diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
index ed35e73ce4cf90..f39ce14bc82cb1 100644
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -2622,6 +2622,18 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// template.
   bool hasSameTemplateName(TemplateName X, TemplateName Y);
 
+  /// Determine whether the two declarations refer to the same entity.
+  bool isSameEntity(NamedDecl *X, NamedDecl *Y);
+
+  /// Determine whether two template parameter lists are similar enough
+  /// that they may be used in declarations of the same template.
+  bool isSameTemplateParameterList(TemplateParameterList *X,
+                                   TemplateParameterList *Y);
+
+  /// Determine whether two template parameters are similar enough
+  /// that they may be used in declarations of the same template.
+  bool isSameTemplateParameter(NamedDecl *X, NamedDecl *Y);
+
   /// Retrieve the "canonical" template argument.
   ///
   /// The canonical template argument is the simplest template argument
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 9d63724c919a09..8a780250b6d8e9 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -6147,6 +6147,376 @@ bool ASTContext::hasSameTemplateName(TemplateName X, TemplateName Y) {
   return X.getAsVoidPointer() == Y.getAsVoidPointer();
 }
 
+bool ASTContext::isSameTemplateParameter(NamedDecl *X, NamedDecl *Y) {
+  if (X->getKind() != Y->getKind())
+    return false;
+
+  if (auto *TX = dyn_cast<TemplateTypeParmDecl>(X)) {
+    auto *TY = cast<TemplateTypeParmDecl>(Y);
+    if (TX->isParameterPack() != TY->isParameterPack())
+      return false;
+    if (TX->hasTypeConstraint() != TY->hasTypeConstraint())
+      return false;
+    const TypeConstraint *TXTC = TX->getTypeConstraint();
+    const TypeConstraint *TYTC = TY->getTypeConstraint();
+    if (!TXTC != !TYTC)
+      return false;
+    if (TXTC && TYTC) {
+      auto *NCX = TXTC->getNamedConcept();
+      auto *NCY = TYTC->getNamedConcept();
+      if (!NCX || !NCY || !isSameEntity(NCX, NCY))
+        return false;
+      if (TXTC->hasExplicitTemplateArgs() != TYTC->hasExplicitTemplateArgs())
+        return false;
+      if (TXTC->hasExplicitTemplateArgs()) {
+        auto *TXTCArgs = TXTC->getTemplateArgsAsWritten();
+        auto *TYTCArgs = TYTC->getTemplateArgsAsWritten();
+        if (TXTCArgs->NumTemplateArgs != TYTCArgs->NumTemplateArgs)
+          return false;
+        llvm::FoldingSetNodeID XID, YID;
+        for (auto &ArgLoc : TXTCArgs->arguments())
+          ArgLoc.getArgument().Profile(XID, X->getASTContext());
+        for (auto &ArgLoc : TYTCArgs->arguments())
+          ArgLoc.getArgument().Profile(YID, Y->getASTContext());
+        if (XID != YID)
+          return false;
+      }
+    }
+    return true;
+  }
+
+  if (auto *TX = dyn_cast<NonTypeTemplateParmDecl>(X)) {
+    auto *TY = cast<NonTypeTemplateParmDecl>(Y);
+    return TX->isParameterPack() == TY->isParameterPack() &&
+           TX->getASTContext().hasSameType(TX->getType(), TY->getType());
+  }
+
+  auto *TX = cast<TemplateTemplateParmDecl>(X);
+  auto *TY = cast<TemplateTemplateParmDecl>(Y);
+  return TX->isParameterPack() == TY->isParameterPack() &&
+         isSameTemplateParameterList(TX->getTemplateParameters(),
+                                     TY->getTemplateParameters());
+}
+
+bool ASTContext::isSameTemplateParameterList(TemplateParameterList *X,
+                                             TemplateParameterList *Y) {
+  if (X->size() != Y->size())
+    return false;
+
+  for (unsigned I = 0, N = X->size(); I != N; ++I)
+    if (!isSameTemplateParameter(X->getParam(I), Y->getParam(I)))
+      return false;
+
+  const Expr *XRC = X->getRequiresClause();
+  const Expr *YRC = Y->getRequiresClause();
+  if (!XRC != !YRC)
+    return false;
+  if (XRC) {
+    llvm::FoldingSetNodeID XRCID, YRCID;
+    XRC->Profile(XRCID, *this, /*Canonical=*/true);
+    YRC->Profile(YRCID, *this, /*Canonical=*/true);
+    if (XRCID != YRCID)
+      return false;
+  }
+
+  return true;
+}
+
+static NamespaceDecl *getNamespace(const NestedNameSpecifier *X) {
+  if (auto *NS = X->getAsNamespace())
+    return NS;
+  if (auto *NAS = X->getAsNamespaceAlias())
+    return NAS->getNamespace();
+  return nullptr;
+}
+
+static bool isSameQualifier(const NestedNameSpecifier *X,
+                            const NestedNameSpecifier *Y) {
+  if (auto *NSX = getNamespace(X)) {
+    auto *NSY = getNamespace(Y);
+    if (!NSY || NSX->getCanonicalDecl() != NSY->getCanonicalDecl())
+      return false;
+  } else if (X->getKind() != Y->getKind())
+    return false;
+
+  // FIXME: For namespaces and types, we're permitted to check that the entity
+  // is named via the same tokens. We should probably do so.
