true;
+      if (D->declarationReplaces(OldD, IsKnownNewer)) {
+        Data.setPointer(D);
+        return;
+      }
+
+      // Add D after OldD.
+      ASTContext &C = D->getASTContext();
+      DeclListNode *Node = C.AllocateDeclListNode(OldD);
+      Node->Rest = D;
+      Data.setPointer(Node);
+      return;
     }
 
+    // FIXME: Move the assert before the single decl case when we fix the
+    // duplication coming from the ASTReader reading builtin types.
+    assert(!llvm::is_contained(getLookupResult(), D) && "Already exists!");
     // Determine if this declaration is actually a redeclaration.
-    DeclsTy &Vec = *getAsVector();
-    for (DeclsTy::iterator OD = Vec.begin(), ODEnd = Vec.end();
-         OD != ODEnd; ++OD) {
-      NamedDecl *OldD = *OD;
-      if (D->declarationReplaces(OldD, IsKnownNewer)) {
-        *OD = D;
-        return true;
+    for (DeclListNode *N = getAsList(); /*return in loop*/;
+         N = N->Rest.dyn_cast<DeclListNode *>()) {
+      if (D->declarationReplaces(N->D, IsKnownNewer)) {
+        N->D = D;
+        return;
+      }
+      if (auto *ND = N->Rest.dyn_cast<NamedDecl *>()) {
+        if (D->declarationReplaces(ND, IsKnownNewer)) {
+          N->Rest = D;
+          return;
+        }
+
+        // Add D after ND.
+        ASTContext &C = D->getASTContext();
+        DeclListNode *Node = C.AllocateDeclListNode(ND);
+        N->Rest = Node;
+        Node->Rest = D;
+        return;
       }
     }
-
-    return false;
   }
 
-  /// AddSubsequentDecl - This is called on the second and later decl when it is
-  /// not a redeclaration to merge it into the appropriate place in our list.
-  void AddSubsequentDecl(NamedDecl *D) {
-    assert(!isNull() && "don't AddSubsequentDecl when we have no decls");
+  /// Add a declaration to the list without checking if it replaces anything.
+  void prependDeclNoReplace(NamedDecl *D) {
+    if (isNull()) {
+      Data.setPointer(D);
+      return;
+    }
 
-    // If this is the second decl added to the list, convert this to vector
-    // form.
-    if (NamedDecl *OldD = getAsDecl()) {
-      DeclsTy *VT = new DeclsTy();
-      VT->push_back(OldD);
-      Data = DeclsAndHasExternalTy(VT, false);
+    ASTContext &C = D->getASTContext();
+    DeclListNode *Node = C.AllocateDeclListNode(D);
+    Node->Rest = Data.getPointer();
+    Data.setPointer(Node);
+  }
+
+  LLVM_DUMP_METHOD void dump() const {
+    Decls D = Data.getPointer();
+    if (!D) {
+      llvm::errs() << "<null>\n";
+      return;
     }
 
-    DeclsTy &Vec = *getAsVector();
-
-    // Using directives end up in a special entry which contains only
-    // other using directives, so all this logic is wasted for them.
-    // But avoiding the logic wastes time in the far-more-common case
-    // that we're *not* adding a new using directive.
-
-    // Tag declarations always go at the end of the list so that an
-    // iterator which points at the first tag will start a span of
-    // decls that only contains tags.
-    if (D->hasTagIdentifierNamespace())
-      Vec.push_back(D);
-
-    // Resolved using declarations go at the front of the list so that
-    // they won't show up in other lookup results.  Unresolved using
-    // declarations (which are always in IDNS_Using | IDNS_Ordinary)
-    // follow that so that the using declarations will be contiguous.
-    else if (D->getIdentifierNamespace() & Decl::IDNS_Using) {
-      DeclsTy::iterator I = Vec.begin();
-      if (D->getIdentifierNamespace() != Decl::IDNS_Using) {
-        while (I != Vec.end() &&
-               (*I)->getIdentifierNamespace() == Decl::IDNS_Using)
-          ++I;
+    while (true) {
+      if (auto *Node = D.dyn_cast<DeclListNode*>()) {
+        llvm::errs() << '[' << Node->D << "] -> ";
+        D = Node->Rest;
+      } else {
+        llvm::errs() << '[' << D.get<NamedDecl*>() << "]\n";
+        return;
       }
-      Vec.insert(I, D);
-
-    // All other declarations go at the end of the list, but before any
-    // tag declarations.  But we can be clever about tag declarations
-    // because there can only ever be one in a scope.
-    } else if (!Vec.empty() && Vec.back()->hasTagIdentifierNamespace()) {
-      NamedDecl *TagD = Vec.back();
-      Vec.back() = D;
-      Vec.push_back(TagD);
-    } else
-      Vec.push_back(D);
+    }
   }
 };
 
 class StoredDeclsMap
     : public llvm::SmallDenseMap<DeclarationName, StoredDeclsList, 4> {
-public:
-  static void DestroyAll(StoredDeclsMap *Map, bool Dependent);
-
-private:
   friend class ASTContext; // walks the chain deleting these
   friend class DeclContext;
 
   llvm::PointerIntPair<StoredDeclsMap*, 1> Previous;
+public:
+  static void DestroyAll(StoredDeclsMap *Map, bool Dependent);
 };
 
 class DependentStoredDeclsMap : public StoredDeclsMap {
-public:
-  DependentStoredDeclsMap() = default;
-
-private:
   friend class DeclContext; // iterates over diagnostics
   friend class DependentDiagnostic;
 
   DependentDiagnostic *FirstDiagnostic = nullptr;
+public:
+  DependentStoredDeclsMap() = default;
 };
 
