From 6bb02a27b9161e772ec3fd64c8ee9955787d583e Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Wed, 4 Nov 2015 03:40:30 +0000
Subject: [PATCH] [Sema] Implement __make_integer_seq

This new builtin template allows for incredibly fast instantiations of
templates like std::integer_sequence.

Performance numbers follow:
My work station has 64 GB of ram + 20 Xeon Cores at 2.8 GHz.

__make_integer_seq<std::integer_sequence, int, 90000> takes 0.25
seconds.

std::make_integer_sequence<int, 90000> takes unbound time, it is still
running.  Clang is consuming gigabytes of memory.

Differential Revision: http://reviews.llvm.org/D13786

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252036 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/clang/AST/ASTContext.h              | 15 +++++
 include/clang/AST/DataRecursiveASTVisitor.h |  4 ++
 include/clang/AST/DeclTemplate.h            | 30 +++++++++
 include/clang/AST/RecursiveASTVisitor.h     |  4 ++
 include/clang/Basic/Builtins.h              |  7 +++
 include/clang/Basic/DeclNodes.td            |  1 +
 include/clang/Basic/DiagnosticSemaKinds.td  |  8 ++-
 include/clang/Serialization/ASTBitCodes.h   |  5 +-
 lib/AST/ASTContext.cpp                      | 20 +++++-
 lib/AST/ASTDumper.cpp                       |  7 +++
 lib/AST/DeclBase.cpp                        |  1 +
 lib/AST/DeclTemplate.cpp                    | 67 +++++++++++++++++++++
 lib/CodeGen/CGDecl.cpp                      |  1 +
 lib/Sema/SemaLookup.cpp                     |  5 ++
 lib/Sema/SemaTemplate.cpp                   | 59 +++++++++++++++++-
 lib/Sema/SemaTemplateInstantiateDecl.cpp    |  5 ++
 lib/Serialization/ASTCommon.cpp             |  1 +
 lib/Serialization/ASTReader.cpp             |  3 +
 lib/Serialization/ASTReaderDecl.cpp         |  5 ++
 lib/Serialization/ASTWriter.cpp             |  2 +
 test/PCH/make-integer-seq.cpp               | 14 +++++
 test/SemaCXX/make_integer_seq.cpp           | 47 +++++++++++++++
 tools/libclang/CIndex.cpp                   |  1 +
 23 files changed, 308 insertions(+), 4 deletions(-)
 create mode 100644 test/PCH/make-integer-seq.cpp
 create mode 100644 test/SemaCXX/make_integer_seq.cpp

diff --git a/include/clang/AST/ASTContext.h b/include/clang/AST/ASTContext.h
index a16c74ed51..72bbb606df 100644
--- a/include/clang/AST/ASTContext.h
+++ b/include/clang/AST/ASTContext.h
@@ -70,6 +70,7 @@ namespace clang {
   class VTableContextBase;
 
   namespace Builtin { class Context; }
+  enum BuiltinTemplateKind : int;
 
   namespace comments {
     class FullComment;
@@ -246,6 +247,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// The identifier 'NSCopying'.
   IdentifierInfo *NSCopyingName = nullptr;
 
+  /// The identifier '__make_integer_seq'.
+  mutable IdentifierInfo *MakeIntegerSeqName = nullptr;
+
   QualType ObjCConstantStringType;
   mutable RecordDecl *CFConstantStringTypeDecl;
   
@@ -399,6 +403,7 @@ private:
   
   TranslationUnitDecl *TUDecl;
   mutable ExternCContextDecl *ExternCContext;
+  mutable BuiltinTemplateDecl *MakeIntegerSeqDecl;
 
   /// \brief The associated SourceManager object.a
   SourceManager &SourceMgr;
@@ -868,6 +873,7 @@ public:
   TranslationUnitDecl *getTranslationUnitDecl() const { return TUDecl; }
 
   ExternCContextDecl *getExternCContextDecl() const;
+  BuiltinTemplateDecl *getMakeIntegerSeqDecl() const;
 
   // Builtin Types.
   CanQualType VoidTy;
@@ -941,6 +947,9 @@ public:
   void PrintStats() const;
   const SmallVectorImpl<Type *>& getTypes() const { return Types; }
 
+  BuiltinTemplateDecl *buildBuiltinTemplateDecl(BuiltinTemplateKind BTK,
+                                                const IdentifierInfo *II) const;
+
   /// \brief Create a new implicit TU-level CXXRecordDecl or RecordDecl
   /// declaration.
   RecordDecl *buildImplicitRecord(StringRef Name,
@@ -1444,6 +1453,12 @@ public:
     return NSCopyingName;
   }
 
+  IdentifierInfo *getMakeIntegerSeqName() const {
+    if (!MakeIntegerSeqName)
+      MakeIntegerSeqName = &Idents.get("__make_integer_seq");
+    return MakeIntegerSeqName;
+  }
+
   /// \brief Retrieve the Objective-C "instancetype" type, if already known;
   /// otherwise, returns a NULL type;
   QualType getObjCInstanceType() {
diff --git a/include/clang/AST/DataRecursiveASTVisitor.h b/include/clang/AST/DataRecursiveASTVisitor.h
index 25e2c069ba..cc80a68436 100644
--- a/include/clang/AST/DataRecursiveASTVisitor.h
+++ b/include/clang/AST/DataRecursiveASTVisitor.h
@@ -1551,6 +1551,10 @@ DEF_TRAVERSE_DECL(FunctionTemplateDecl, {
     TRY_TO(TraverseFunctionInstantiations(D));
 })
 