+  switch (X->getKind()) {
+  case NestedNameSpecifier::Identifier:
+    if (X->getAsIdentifier() != Y->getAsIdentifier())
+      return false;
+    break;
+  case NestedNameSpecifier::Namespace:
+  case NestedNameSpecifier::NamespaceAlias:
+    // We've already checked that we named the same namespace.
+    break;
+  case NestedNameSpecifier::TypeSpec:
+  case NestedNameSpecifier::TypeSpecWithTemplate:
+    if (X->getAsType()->getCanonicalTypeInternal() !=
+        Y->getAsType()->getCanonicalTypeInternal())
+      return false;
+    break;
+  case NestedNameSpecifier::Global:
+  case NestedNameSpecifier::Super:
+    return true;
+  }
+
+  // Recurse into earlier portion of NNS, if any.
+  auto *PX = X->getPrefix();
+  auto *PY = Y->getPrefix();
+  if (PX && PY)
+    return isSameQualifier(PX, PY);
+  return !PX && !PY;
+}
+
+/// Determine whether the attributes we can overload on are identical for A and
+/// B. Will ignore any overloadable attrs represented in the type of A and B.
+static bool hasSameOverloadableAttrs(const FunctionDecl *A,
+                                     const FunctionDecl *B) {
+  // Note that pass_object_size attributes are represented in the function's
+  // ExtParameterInfo, so we don't need to check them here.
+
+  llvm::FoldingSetNodeID Cand1ID, Cand2ID;
+  auto AEnableIfAttrs = A->specific_attrs<EnableIfAttr>();
+  auto BEnableIfAttrs = B->specific_attrs<EnableIfAttr>();
+
+  for (auto Pair : zip_longest(AEnableIfAttrs, BEnableIfAttrs)) {
+    Optional<EnableIfAttr *> Cand1A = std::get<0>(Pair);
+    Optional<EnableIfAttr *> Cand2A = std::get<1>(Pair);
+
+    // Return false if the number of enable_if attributes is different.
+    if (!Cand1A || !Cand2A)
+      return false;
+
+    Cand1ID.clear();
+    Cand2ID.clear();
+
+    (*Cand1A)->getCond()->Profile(Cand1ID, A->getASTContext(), true);
+    (*Cand2A)->getCond()->Profile(Cand2ID, B->getASTContext(), true);
+
+    // Return false if any of the enable_if expressions of A and B are
+    // different.
+    if (Cand1ID != Cand2ID)
+      return false;
+  }
+  return true;
+}
+
+bool ASTContext::isSameEntity(NamedDecl *X, NamedDecl *Y) {
+  if (X == Y)
+    return true;
+
+  if (X->getDeclName() != Y->getDeclName())
+    return false;
+
+  // Must be in the same context.
+  //
+  // Note that we can't use DeclContext::Equals here, because the DeclContexts
+  // could be two different declarations of the same function. (We will fix the
+  // semantic DC to refer to the primary definition after merging.)
+  if (!declaresSameEntity(cast<Decl>(X->getDeclContext()->getRedeclContext()),
+                          cast<Decl>(Y->getDeclContext()->getRedeclContext())))
+    return false;
+
+  // Two typedefs refer to the same entity if they have the same underlying
+  // type.
+  if (const auto *TypedefX = dyn_cast<TypedefNameDecl>(X))
+    if (const auto *TypedefY = dyn_cast<TypedefNameDecl>(Y))
+      return hasSameType(TypedefX->getUnderlyingType(),
+                         TypedefY->getUnderlyingType());
+
+  // Must have the same kind.
+  if (X->getKind() != Y->getKind())
+    return false;
+
+  // Objective-C classes and protocols with the same name always match.
+  if (isa<ObjCInterfaceDecl>(X) || isa<ObjCProtocolDecl>(X))
+    return true;
+
+  if (isa<ClassTemplateSpecializationDecl>(X)) {
+    // No need to handle these here: we merge them when adding them to the
+    // template.
+    return false;
+  }
+
+  // Compatible tags match.
+  if (const auto *TagX = dyn_cast<TagDecl>(X)) {
+    const auto *TagY = cast<TagDecl>(Y);
+    return (TagX->getTagKind() == TagY->getTagKind()) ||
+           ((TagX->getTagKind() == TTK_Struct ||
+             TagX->getTagKind() == TTK_Class ||
+             TagX->getTagKind() == TTK_Interface) &&
+            (TagY->getTagKind() == TTK_Struct ||
+             TagY->getTagKind() == TTK_Class ||
+             TagY->getTagKind() == TTK_Interface));
+  }
+
+  // Functions with the same type and linkage match.