 } // namespace clang
diff --git a/clang/include/clang/Serialization/ASTWriter.h b/clang/include/clang/Serialization/ASTWriter.h
index 12073a38a77a51..ea67d6990a8a0b 100644
--- a/clang/include/clang/Serialization/ASTWriter.h
+++ b/clang/include/clang/Serialization/ASTWriter.h
@@ -84,7 +84,7 @@ class RecordDecl;
 class Sema;
 class SourceManager;
 class Stmt;
-struct StoredDeclsList;
+class StoredDeclsList;
 class SwitchCase;
 class TemplateParameterList;
 class Token;
diff --git a/clang/lib/ARCMigrate/ObjCMT.cpp b/clang/lib/ARCMigrate/ObjCMT.cpp
index 68a51a49c71823..c8069b51567c2e 100644
--- a/clang/lib/ARCMigrate/ObjCMT.cpp
+++ b/clang/lib/ARCMigrate/ObjCMT.cpp
@@ -613,7 +613,7 @@ ClassImplementsAllMethodsAndProperties(ASTContext &Ctx,
         continue;
       HasAtleastOneRequiredProperty = true;
       DeclContext::lookup_result R = IDecl->lookup(Property->getDeclName());
-      if (R.size() == 0) {
+      if (R.empty()) {
         // Relax the rule and look into class's implementation for a synthesize
         // or dynamic declaration. Class is implementing a property coming from
         // another protocol. This still makes the target protocol as conforming.
@@ -621,14 +621,12 @@ ClassImplementsAllMethodsAndProperties(ASTContext &Ctx,
                                   Property->getDeclName().getAsIdentifierInfo(),
                                   Property->getQueryKind()))
           return false;
-      }
-      else if (ObjCPropertyDecl *ClassProperty = dyn_cast<ObjCPropertyDecl>(R[0])) {
-          if ((ClassProperty->getPropertyAttributes()
-              != Property->getPropertyAttributes()) ||
-              !Ctx.hasSameType(ClassProperty->getType(), Property->getType()))
-            return false;
-      }
-      else
+      } else if (auto *ClassProperty = R.find_first<ObjCPropertyDecl>()) {
+        if ((ClassProperty->getPropertyAttributes() !=
+             Property->getPropertyAttributes()) ||
+            !Ctx.hasSameType(ClassProperty->getType(), Property->getType()))
+          return false;
+      } else
         return false;
     }
 
@@ -645,12 +643,12 @@ ClassImplementsAllMethodsAndProperties(ASTContext &Ctx,
       if (MD->getImplementationControl() == ObjCMethodDecl::Optional)
         continue;
       DeclContext::lookup_result R = ImpDecl->lookup(MD->getDeclName());
-      if (R.size() == 0)
+      if (R.empty())
         return false;
       bool match = false;
       HasAtleastOneRequiredMethod = true;
-      for (unsigned I = 0, N = R.size(); I != N; ++I)
-        if (ObjCMethodDecl *ImpMD = dyn_cast<ObjCMethodDecl>(R[0]))
+      for (NamedDecl *ND : R)
+        if (ObjCMethodDecl *ImpMD = dyn_cast<ObjCMethodDecl>(ND))
           if (Ctx.ObjCMethodsAreEqual(MD, ImpMD)) {
             match = true;
             break;
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index b8231f66908a91..d52aea88e092a1 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -10892,6 +10892,9 @@ void ASTContext::forEachMultiversionedFunctionVersion(
   assert(FD->isMultiVersion() && "Only valid for multiversioned functions");
   llvm::SmallDenseSet<const FunctionDecl*, 4> SeenDecls;
   FD = FD->getMostRecentDecl();
+  // FIXME: The order of traversal here matters and depends on the order of
+  // lookup results, which happens to be (mostly) oldest-to-newest, but we
+  // shouldn't rely on that.
   for (auto *CurDecl :
        FD->getDeclContext()->getRedeclContext()->lookup(FD->getDeclName())) {
     FunctionDecl *CurFD = CurDecl->getAsFunction()->getMostRecentDecl();
diff --git a/clang/lib/AST/CXXInheritance.cpp b/clang/lib/AST/CXXInheritance.cpp
index c87bcf31d1209c..9027fa7a7515d3 100644
--- a/clang/lib/AST/CXXInheritance.cpp
+++ b/clang/lib/AST/CXXInheritance.cpp
@@ -386,9 +386,9 @@ static bool isOrdinaryMember(const NamedDecl *ND) {
 
 static bool findOrdinaryMember(const CXXRecordDecl *RD, CXXBasePath &Path,
                                DeclarationName Name) {
-  Path.Decls = RD->lookup(Name);
-  for (NamedDecl *ND : Path.Decls)
-    if (isOrdinaryMember(ND))
+  Path.Decls = RD->lookup(Name).begin();
+  for (DeclContext::lookup_iterator I = Path.Decls, E = I.end(); I != E; ++I)
+    if (isOrdinaryMember(*I))
       return true;
 
   return false;
@@ -453,9 +453,10 @@ std::vector<const NamedDecl *> CXXRecordDecl::lookupDependentName(
           },
           Paths, /*LookupInDependent=*/true))
     return Results;
-  for (const NamedDecl *ND : Paths.front().Decls) {
-    if (isOrdinaryMember(ND) && Filter(ND))
-      Results.push_back(ND);
+  for (DeclContext::lookup_iterator I = Paths.front().Decls, E = I.end();
+       I != E; ++I) {
+    if (isOrdinaryMember(*I) && Filter(*I))
+      Results.push_back(*I);
   }
   return Results;
 }
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index cae092ac369d03..fcda07dab6045a 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1612,8 +1612,7 @@ void NamedDecl::printNestedNameSpecifier(raw_ostream &OS,
 