+  // FIXME: This needs to cope with merging of prototyped/non-prototyped
+  // functions, etc.
+  if (const auto *FuncX = dyn_cast<FunctionDecl>(X)) {
+    const auto *FuncY = cast<FunctionDecl>(Y);
+    if (const auto *CtorX = dyn_cast<CXXConstructorDecl>(X)) {
+      const auto *CtorY = cast<CXXConstructorDecl>(Y);
+      if (CtorX->getInheritedConstructor() &&
+          !isSameEntity(CtorX->getInheritedConstructor().getConstructor(),
+                        CtorY->getInheritedConstructor().getConstructor()))
+        return false;
+    }
+
+    if (FuncX->isMultiVersion() != FuncY->isMultiVersion())
+      return false;
+
+    // Multiversioned functions with different feature strings are represented
+    // as separate declarations.
+    if (FuncX->isMultiVersion()) {
+      const auto *TAX = FuncX->getAttr<TargetAttr>();
+      const auto *TAY = FuncY->getAttr<TargetAttr>();
+      assert(TAX && TAY && "Multiversion Function without target attribute");
+
+      if (TAX->getFeaturesStr() != TAY->getFeaturesStr())
+        return false;
+    }
+
+    const Expr *XRC = FuncX->getTrailingRequiresClause();
+    const Expr *YRC = FuncY->getTrailingRequiresClause();
+    if (!XRC != !YRC)
+      return false;
+    if (XRC) {
+      llvm::FoldingSetNodeID XRCID, YRCID;
+      XRC->Profile(XRCID, *this, /*Canonical=*/true);
+      YRC->Profile(YRCID, *this, /*Canonical=*/true);
+      if (XRCID != YRCID)
+        return false;
+    }
+
+    auto GetTypeAsWritten = [](const FunctionDecl *FD) {
+      // Map to the first declaration that we've already merged into this one.
+      // The TSI of redeclarations might not match (due to calling conventions
+      // being inherited onto the type but not the TSI), but the TSI type of
+      // the first declaration of the function should match across modules.
+      FD = FD->getCanonicalDecl();
+      return FD->getTypeSourceInfo() ? FD->getTypeSourceInfo()->getType()
+                                     : FD->getType();
+    };
+    QualType XT = GetTypeAsWritten(FuncX), YT = GetTypeAsWritten(FuncY);
+    if (!hasSameType(XT, YT)) {
+      // We can get functions with different types on the redecl chain in C++17
+      // if they have differing exception specifications and at least one of
+      // the excpetion specs is unresolved.
+      auto *XFPT = XT->getAs<FunctionProtoType>();
+      auto *YFPT = YT->getAs<FunctionProtoType>();
+      if (getLangOpts().CPlusPlus17 && XFPT && YFPT &&
+          (isUnresolvedExceptionSpec(XFPT->getExceptionSpecType()) ||
+           isUnresolvedExceptionSpec(YFPT->getExceptionSpecType())) &&
+          hasSameFunctionTypeIgnoringExceptionSpec(XT, YT))
+        return true;
+      return false;
+    }
+
+    return FuncX->getLinkageInternal() == FuncY->getLinkageInternal() &&
+           hasSameOverloadableAttrs(FuncX, FuncY);
+  }
+
+  // Variables with the same type and linkage match.
+  if (const auto *VarX = dyn_cast<VarDecl>(X)) {
+    const auto *VarY = cast<VarDecl>(Y);
+    if (VarX->getLinkageInternal() == VarY->getLinkageInternal()) {
+      if (hasSameType(VarX->getType(), VarY->getType()))
+        return true;
+
+      // We can get decls with different types on the redecl chain. Eg.
+      // template <typename T> struct S { static T Var[]; }; // #1
+      // template <typename T> T S<T>::Var[sizeof(T)]; // #2
+      // Only? happens when completing an incomplete array type. In this case
+      // when comparing #1 and #2 we should go through their element type.
+      const ArrayType *VarXTy = getAsArrayType(VarX->getType());
+      const ArrayType *VarYTy = getAsArrayType(VarY->getType());
+      if (!VarXTy || !VarYTy)
+        return false;
+      if (VarXTy->isIncompleteArrayType() || VarYTy->isIncompleteArrayType())
+        return hasSameType(VarXTy->getElementType(), VarYTy->getElementType());
+    }
+    return false;
+  }
+
+  // Namespaces with the same name and inlinedness match.
+  if (const auto *NamespaceX = dyn_cast<NamespaceDecl>(X)) {
+    const auto *NamespaceY = cast<NamespaceDecl>(Y);
+    return NamespaceX->isInline() == NamespaceY->isInline();
+  }
+
+  // Identical template names and kinds match if their template parameter lists
+  // and patterns match.
+  if (const auto *TemplateX = dyn_cast<TemplateDecl>(X)) {
+    const auto *TemplateY = cast<TemplateDecl>(Y);
+    return isSameEntity(TemplateX->getTemplatedDecl(),
+                        TemplateY->getTemplatedDecl()) &&
+           isSameTemplateParameterList(TemplateX->getTemplateParameters(),
+                                       TemplateY->getTemplateParameters());
+  }
+
+  // Fields with the same name and the same type match.
+  if (const auto *FDX = dyn_cast<FieldDecl>(X)) {
+    const auto *FDY = cast<FieldDecl>(Y);
+    // FIXME: Also check the bitwidth is odr-equivalent, if any.
+    return hasSameType(FDX->getType(), FDY->getType());
+  }
+
+  // Indirect fields with the same target field match.
+  if (const auto *IFDX = dyn_cast<IndirectFieldDecl>(X)) {
+    const auto *IFDY = cast<IndirectFieldDecl>(Y);
+    return IFDX->getAnonField()->getCanonicalDecl() ==
+           IFDY->getAnonField()->getCanonicalDecl();
+  }
+
+  // Enumerators with the same name match.
+  if (isa<EnumConstantDecl>(X))
+    // FIXME: Also check the value is odr-equivalent.
+    return true;
+
+  // Using shadow declarations with the same target match.
+  if (const auto *USX = dyn_cast<UsingShadowDecl>(X)) {
+    const auto *USY = cast<UsingShadowDecl>(Y);
+    return USX->getTargetDecl() == USY->getTargetDecl();
+  }
+
+  // Using declarations with the same qualifier match. (We already know that
+  // the name matches.)
+  if (const auto *UX = dyn_cast<UsingDecl>(X)) {
+    const auto *UY = cast<UsingDecl>(Y);
+    return isSameQualifier(UX->getQualifier(), UY->getQualifier()) &&
+           UX->hasTypename() == UY->hasTypename() &&
+           UX->isAccessDeclaration() == UY->isAccessDeclaration();
+  }
+  if (const auto *UX = dyn_cast<UnresolvedUsingValueDecl>(X)) {
+    const auto *UY = cast<UnresolvedUsingValueDecl>(Y);
+    return isSameQualifier(UX->getQualifier(), UY->getQualifier()) &&
+           UX->isAccessDeclaration() == UY->isAccessDeclaration();
+  }
+  if (const auto *UX = dyn_cast<UnresolvedUsingTypenameDecl>(X)) {
+    return isSameQualifier(
+        UX->getQualifier(),
+        cast<UnresolvedUsingTypenameDecl>(Y)->getQualifier());
+  }
+
+  // Using-pack declarations are only created by instantiation, and match if
+  // they're instantiated from matching UnresolvedUsing...Decls.
+  if (const auto *UX = dyn_cast<UsingPackDecl>(X)) {
+    return declaresSameEntity(
+        UX->getInstantiatedFromUsingDecl(),
+        cast<UsingPackDecl>(Y)->getInstantiatedFromUsingDecl());
+  }
+
+  // Namespace alias definitions with the same target match.
+  if (const auto *NAX = dyn_cast<NamespaceAliasDecl>(X)) {
+    const auto *NAY = cast<NamespaceAliasDecl>(Y);
+    return NAX->getNamespace()->Equals(NAY->getNamespace());
+  }
+
+  return false;
+}
+
 TemplateArgument
 ASTContext::getCanonicalTemplateArgument(const TemplateArgument &Arg) const {
   switch (Arg.getKind()) {
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 310bbc0f3cbbc4..1ab26e58a40401 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2945,391 +2945,6 @@ uint64_t ASTReader::getGlobalBitOffset(ModuleFile &M, uint64_t LocalOffset) {
   return LocalOffset + M.GlobalBitOffset;
 }
 
-static bool isSameTemplateParameterList(const ASTContext &C,
-                                        const TemplateParameterList *X,
-                                        const TemplateParameterList *Y);
-static bool isSameEntity(NamedDecl *X, NamedDecl *Y);
-
-/// Determine whether two template parameters are similar enough
-/// that they may be used in declarations of the same template.