     // Suppress inline namespace if it doesn't make the result ambiguous.
     if (P.SuppressInlineNamespace && Ctx->isInlineNamespace() && NameInScope &&
-        Ctx->lookup(NameInScope).size() ==
-            Ctx->getParent()->lookup(NameInScope).size())
+        cast<NamespaceDecl>(Ctx)->isRedundantInlineQualifierFor(NameInScope))
       continue;
 
     // Skip non-named contexts such as linkage specifications and ExportDecls.
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index c26d6d1a42ea03..6d438cf05590af 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -1394,39 +1394,7 @@ ExternalASTSource::SetExternalVisibleDeclsForName(const DeclContext *DC,
     DC->reconcileExternalVisibleStorage();
 
   StoredDeclsList &List = (*Map)[Name];
-
-  // Clear out any old external visible declarations, to avoid quadratic
-  // performance in the redeclaration checks below.
-  List.removeExternalDecls();
-
-  if (!List.isNull()) {
-    // We have both existing declarations and new declarations for this name.
-    // Some of the declarations may simply replace existing ones. Handle those
-    // first.
-    llvm::SmallVector<unsigned, 8> Skip;
-    for (unsigned I = 0, N = Decls.size(); I != N; ++I)
-      if (List.HandleRedeclaration(Decls[I], /*IsKnownNewer*/false))
-        Skip.push_back(I);
-    Skip.push_back(Decls.size());
-
-    // Add in any new declarations.
-    unsigned SkipPos = 0;
-    for (unsigned I = 0, N = Decls.size(); I != N; ++I) {
-      if (I == Skip[SkipPos])
-        ++SkipPos;
-      else
-        List.AddSubsequentDecl(Decls[I]);
-    }
-  } else {
-    // Convert the array to a StoredDeclsList.
-    for (auto *D : Decls) {
-      if (List.isNull())
-        List.setOnlyValue(D);
-      else
-        List.AddSubsequentDecl(D);
-    }
-  }
-
+  List.replaceExternalDecls(Decls);
   return List.getLookupResult();
 }
 
@@ -1538,10 +1506,7 @@ void DeclContext::removeDecl(Decl *D) {
       if (Map) {
         StoredDeclsMap::iterator Pos = Map->find(ND->getDeclName());
         assert(Pos != Map->end() && "no lookup entry for decl");
-        // Remove the decl only if it is contained.
-        StoredDeclsList::DeclsTy *Vec = Pos->second.getAsVector();
-        if ((Vec && is_contained(*Vec, ND)) || Pos->second.getAsDecl() == ND)
-          Pos->second.remove(ND);
+        Pos->second.remove(ND);
       }
     } while (DC->isTransparentContext() && (DC = DC->getParent()));
   }
@@ -1658,8 +1623,6 @@ void DeclContext::buildLookupImpl(DeclContext *DCtx, bool Internal) {
   }
 }
 
-NamedDecl *const DeclContextLookupResult::SingleElementDummyList = nullptr;
-
 DeclContext::lookup_result
 DeclContext::lookup(DeclarationName Name) const {
   assert(getDeclKind() != Decl::LinkageSpec &&
@@ -1935,23 +1898,11 @@ void DeclContext::makeDeclVisibleInContextImpl(NamedDecl *D, bool Internal) {
     // In this case, we never try to replace an existing declaration; we'll
     // handle that when we finalize the list of declarations for this name.
     DeclNameEntries.setHasExternalDecls();
-    DeclNameEntries.AddSubsequentDecl(D);
-    return;
-  }
-
-  if (DeclNameEntries.isNull()) {
-    DeclNameEntries.setOnlyValue(D);
-    return;
-  }
-
-  if (DeclNameEntries.HandleRedeclaration(D, /*IsKnownNewer*/!Internal)) {
-    // This declaration has replaced an existing one for which
-    // declarationReplaces returns true.
+    DeclNameEntries.prependDeclNoReplace(D);
     return;
   }
 
-  // Put this declaration into the appropriate slot.
-  DeclNameEntries.AddSubsequentDecl(D);
+  DeclNameEntries.addOrReplaceDecl(D);
 }
 
 UsingDirectiveDecl *DeclContext::udir_iterator::operator*() const {
diff --git a/clang/lib/AST/ExternalASTMerger.cpp b/clang/lib/AST/ExternalASTMerger.cpp
index 88bbe90a4e909b..c7789b707b2199 100644
--- a/clang/lib/AST/ExternalASTMerger.cpp
+++ b/clang/lib/AST/ExternalASTMerger.cpp
@@ -64,24 +64,24 @@ LookupSameContext(Source<TranslationUnitDecl *> SourceTU, const DeclContext *DC,
   Source<DeclarationName> SourceName = *SourceNameOrErr;
   DeclContext::lookup_result SearchResult =
       SourceParentDC.get()->lookup(SourceName.get());
-  size_t SearchResultSize = SearchResult.size();
-  if (SearchResultSize == 0 || SearchResultSize > 1) {
-    // There are two cases here.  First, we might not find the name.
-    // We might also find multiple copies, in which case we have no
-    // guarantee that the one we wanted is the one we pick.  (E.g.,
-    // if we have two specializations of the same template it is
-    // very hard to determine which is the one you want.)
-    //
-    // The Origins map fixes this problem by allowing the origin to be
-    // explicitly recorded, so we trigger that recording by returning
-    // nothing (rather than a possibly-inaccurate guess) here.
-    return nullptr;
-  } else {
-    NamedDecl *SearchResultDecl = SearchResult[0];
+
+  // There are two cases here. First, we might not find the name.
+  // We might also find multiple copies, in which case we have no
+  // guarantee that the one we wanted is the one we pick.  (E.g.,
+  // if we have two specializations of the same template it is
+  // very hard to determine which is the one you want.)
+  //
+  // The Origins map fixes this problem by allowing the origin to be
+  // explicitly recorded, so we trigger that recording by returning
+  // nothing (rather than a possibly-inaccurate guess) here.
+  if (SearchResult.isSingleResult()) {
+    NamedDecl *SearchResultDecl = SearchResult.front();
     if (isa<DeclContext>(SearchResultDecl) &&
         SearchResultDecl->getKind() == DC->getDeclKind())
       return cast<DeclContext>(SearchResultDecl)->getPrimaryContext();
     return nullptr; // This type of lookup is unsupported
+  } else {
+    return nullptr;
   }
 }
 