-static bool isSameTemplateParameter(const NamedDecl *X,
-                                    const NamedDecl *Y) {
-  if (X->getKind() != Y->getKind())
-    return false;
-
-  if (const auto *TX = dyn_cast<TemplateTypeParmDecl>(X)) {
-    const auto *TY = cast<TemplateTypeParmDecl>(Y);
-    if (TX->isParameterPack() != TY->isParameterPack())
-      return false;
-    if (TX->hasTypeConstraint() != TY->hasTypeConstraint())
-      return false;
-    const TypeConstraint *TXTC = TX->getTypeConstraint();
-    const TypeConstraint *TYTC = TY->getTypeConstraint();
-    if (!TXTC != !TYTC)
-      return false;
-    if (TXTC && TYTC) {
-      auto *NCX = TXTC->getNamedConcept();
-      auto *NCY = TYTC->getNamedConcept();
-      if (!NCX || !NCY || !isSameEntity(NCX, NCY))
-        return false;
-      if (TXTC->hasExplicitTemplateArgs() != TYTC->hasExplicitTemplateArgs())
-        return false;
-      if (TXTC->hasExplicitTemplateArgs()) {
-        const auto *TXTCArgs = TXTC->getTemplateArgsAsWritten();
-        const auto *TYTCArgs = TYTC->getTemplateArgsAsWritten();
-        if (TXTCArgs->NumTemplateArgs != TYTCArgs->NumTemplateArgs)
-          return false;
-        llvm::FoldingSetNodeID XID, YID;
-        for (const auto &ArgLoc : TXTCArgs->arguments())
-          ArgLoc.getArgument().Profile(XID, X->getASTContext());
-        for (const auto &ArgLoc : TYTCArgs->arguments())
-          ArgLoc.getArgument().Profile(YID, Y->getASTContext());
-        if (XID != YID)
-          return false;
-      }
-    }
-    return true;
-  }
-
-  if (const auto *TX = dyn_cast<NonTypeTemplateParmDecl>(X)) {
-    const auto *TY = cast<NonTypeTemplateParmDecl>(Y);
-    return TX->isParameterPack() == TY->isParameterPack() &&
-           TX->getASTContext().hasSameType(TX->getType(), TY->getType());
-  }
-
-  const auto *TX = cast<TemplateTemplateParmDecl>(X);
-  const auto *TY = cast<TemplateTemplateParmDecl>(Y);
-  return TX->isParameterPack() == TY->isParameterPack() &&
-         isSameTemplateParameterList(TX->getASTContext(),
-                                     TX->getTemplateParameters(),
-                                     TY->getTemplateParameters());
-}
-
-static NamespaceDecl *getNamespace(const NestedNameSpecifier *X) {
-  if (auto *NS = X->getAsNamespace())
-    return NS;
-  if (auto *NAS = X->getAsNamespaceAlias())
-    return NAS->getNamespace();
-  return nullptr;
-}
-
-static bool isSameQualifier(const NestedNameSpecifier *X,
-                            const NestedNameSpecifier *Y) {
-  if (auto *NSX = getNamespace(X)) {
-    auto *NSY = getNamespace(Y);
-    if (!NSY || NSX->getCanonicalDecl() != NSY->getCanonicalDecl())
-      return false;
-  } else if (X->getKind() != Y->getKind())
-    return false;
-
-  // FIXME: For namespaces and types, we're permitted to check that the entity
-  // is named via the same tokens. We should probably do so.
-  switch (X->getKind()) {
-  case NestedNameSpecifier::Identifier:
-    if (X->getAsIdentifier() != Y->getAsIdentifier())
-      return false;
-    break;
-  case NestedNameSpecifier::Namespace:
-  case NestedNameSpecifier::NamespaceAlias:
-    // We've already checked that we named the same namespace.
-    break;
-  case NestedNameSpecifier::TypeSpec:
-  case NestedNameSpecifier::TypeSpecWithTemplate:
-    if (X->getAsType()->getCanonicalTypeInternal() !=
-        Y->getAsType()->getCanonicalTypeInternal())
-      return false;
-    break;
-  case NestedNameSpecifier::Global:
-  case NestedNameSpecifier::Super:
-    return true;
-  }
-
-  // Recurse into earlier portion of NNS, if any.
-  auto *PX = X->getPrefix();
-  auto *PY = Y->getPrefix();
-  if (PX && PY)
-    return isSameQualifier(PX, PY);
-  return !PX && !PY;
-}
-
-/// Determine whether two template parameter lists are similar enough
-/// that they may be used in declarations of the same template.
-static bool isSameTemplateParameterList(const ASTContext &C,
-                                        const TemplateParameterList *X,
-                                        const TemplateParameterList *Y) {
-  if (X->size() != Y->size())
-    return false;
-
-  for (unsigned I = 0, N = X->size(); I != N; ++I)
-    if (!isSameTemplateParameter(X->getParam(I), Y->getParam(I)))
-      return false;
-
-  const Expr *XRC = X->getRequiresClause();
-  const Expr *YRC = Y->getRequiresClause();
-  if (!XRC != !YRC)
-    return false;
-  if (XRC) {
-    llvm::FoldingSetNodeID XRCID, YRCID;
-    XRC->Profile(XRCID, C, /*Canonical=*/true);
-    YRC->Profile(YRCID, C, /*Canonical=*/true);
-    if (XRCID != YRCID)
-      return false;
-  }
-
-  return true;
-}
-
-/// Determine whether the attributes we can overload on are identical for A and
-/// B. Will ignore any overloadable attrs represented in the type of A and B.
-static bool hasSameOverloadableAttrs(const FunctionDecl *A,
-                                     const FunctionDecl *B) {
-  // Note that pass_object_size attributes are represented in the function's
-  // ExtParameterInfo, so we don't need to check them here.
-
-  llvm::FoldingSetNodeID Cand1ID, Cand2ID;
-  auto AEnableIfAttrs = A->specific_attrs<EnableIfAttr>();
-  auto BEnableIfAttrs = B->specific_attrs<EnableIfAttr>();
-
-  for (auto Pair : zip_longest(AEnableIfAttrs, BEnableIfAttrs)) {
-    Optional<EnableIfAttr *> Cand1A = std::get<0>(Pair);
-    Optional<EnableIfAttr *> Cand2A = std::get<1>(Pair);
-
-    // Return false if the number of enable_if attributes is different.
-    if (!Cand1A || !Cand2A)
-      return false;
-
-    Cand1ID.clear();
-    Cand2ID.clear();
-
-    (*Cand1A)->getCond()->Profile(Cand1ID, A->getASTContext(), true);
-    (*Cand2A)->getCond()->Profile(Cand2ID, B->getASTContext(), true);
-
-    // Return false if any of the enable_if expressions of A and B are
-    // different.
-    if (Cand1ID != Cand2ID)
-      return false;
-  }
-  return true;
-}
-
-/// Determine whether the two declarations refer to the same entity.
-static bool isSameEntity(NamedDecl *X, NamedDecl *Y) {
-  if (X == Y)
-    return true;
-
-  if (X->getDeclName() != Y->getDeclName())
-    return false;
-
-  // Must be in the same context.
-  //
-  // Note that we can't use DeclContext::Equals here, because the DeclContexts
-  // could be two different declarations of the same function. (We will fix the
-  // semantic DC to refer to the primary definition after merging.)
-  if (!declaresSameEntity(cast<Decl>(X->getDeclContext()->getRedeclContext()),
-                          cast<Decl>(Y->getDeclContext()->getRedeclContext())))
-    return false;
-
-  // Two typedefs refer to the same entity if they have the same underlying
-  // type.
-  if (const auto *TypedefX = dyn_cast<TypedefNameDecl>(X))
-    if (const auto *TypedefY = dyn_cast<TypedefNameDecl>(Y))
-      return X->getASTContext().hasSameType(TypedefX->getUnderlyingType(),
-                                            TypedefY->getUnderlyingType());
-
-  // Must have the same kind.
-  if (X->getKind() != Y->getKind())
-    return false;
-
-  // Objective-C classes and protocols with the same name always match.
-  if (isa<ObjCInterfaceDecl>(X) || isa<ObjCProtocolDecl>(X))
-    return true;
-
-  if (isa<ClassTemplateSpecializationDecl>(X)) {
-    // No need to handle these here: we merge them when adding them to the
-    // template.
-    return false;
-  }
-
-  // Compatible tags match.
-  if (const auto *TagX = dyn_cast<TagDecl>(X)) {
-    const auto *TagY = cast<TagDecl>(Y);
-    return (TagX->getTagKind() == TagY->getTagKind()) ||
-      ((TagX->getTagKind() == TTK_Struct || TagX->getTagKind() == TTK_Class ||
-        TagX->getTagKind() == TTK_Interface) &&
-       (TagY->getTagKind() == TTK_Struct || TagY->getTagKind() == TTK_Class ||
-        TagY->getTagKind() == TTK_Interface));
-  }
-
-  // Functions with the same type and linkage match.
-  // FIXME: This needs to cope with merging of prototyped/non-prototyped
-  // functions, etc.
-  if (const auto *FuncX = dyn_cast<FunctionDecl>(X)) {
-    const auto *FuncY = cast<FunctionDecl>(Y);
-    if (const auto *CtorX = dyn_cast<CXXConstructorDecl>(X)) {
-      const auto *CtorY = cast<CXXConstructorDecl>(Y);
-      if (CtorX->getInheritedConstructor() &&
-          !isSameEntity(CtorX->getInheritedConstructor().getConstructor(),
-                        CtorY->getInheritedConstructor().getConstructor()))
-        return false;
-    }
-
-    if (FuncX->isMultiVersion() != FuncY->isMultiVersion())
-      return false;
-
-    // Multiversioned functions with different feature strings are represented
-    // as separate declarations.