diff --git a/clang/lib/AST/TypePrinter.cpp b/clang/lib/AST/TypePrinter.cpp
index 3e8e94300caeb4..17e9b3f6a023d6 100644
--- a/clang/lib/AST/TypePrinter.cpp
+++ b/clang/lib/AST/TypePrinter.cpp
@@ -1235,8 +1235,7 @@ void TypePrinter::AppendScope(DeclContext *DC, raw_ostream &OS,
     // Only suppress an inline namespace if the name has the same lookup
     // results in the enclosing namespace.
     if (Policy.SuppressInlineNamespace && NS->isInline() && NameInScope &&
-        DC->getParent()->lookup(NameInScope).size() ==
-            DC->lookup(NameInScope).size())
+        NS->isRedundantInlineQualifierFor(NameInScope))
       return AppendScope(DC->getParent(), OS, NameInScope);
 
     AppendScope(DC->getParent(), OS, NS->getDeclName());
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index fc342bbdbd711e..4bca21a51f9c74 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -4673,7 +4673,6 @@ class CodeGenFunction : public CodeGenTypeCache {
 
   struct MultiVersionResolverOption {
     llvm::Function *Function;
-    FunctionDecl *FD;
     struct Conds {
       StringRef Architecture;
       llvm::SmallVector<StringRef, 8> Features;
diff --git a/clang/lib/Sema/MultiplexExternalSemaSource.cpp b/clang/lib/Sema/MultiplexExternalSemaSource.cpp
index 252008cda15d20..3a993e24b134de 100644
--- a/clang/lib/Sema/MultiplexExternalSemaSource.cpp
+++ b/clang/lib/Sema/MultiplexExternalSemaSource.cpp
@@ -10,7 +10,6 @@
 //
 //===----------------------------------------------------------------------===//
 #include "clang/Sema/MultiplexExternalSemaSource.h"
-#include "clang/AST/DeclContextInternals.h"
 #include "clang/Sema/Lookup.h"
 
 using namespace clang;
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 0365d77cfc4e3a..10f61d8c649e3e 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -4130,13 +4130,9 @@ ValueDecl *Sema::tryLookupCtorInitMemberDecl(CXXRecordDecl *ClassDecl,
                                              IdentifierInfo *MemberOrBase) {
   if (SS.getScopeRep() || TemplateTypeTy)
     return nullptr;
-  DeclContext::lookup_result Result = ClassDecl->lookup(MemberOrBase);
-  if (Result.empty())
-    return nullptr;
-  ValueDecl *Member;
-  if ((Member = dyn_cast<FieldDecl>(Result.front())) ||
-      (Member = dyn_cast<IndirectFieldDecl>(Result.front())))
-    return Member;
+  for (auto *D : ClassDecl->lookup(MemberOrBase))
+    if (isa<FieldDecl>(D) || isa<IndirectFieldDecl>(D))
+      return cast<ValueDecl>(D);
   return nullptr;
 }
 
@@ -9672,9 +9668,9 @@ struct FindHiddenVirtualMethod {
 
     bool foundSameNameMethod = false;
     SmallVector<CXXMethodDecl *, 8> overloadedMethods;
-    for (Path.Decls = BaseRecord->lookup(Name); !Path.Decls.empty();
-         Path.Decls = Path.Decls.slice(1)) {
-      NamedDecl *D = Path.Decls.front();
+    for (Path.Decls = BaseRecord->lookup(Name).begin();
+         Path.Decls != DeclContext::lookup_iterator(); ++Path.Decls) {
+      NamedDecl *D = *Path.Decls;
       if (CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(D)) {
         MD = MD->getCanonicalDecl();
         foundSameNameMethod = true;
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index b6bf88203745b4..fef96b2eb11fd4 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -638,8 +638,8 @@ void LookupResult::resolveKind() {
 void LookupResult::addDeclsFromBasePaths(const CXXBasePaths &P) {
   CXXBasePaths::const_paths_iterator I, E;
   for (I = P.begin(), E = P.end(); I != E; ++I)
-    for (DeclContext::lookup_iterator DI = I->Decls.begin(),
-         DE = I->Decls.end(); DI != DE; ++DI)
+    for (DeclContext::lookup_iterator DI = I->Decls, DE = DI.end(); DI != DE;
+         ++DI)
       addDecl(*DI);
 }
 
@@ -2230,9 +2230,9 @@ bool Sema::LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
     CXXRecordDecl *BaseRecord = Specifier->getType()->getAsCXXRecordDecl();
     // Drop leading non-matching lookup results from the declaration list so
     // we don't need to consider them again below.
-    for (Path.Decls = BaseRecord->lookup(Name); !Path.Decls.empty();
-         Path.Decls = Path.Decls.slice(1)) {
-      if (Path.Decls.front()->isInIdentifierNamespace(IDNS))
+    for (Path.Decls = BaseRecord->lookup(Name).begin();
+         Path.Decls != Path.Decls.end(); ++Path.Decls) {
+      if ((*Path.Decls)->isInIdentifierNamespace(IDNS))
         return true;
     }
     return false;
@@ -2256,9 +2256,9 @@ bool Sema::LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
   AccessSpecifier SubobjectAccess = AS_none;
 