-    if (FuncX->isMultiVersion()) {
-      const auto *TAX = FuncX->getAttr<TargetAttr>();
-      const auto *TAY = FuncY->getAttr<TargetAttr>();
-      assert(TAX && TAY && "Multiversion Function without target attribute");
-
-      if (TAX->getFeaturesStr() != TAY->getFeaturesStr())
-        return false;
-    }
-
-    ASTContext &C = FuncX->getASTContext();
-
-    const Expr *XRC = FuncX->getTrailingRequiresClause();
-    const Expr *YRC = FuncY->getTrailingRequiresClause();
-    if (!XRC != !YRC)
-      return false;
-    if (XRC) {
-      llvm::FoldingSetNodeID XRCID, YRCID;
-      XRC->Profile(XRCID, C, /*Canonical=*/true);
-      YRC->Profile(YRCID, C, /*Canonical=*/true);
-      if (XRCID != YRCID)
-        return false;
-    }
-
-    auto GetTypeAsWritten = [](const FunctionDecl *FD) {
-      // Map to the first declaration that we've already merged into this one.
-      // The TSI of redeclarations might not match (due to calling conventions
-      // being inherited onto the type but not the TSI), but the TSI type of
-      // the first declaration of the function should match across modules.
-      FD = FD->getCanonicalDecl();
-      return FD->getTypeSourceInfo() ? FD->getTypeSourceInfo()->getType()
-                                     : FD->getType();
-    };
-    QualType XT = GetTypeAsWritten(FuncX), YT = GetTypeAsWritten(FuncY);
-    if (!C.hasSameType(XT, YT)) {
-      // We can get functions with different types on the redecl chain in C++17
-      // if they have differing exception specifications and at least one of
-      // the excpetion specs is unresolved.
-      auto *XFPT = XT->getAs<FunctionProtoType>();
-      auto *YFPT = YT->getAs<FunctionProtoType>();
-      if (C.getLangOpts().CPlusPlus17 && XFPT && YFPT &&
-          (isUnresolvedExceptionSpec(XFPT->getExceptionSpecType()) ||
-           isUnresolvedExceptionSpec(YFPT->getExceptionSpecType())) &&
-          C.hasSameFunctionTypeIgnoringExceptionSpec(XT, YT))
-        return true;
-      return false;
-    }
-
-    return FuncX->getLinkageInternal() == FuncY->getLinkageInternal() &&
-           hasSameOverloadableAttrs(FuncX, FuncY);
-  }
-
-  // Variables with the same type and linkage match.
-  if (const auto *VarX = dyn_cast<VarDecl>(X)) {
-    const auto *VarY = cast<VarDecl>(Y);
-    if (VarX->getLinkageInternal() == VarY->getLinkageInternal()) {
-      ASTContext &C = VarX->getASTContext();
-      if (C.hasSameType(VarX->getType(), VarY->getType()))
-        return true;
-
-      // We can get decls with different types on the redecl chain. Eg.
-      // template <typename T> struct S { static T Var[]; }; // #1
-      // template <typename T> T S<T>::Var[sizeof(T)]; // #2
-      // Only? happens when completing an incomplete array type. In this case
-      // when comparing #1 and #2 we should go through their element type.
-      const ArrayType *VarXTy = C.getAsArrayType(VarX->getType());
-      const ArrayType *VarYTy = C.getAsArrayType(VarY->getType());
-      if (!VarXTy || !VarYTy)
-        return false;
-      if (VarXTy->isIncompleteArrayType() || VarYTy->isIncompleteArrayType())
-        return C.hasSameType(VarXTy->getElementType(), VarYTy->getElementType());
-    }
-    return false;
-  }
-
-  // Namespaces with the same name and inlinedness match.
-  if (const auto *NamespaceX = dyn_cast<NamespaceDecl>(X)) {
-    const auto *NamespaceY = cast<NamespaceDecl>(Y);
-    return NamespaceX->isInline() == NamespaceY->isInline();
-  }
-
-  // Identical template names and kinds match if their template parameter lists
-  // and patterns match.
-  if (const auto *TemplateX = dyn_cast<TemplateDecl>(X)) {
-    const auto *TemplateY = cast<TemplateDecl>(Y);
-    return isSameEntity(TemplateX->getTemplatedDecl(),
-                        TemplateY->getTemplatedDecl()) &&
-           isSameTemplateParameterList(TemplateX->getASTContext(),
-                                       TemplateX->getTemplateParameters(),
-                                       TemplateY->getTemplateParameters());
-  }
-
-  // Fields with the same name and the same type match.
-  if (const auto *FDX = dyn_cast<FieldDecl>(X)) {
-    const auto *FDY = cast<FieldDecl>(Y);
-    // FIXME: Also check the bitwidth is odr-equivalent, if any.
-    return X->getASTContext().hasSameType(FDX->getType(), FDY->getType());
-  }
-
-  // Indirect fields with the same target field match.
-  if (const auto *IFDX = dyn_cast<IndirectFieldDecl>(X)) {
-    const auto *IFDY = cast<IndirectFieldDecl>(Y);
-    return IFDX->getAnonField()->getCanonicalDecl() ==
-           IFDY->getAnonField()->getCanonicalDecl();
-  }
-
-  // Enumerators with the same name match.
-  if (isa<EnumConstantDecl>(X))
-    // FIXME: Also check the value is odr-equivalent.