   // Check whether the given lookup result contains only static members.
-  auto HasOnlyStaticMembers = [&](DeclContextLookupResult Result) {
-    for (NamedDecl *ND : Result)
-      if (ND->isInIdentifierNamespace(IDNS) && ND->isCXXInstanceMember())
+  auto HasOnlyStaticMembers = [&](DeclContext::lookup_iterator Result) {
+    for (DeclContext::lookup_iterator I = Result, E = I.end(); I != E; ++I)
+      if ((*I)->isInIdentifierNamespace(IDNS) && (*I)->isCXXInstanceMember())
         return false;
     return true;
   };
@@ -2267,8 +2267,8 @@ bool Sema::LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
 
   // Determine whether two sets of members contain the same members, as
   // required by C++ [class.member.lookup]p6.
-  auto HasSameDeclarations = [&](DeclContextLookupResult A,
-                                 DeclContextLookupResult B) {
+  auto HasSameDeclarations = [&](DeclContext::lookup_iterator A,
+                                 DeclContext::lookup_iterator B) {
     using Iterator = DeclContextLookupResult::iterator;
     using Result = const void *;
 
@@ -2305,7 +2305,7 @@ bool Sema::LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
 
     // We'll often find the declarations are in the same order. Handle this
     // case (and the special case of only one declaration) efficiently.
-    Iterator AIt = A.begin(), BIt = B.begin(), AEnd = A.end(), BEnd = B.end();
+    Iterator AIt = A, BIt = B, AEnd, BEnd;
     while (true) {
       Result AResult = Next(AIt, AEnd);
       Result BResult = Next(BIt, BEnd);
@@ -2388,10 +2388,11 @@ bool Sema::LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
 
   // Lookup in a base class succeeded; return these results.
 
-  for (auto *D : Paths.front().Decls) {
+  for (DeclContext::lookup_iterator I = Paths.front().Decls, E = I.end();
+       I != E; ++I) {
     AccessSpecifier AS = CXXRecordDecl::MergeAccess(SubobjectAccess,
-                                                    D->getAccess());
-    if (NamedDecl *ND = R.getAcceptableDecl(D))
+                                                    (*I)->getAccess());
+    if (NamedDecl *ND = R.getAcceptableDecl(*I))
       R.addDecl(ND, AS);
   }
   R.resolveKind();
@@ -2534,7 +2535,7 @@ void Sema::DiagnoseAmbiguousLookup(LookupResult &Result) {
       << Name << SubobjectType << getAmbiguousPathsDisplayString(*Paths)
       << LookupRange;
 
-    DeclContext::lookup_iterator Found = Paths->front().Decls.begin();
+    DeclContext::lookup_iterator Found = Paths->front().Decls;
     while (isa<CXXMethodDecl>(*Found) &&
            cast<CXXMethodDecl>(*Found)->isStatic())
       ++Found;
@@ -2552,7 +2553,7 @@ void Sema::DiagnoseAmbiguousLookup(LookupResult &Result) {
     for (CXXBasePaths::paths_iterator Path = Paths->begin(),
                                       PathEnd = Paths->end();
          Path != PathEnd; ++Path) {
-      const NamedDecl *D = Path->Decls.front();
+      const NamedDecl *D = *Path->Decls;
       if (!D->isInIdentifierNamespace(Result.getIdentifierNamespace()))
         continue;
       if (DeclsPrinted.insert(D).second) {
diff --git a/clang/lib/Sema/SemaObjCProperty.cpp b/clang/lib/Sema/SemaObjCProperty.cpp
index fdc30fe6f6576f..db999270219cb8 100644
--- a/clang/lib/Sema/SemaObjCProperty.cpp
+++ b/clang/lib/Sema/SemaObjCProperty.cpp
@@ -112,12 +112,10 @@ CheckPropertyAgainstProtocol(Sema &S, ObjCPropertyDecl *Prop,
     return;
 
   // Look for a property with the same name.
-  DeclContext::lookup_result R = Proto->lookup(Prop->getDeclName());
-  for (unsigned I = 0, N = R.size(); I != N; ++I) {
-    if (ObjCPropertyDecl *ProtoProp = dyn_cast<ObjCPropertyDecl>(R[I])) {
-      S.DiagnosePropertyMismatch(Prop, ProtoProp, Proto->getIdentifier(), true);
-      return;
-    }
+  if (ObjCPropertyDecl *ProtoProp =
+      Proto->lookup(Prop->getDeclName()).find_first<ObjCPropertyDecl>()) {
+    S.DiagnosePropertyMismatch(Prop, ProtoProp, Proto->getIdentifier(), true);
+    return;
   }
 
   // Check this property against any protocols we inherit.
@@ -233,18 +231,13 @@ Decl *Sema::ActOnProperty(Scope *S, SourceLocation AtLoc,
     bool FoundInSuper = false;
     ObjCInterfaceDecl *CurrentInterfaceDecl = IFace;
     while (ObjCInterfaceDecl *Super = CurrentInterfaceDecl->getSuperClass()) {
-      DeclContext::lookup_result R = Super->lookup(Res->getDeclName());
-      for (unsigned I = 0, N = R.size(); I != N; ++I) {
-        if (ObjCPropertyDecl *SuperProp = dyn_cast<ObjCPropertyDecl>(R[I])) {
-          DiagnosePropertyMismatch(Res, SuperProp, Super->getIdentifier(), false);
-          FoundInSuper = true;
-          break;
-        }
-      }
-      if (FoundInSuper)
+      if (ObjCPropertyDecl *SuperProp =
+          Super->lookup(Res->getDeclName()).find_first<ObjCPropertyDecl>()) {
+        DiagnosePropertyMismatch(Res, SuperProp, Super->getIdentifier(), false);
+        FoundInSuper = true;
         break;
-      else
-        CurrentInterfaceDecl = Super;
+      }
+      CurrentInterfaceDecl = Super;
     }
 
     if (FoundInSuper) {
@@ -1149,14 +1142,13 @@ Decl *Sema::ActOnPropertyImplDecl(Scope *S,
       // redeclared 'readwrite', then no warning is to be issued.
       for (auto *Ext : IDecl->known_extensions()) {
         DeclContext::lookup_result R = Ext->lookup(property->getDeclName());
-        if (!R.empty())
-          if (ObjCPropertyDecl *ExtProp = dyn_cast<ObjCPropertyDecl>(R[0])) {
-            PIkind = ExtProp->getPropertyAttributesAsWritten();
-            if (PIkind & ObjCPropertyAttribute::kind_readwrite) {
-              ReadWriteProperty = true;
-              break;
-            }
+        if (auto *ExtProp = R.find_first<ObjCPropertyDecl>()) {
+          PIkind = ExtProp->getPropertyAttributesAsWritten();
+          if (PIkind & ObjCPropertyAttribute::kind_readwrite) {
+            ReadWriteProperty = true;
+            break;
           }
+        }
       }
 
       if (!ReadWriteProperty) {
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 8bd812b39de4d3..578a77aceeda53 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -3420,7 +3420,8 @@ Sema::InstantiateClassMembers(SourceLocation PointOfInstantiation,
             Instantiation->getTemplateInstantiationPattern();
         DeclContext::lookup_result Lookup =
             ClassPattern->lookup(Field->getDeclName());
-        FieldDecl *Pattern = cast<FieldDecl>(Lookup.front());
+        FieldDecl *Pattern = Lookup.find_first<FieldDecl>();
+        assert(Pattern);
         InstantiateInClassInitializer(PointOfInstantiation, Field, Pattern,
                                       TemplateArgs);
       }
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index a4a0a5ced90be3..bc390a696e9a90 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -7649,9 +7649,10 @@ ASTReader::FindExternalVisibleDeclsByName(const DeclContext *DC,
 
   // Load the list of declarations.
   SmallVector<NamedDecl *, 64> Decls;
+  llvm::SmallPtrSet<NamedDecl *, 8> Found;
   for (DeclID ID : It->second.Table.find(Name)) {
     NamedDecl *ND = cast<NamedDecl>(GetDecl(ID));
-    if (ND->getDeclName() == Name)
+    if (ND->getDeclName() == Name && Found.insert(ND).second)
       Decls.push_back(ND);
   }
 
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index 2cb44bf9038b52..2b8278090b058c 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -13,7 +13,6 @@
 #include "ASTCommon.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/DeclCXX.h"
-#include "clang/AST/DeclContextInternals.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclVisitor.h"
 #include "clang/AST/Expr.h"
diff --git a/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
index 38a9d4ba65b6bf..c8b2e3d9152076 100644
--- a/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
@@ -842,7 +842,7 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
     llvm::Optional<QualType> operator()(StringRef Name) {
       IdentifierInfo &II = ACtx.Idents.get(Name);
       auto LookupRes = ACtx.getTranslationUnitDecl()->lookup(&II);
-      if (LookupRes.size() == 0)
+      if (LookupRes.empty())
         return None;
 
       // Prioritze typedef declarations.
@@ -994,7 +994,7 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
         return false;
       IdentifierInfo &II = ACtx.Idents.get(Name);
       auto LookupRes = ACtx.getTranslationUnitDecl()->lookup(&II);
-      if (LookupRes.size() == 0)
+      if (LookupRes.empty())
         return false;
       for (Decl *D : LookupRes) {
         if (auto *FD = dyn_cast<FunctionDecl>(D)) {
diff --git a/clang/test/PCH/cxx-explicit-specifier.cpp b/clang/test/PCH/cxx-explicit-specifier.cpp
index ede0730f7c1399..331c2e84c78158 100644
--- a/clang/test/PCH/cxx-explicit-specifier.cpp
+++ b/clang/test/PCH/cxx-explicit-specifier.cpp
@@ -1,8 +1,10 @@
+// RUN: %clang_cc1 -std=c++2a -include %s %s -ast-print -verify | FileCheck %s
+//
 // RUN: %clang_cc1 -std=c++2a -emit-pch %s -o %t-cxx2a
-// RUN: %clang_cc1 -std=c++2a -DUSE_PCH -include-pch %t-cxx2a %s -ast-print -verify | FileCheck %s
+// RUN: %clang_cc1 -std=c++2a -include-pch %t-cxx2a %s -ast-print -verify | FileCheck %s
 
 // RUN: %clang_cc1 -std=c++2a -emit-pch -fpch-instantiate-templates %s -o %t-cxx2a
-// RUN: %clang_cc1 -std=c++2a -DUSE_PCH -include-pch %t-cxx2a %s -ast-print -verify | FileCheck %s
+// RUN: %clang_cc1 -std=c++2a -include-pch %t-cxx2a %s -ast-print -verify | FileCheck %s
 
 #ifndef USE_PCH
 namespace inheriting_constructor {
@@ -125,3 +127,5 @@ A a1 = { 0 };
 #endif
 
 }
+
+#define USE_PCH
diff --git a/clang/tools/libclang/CXType.cpp b/clang/tools/libclang/CXType.cpp
index 4b9620827002c9..f8c36df35607e3 100644
--- a/clang/tools/libclang/CXType.cpp
+++ b/clang/tools/libclang/CXType.cpp
@@ -1030,7 +1030,7 @@ long long clang_Type_getOffsetOf(CXType PT, const char *S) {
   // and we would return InvalidFieldName instead of Incomplete.
   // But this erroneous results does protects again a hidden assertion failure
   // in the RecordLayoutBuilder
-  if (Res.size() != 1)
+  if (!Res.isSingleResult())
     return CXTypeLayoutError_InvalidFieldName;
   if (const FieldDecl *FD = dyn_cast<FieldDecl>(Res.front()))
     return Ctx.getFieldOffset(FD);
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
index 193523f2fc516c..39612d43799b81 100644
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -2561,9 +2561,9 @@ TEST_P(ImportFriendFunctions, Lookup) {
     auto FromName = FromD->getDeclName();
     auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(FromTU, ClassPattern);
     auto LookupRes = Class->noload_lookup(FromName);
-    ASSERT_EQ(LookupRes.size(), 0u);
+    ASSERT_TRUE(LookupRes.empty());
     LookupRes = FromTU->noload_lookup(FromName);
-    ASSERT_EQ(LookupRes.size(), 1u);
+    ASSERT_TRUE(LookupRes.isSingleResult());
   }
 