-    return true;
-
-  // Using shadow declarations with the same target match.
-  if (const auto *USX = dyn_cast<UsingShadowDecl>(X)) {
-    const auto *USY = cast<UsingShadowDecl>(Y);
-    return USX->getTargetDecl() == USY->getTargetDecl();
-  }
-
-  // Using declarations with the same qualifier match. (We already know that
-  // the name matches.)
-  if (const auto *UX = dyn_cast<UsingDecl>(X)) {
-    const auto *UY = cast<UsingDecl>(Y);
-    return isSameQualifier(UX->getQualifier(), UY->getQualifier()) &&
-           UX->hasTypename() == UY->hasTypename() &&
-           UX->isAccessDeclaration() == UY->isAccessDeclaration();
-  }
-  if (const auto *UX = dyn_cast<UnresolvedUsingValueDecl>(X)) {
-    const auto *UY = cast<UnresolvedUsingValueDecl>(Y);
-    return isSameQualifier(UX->getQualifier(), UY->getQualifier()) &&
-           UX->isAccessDeclaration() == UY->isAccessDeclaration();
-  }
-  if (const auto *UX = dyn_cast<UnresolvedUsingTypenameDecl>(X)) {
-    return isSameQualifier(
-        UX->getQualifier(),
-        cast<UnresolvedUsingTypenameDecl>(Y)->getQualifier());
-  }
-
-  // Using-pack declarations are only created by instantiation, and match if
-  // they're instantiated from matching UnresolvedUsing...Decls.
-  if (const auto *UX = dyn_cast<UsingPackDecl>(X)) {
-    return declaresSameEntity(
-        UX->getInstantiatedFromUsingDecl(),
-        cast<UsingPackDecl>(Y)->getInstantiatedFromUsingDecl());
-  }
-
-  // Namespace alias definitions with the same target match.
-  if (const auto *NAX = dyn_cast<NamespaceAliasDecl>(X)) {
-    const auto *NAY = cast<NamespaceAliasDecl>(Y);
-    return NAX->getNamespace()->Equals(NAY->getNamespace());
-  }
-
-  return false;
-}
-
 /// Find the context in which we should search for previous declarations when
 /// looking for declarations to merge.
 DeclContext *ASTDeclReader::getPrimaryContextForMerging(ASTReader &Reader,
@@ -3511,12 +3126,13 @@ ASTDeclReader::FindExistingResult ASTDeclReader::findExisting(NamedDecl *D) {
     return Result;
   }
 
+  ASTContext &C = Reader.getContext();
   DeclContext *DC = D->getDeclContext()->getRedeclContext();
   if (TypedefNameForLinkage) {
     auto It = Reader.ImportedTypedefNamesForLinkage.find(
         std::make_pair(DC, TypedefNameForLinkage));
     if (It != Reader.ImportedTypedefNamesForLinkage.end())
-      if (isSameEntity(It->second, D))
+      if (C.isSameEntity(It->second, D))
         return FindExistingResult(Reader, D, It->second, AnonymousDeclNumber,
                                   TypedefNameForLinkage);
     // Go on to check in other places in case an existing typedef name
@@ -3528,7 +3144,7 @@ ASTDeclReader::FindExistingResult ASTDeclReader::findExisting(NamedDecl *D) {
     // in its context by number.
     if (auto *Existing = getAnonymousDeclForMerging(
             Reader, D->getLexicalDeclContext(), AnonymousDeclNumber))
-      if (isSameEntity(Existing, D))
+      if (C.isSameEntity(Existing, D))
         return FindExistingResult(Reader, D, Existing, AnonymousDeclNumber,
                                   TypedefNameForLinkage);
   } else if (DC->isTranslationUnit() &&
@@ -3560,7 +3176,7 @@ ASTDeclReader::FindExistingResult ASTDeclReader::findExisting(NamedDecl *D) {
                                    IEnd = IdResolver.end();
          I != IEnd; ++I) {
       if (NamedDecl *Existing = getDeclForMerging(*I, TypedefNameForLinkage))
-        if (isSameEntity(Existing, D))
+        if (C.isSameEntity(Existing, D))
           return FindExistingResult(Reader, D, Existing, AnonymousDeclNumber,
                                     TypedefNameForLinkage);
     }
@@ -3568,7 +3184,7 @@ ASTDeclReader::FindExistingResult ASTDeclReader::findExisting(NamedDecl *D) {
     DeclContext::lookup_result R = MergeDC->noload_lookup(Name);
     for (DeclContext::lookup_iterator I = R.begin(), E = R.end(); I != E; ++I) {
       if (NamedDecl *Existing = getDeclForMerging(*I, TypedefNameForLinkage))
-        if (isSameEntity(Existing, D))
+        if (C.isSameEntity(Existing, D))
           return FindExistingResult(Reader, D, Existing, AnonymousDeclNumber,
                                     TypedefNameForLinkage);
     }