   auto *ToD = cast<FunctionDecl>(Import(FromD, Lang_CXX03));
@@ -2572,9 +2572,9 @@ TEST_P(ImportFriendFunctions, Lookup) {
   TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
   auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(ToTU, ClassPattern);
   auto LookupRes = Class->noload_lookup(ToName);
-  EXPECT_EQ(LookupRes.size(), 0u);
+  EXPECT_TRUE(LookupRes.empty());
   LookupRes = ToTU->noload_lookup(ToName);
-  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_TRUE(LookupRes.isSingleResult());
 
   EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, FunctionPattern), 1u);
   auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
@@ -2608,9 +2608,9 @@ TEST_P(ImportFriendFunctions, LookupWithProtoAfter) {
   auto *FromClass =
       FirstDeclMatcher<CXXRecordDecl>().match(FromTU, ClassPattern);
   auto LookupRes = FromClass->noload_lookup(FromName);
-  ASSERT_EQ(LookupRes.size(), 0u);
+  ASSERT_TRUE(LookupRes.empty());
   LookupRes = FromTU->noload_lookup(FromName);
-  ASSERT_EQ(LookupRes.size(), 1u);
+  ASSERT_TRUE(LookupRes.isSingleResult());
 
   auto *ToFriend = cast<FunctionDecl>(Import(FromFriend, Lang_CXX03));
   auto ToName = ToFriend->getDeclName();
@@ -2618,10 +2618,10 @@ TEST_P(ImportFriendFunctions, LookupWithProtoAfter) {
   TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
   auto *ToClass = FirstDeclMatcher<CXXRecordDecl>().match(ToTU, ClassPattern);
   LookupRes = ToClass->noload_lookup(ToName);
-  EXPECT_EQ(LookupRes.size(), 0u);
+  EXPECT_TRUE(LookupRes.empty());
   LookupRes = ToTU->noload_lookup(ToName);
   // Test is disabled because this result is 2.
-  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_TRUE(LookupRes.isSingleResult());
 
   ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, FunctionPattern), 2u);
   ToFriend = FirstDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
@@ -2652,9 +2652,9 @@ TEST_P(ImportFriendFunctions, LookupWithProtoBefore) {
   auto *FromClass =
       FirstDeclMatcher<CXXRecordDecl>().match(FromTU, ClassPattern);
   auto LookupRes = FromClass->noload_lookup(FromName);
-  ASSERT_EQ(LookupRes.size(), 0u);
+  ASSERT_TRUE(LookupRes.empty());
   LookupRes = FromTU->noload_lookup(FromName);
-  ASSERT_EQ(LookupRes.size(), 1u);
+  ASSERT_TRUE(LookupRes.isSingleResult());
 
   auto *ToNormal = cast<FunctionDecl>(Import(FromNormal, Lang_CXX03));
   auto ToName = ToNormal->getDeclName();
@@ -2662,9 +2662,9 @@ TEST_P(ImportFriendFunctions, LookupWithProtoBefore) {
 
   auto *ToClass = FirstDeclMatcher<CXXRecordDecl>().match(ToTU, ClassPattern);
   LookupRes = ToClass->noload_lookup(ToName);
-  EXPECT_EQ(LookupRes.size(), 0u);
+  EXPECT_TRUE(LookupRes.empty());
   LookupRes = ToTU->noload_lookup(ToName);
-  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_TRUE(LookupRes.isSingleResult());
 
   EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, FunctionPattern), 2u);
   ToNormal = FirstDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
@@ -2694,9 +2694,9 @@ TEST_P(ImportFriendFunctions, ImportFriendChangesLookup) {
   ASSERT_FALSE(FromFriendF->isInIdentifierNamespace(Decl::IDNS_Ordinary));
   ASSERT_TRUE(FromFriendF->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
   auto LookupRes = FromNormalTU->noload_lookup(FromNormalName);
-  ASSERT_EQ(LookupRes.size(), 1u);
+  ASSERT_TRUE(LookupRes.isSingleResult());
   LookupRes = FromFriendTU->noload_lookup(FromFriendName);
-  ASSERT_EQ(LookupRes.size(), 1u);
+  ASSERT_TRUE(LookupRes.isSingleResult());
 
   auto *ToNormalF = cast<FunctionDecl>(Import(FromNormalF, Lang_CXX03));
   TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
@@ -2704,12 +2704,12 @@ TEST_P(ImportFriendFunctions, ImportFriendChangesLookup) {
   EXPECT_TRUE(ToNormalF->isInIdentifierNamespace(Decl::IDNS_Ordinary));
   EXPECT_FALSE(ToNormalF->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
   LookupRes = ToTU->noload_lookup(ToName);
-  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_TRUE(LookupRes.isSingleResult());
   EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
 
   auto *ToFriendF = cast<FunctionDecl>(Import(FromFriendF, Lang_CXX03));
   LookupRes = ToTU->noload_lookup(ToName);
-  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_TRUE(LookupRes.isSingleResult());
   EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
 
   EXPECT_TRUE(ToNormalF->isInIdentifierNamespace(Decl::IDNS_Ordinary));
@@ -4031,11 +4031,11 @@ TEST_P(DeclContextTest,
 
   ASSERT_TRUE(L.getAsDecl());
   // Simulate the private function DeclContext::reconcileExternalVisibleStorage.
-  // The point here is to have a Vec with only one element, which is not the
-  // one we are going to delete from the DC later.
+  // We do not have a list with one element.
   L.setHasExternalDecls();
-  ASSERT_TRUE(L.getAsVector());
-  ASSERT_EQ(1u, L.getAsVector()->size());
+  ASSERT_FALSE(L.getAsList());
+  auto Results = L.getLookupResult();
+  ASSERT_EQ(1u, std::distance(Results.begin(), Results.end()));
 
   // This asserts in the old implementation.
   DC->removeDecl(A0);
diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCDeclVendor.cpp b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCDeclVendor.cpp
index 7b331307c0f783..9bc40c16e5d05b 100644
--- a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCDeclVendor.cpp
+++ b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCDeclVendor.cpp
@@ -59,7 +59,7 @@ class lldb_private::AppleObjCExternalASTSource
       clang::DeclContext::lookup_result result =
           non_const_interface_decl->lookup(name);
 
-      return (result.size() != 0);
+      return (!result.empty());
     } while (false);
 
     SetNoExternalVisibleDeclsForName(decl_ctx, name);
@@ -555,7 +555,7 @@ uint32_t AppleObjCDeclVendor::FindDecls(ConstString name, bool append,
 
     if (!lookup_result.empty()) {
       if (clang::ObjCInterfaceDecl *result_iface_decl =
-              llvm::dyn_cast<clang::ObjCInterfaceDecl>(lookup_result[0])) {
+             llvm::dyn_cast<clang::ObjCInterfaceDecl>(*lookup_result.begin())) {
         if (log) {
           clang::QualType result_iface_type =
               ast_ctx.getObjCInterfaceType(result_iface_decl);
diff --git a/lldb/source/Plugins/SymbolFile/PDB/PDBASTParser.cpp b/lldb/source/Plugins/SymbolFile/PDB/PDBASTParser.cpp
index f9c12e63414072..ef80d764eb4956 100644
--- a/lldb/source/Plugins/SymbolFile/PDB/PDBASTParser.cpp
+++ b/lldb/source/Plugins/SymbolFile/PDB/PDBASTParser.cpp
@@ -326,7 +326,7 @@ GetDeclFromContextByName(const clang::ASTContext &ast,
   if (result.empty())
     return nullptr;
 
-  return result[0];
+  return *result.begin();
 }
 
 static bool IsAnonymousNamespaceName(llvm::StringRef name) {
diff --git a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
index c94cb8d16ee9e7..a61666adebaadb 100644
--- a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
+++ b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
@@ -163,7 +163,6 @@ void addOverridesForMethod(clang::CXXMethodDecl *decl) {
           if (name.getNameKind() == clang::DeclarationName::CXXDestructorName)
             if (auto *baseDtorDecl = base_record->getDestructor()) {
               if (baseDtorDecl->isVirtual()) {
-                path.Decls = baseDtorDecl;
                 decls.push_back(baseDtorDecl);
                 return true;
               } else
@@ -171,12 +170,11 @@ void addOverridesForMethod(clang::CXXMethodDecl *decl) {
             }
 
           // Otherwise, search for name in the base class.
-          for (path.Decls = base_record->lookup(name); !path.Decls.empty();
-               path.Decls = path.Decls.slice(1)) {
+          for (path.Decls = base_record->lookup(name).begin();
+               path.Decls != path.Decls.end(); ++path.Decls) {
             if (auto *method_decl =
-                    llvm::dyn_cast<clang::CXXMethodDecl>(path.Decls.front()))
+                    llvm::dyn_cast<clang::CXXMethodDecl>(*path.Decls))
               if (method_decl->isVirtual() && !isOverload(decl, method_decl)) {
-                path.Decls = method_decl;
                 decls.push_back(method_decl);
                 return true;
               }
@@ -6605,10 +6603,11 @@ size_t TypeSystemClang::GetIndexOfChildMemberWithName(
           if (cxx_record_decl->lookupInBases(
                   [decl_name](const clang::CXXBaseSpecifier *specifier,
                               clang::CXXBasePath &path) {
-                    path.Decls =
-                        specifier->getType()->getAsCXXRecordDecl()->lookup(
-                            decl_name);
-                    return !path.Decls.empty();
+                    CXXRecordDecl *record =
+                      specifier->getType()->getAsCXXRecordDecl();
+                    auto r = record->lookup(decl_name);
+                    path.Decls = r.begin();
+                    return !r.empty();
                   },
                   paths)) {
             clang::CXXBasePaths::const_paths_iterator path,
@@ -6631,9 +6630,10 @@ size_t TypeSystemClang::GetIndexOfChildMemberWithName(
                           ->getDecl());
                 }
               }
-              for (clang::NamedDecl *path_decl : path->Decls) {
+              for (clang::DeclContext::lookup_iterator I = path->Decls, E;
+                   I != E; ++I) {
                 child_idx = GetIndexForRecordChild(
-                    parent_record_decl, path_decl, omit_empty_base_classes);
+                    parent_record_decl, *I, omit_empty_base_classes);
                 if (child_idx == UINT32_MAX) {
                   child_indexes.clear();
                   return 0;
diff --git a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.h b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.h
index f37652ff477a6b..45c84e7447acba 100644
--- a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.h
+++ b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.h
@@ -265,7 +265,7 @@ class TypeSystemClang : public TypeSystem {
       clang::DeclContext::lookup_result result = decl_context->lookup(myName);
 
       if (!result.empty()) {
-        clang::NamedDecl *named_decl = result[0];
+        clang::NamedDecl *named_decl = *result.begin();
         if (const RecordDeclType *record_decl =
                 llvm::dyn_cast<RecordDeclType>(named_decl))
           compiler_type.SetCompilerType(