+DEF_TRAVERSE_DECL(BuiltinTemplateDecl, {
+  TRY_TO(TraverseTemplateParameterListHelper(D->getTemplateParameters()));
+})
+
 DEF_TRAVERSE_DECL(TemplateTemplateParmDecl, {
   // D is the "T" in something like
   //   template <template <typename> class T> class container { };
diff --git a/include/clang/AST/DeclTemplate.h b/include/clang/AST/DeclTemplate.h
index bd38ac8823..a23fe4b5e5 100644
--- a/include/clang/AST/DeclTemplate.h
+++ b/include/clang/AST/DeclTemplate.h
@@ -25,6 +25,7 @@
 
 namespace clang {
 
+enum BuiltinTemplateKind : int;
 class TemplateParameterList;
 class TemplateDecl;
 class RedeclarableTemplateDecl;
@@ -1490,6 +1491,35 @@ public:
   friend TrailingObjects;
 };
 
+/// \brief Represents the builtin template declaration which is used to
+/// implement __make_integer_seq.  It serves no real purpose beyond existing as
+/// a place to hold template parameters.
+class BuiltinTemplateDecl : public TemplateDecl {
+  void anchor() override;
+
+  BuiltinTemplateDecl(const ASTContext &C, DeclContext *DC,
+                      DeclarationName Name, BuiltinTemplateKind BTK);
+
+  BuiltinTemplateKind BTK;
+
+public:
+  // Implement isa/cast/dyncast support
+  static bool classof(const Decl *D) { return classofKind(D->getKind()); }
+  static bool classofKind(Kind K) { return K == BuiltinTemplate; }
+
+  static BuiltinTemplateDecl *Create(const ASTContext &C, DeclContext *DC,
+                                     DeclarationName Name,
+                                     BuiltinTemplateKind BTK) {
+    return new (C, DC) BuiltinTemplateDecl(C, DC, Name, BTK);
+  }
+
+  SourceRange getSourceRange() const override LLVM_READONLY {
+    return SourceRange();
+  }
+
+  BuiltinTemplateKind getBuiltinTemplateKind() const { return BTK; }
+};
+
 /// \brief Represents a class template specialization, which refers to
 /// a class template with a given set of template arguments.
 ///
diff --git a/include/clang/AST/RecursiveASTVisitor.h b/include/clang/AST/RecursiveASTVisitor.h
index 341acd99e0..ab0aa96876 100644
--- a/include/clang/AST/RecursiveASTVisitor.h
+++ b/include/clang/AST/RecursiveASTVisitor.h
@@ -1603,6 +1603,10 @@ DEF_TRAVERSE_DECL(TemplateTemplateParmDecl, {
   TRY_TO(TraverseTemplateParameterListHelper(D->getTemplateParameters()));
 })
 
+DEF_TRAVERSE_DECL(BuiltinTemplateDecl, {
+  TRY_TO(TraverseTemplateParameterListHelper(D->getTemplateParameters()));
+})
+
 DEF_TRAVERSE_DECL(TemplateTypeParmDecl, {
   // D is the "T" in something like "template<typename T> class vector;"
   if (D->getTypeForDecl())
diff --git a/include/clang/Basic/Builtins.h b/include/clang/Basic/Builtins.h
index 13849fd5f2..41f19e7cbf 100644
--- a/include/clang/Basic/Builtins.h
+++ b/include/clang/Basic/Builtins.h
@@ -205,5 +205,12 @@ private:
 };
 
 }
+
+/// \brief Kinds of BuiltinTemplateDecl.
+enum BuiltinTemplateKind : int {
+  /// \brief This names the __make_integer_seq BuiltinTemplateDecl.
+  BTK__make_integer_seq
+};
+
 } // end namespace clang
 #endif
diff --git a/include/clang/Basic/DeclNodes.td b/include/clang/Basic/DeclNodes.td
index e1a23120c1..723ea547d8 100644
--- a/include/clang/Basic/DeclNodes.td
+++ b/include/clang/Basic/DeclNodes.td
@@ -59,6 +59,7 @@ def Named : Decl<1>;
       def VarTemplate : DDecl<RedeclarableTemplate>;
       def TypeAliasTemplate : DDecl<RedeclarableTemplate>;
     def TemplateTemplateParm : DDecl<Template>;
+    def BuiltinTemplate : DDecl<Template>;
   def Using : DDecl<Named>;
   def UsingShadow : DDecl<Named>;
   def ObjCMethod : DDecl<Named>, DeclContext;
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index 542ccbef76..0862a96eb2 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1995,7 +1995,13 @@ def err_concept_decl_invalid_specifiers : Error<
 def warn_cxx98_compat_unicode_type : Warning<
   "'%0' type specifier is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
- 
+
+// __make_integer_seq
+def err_integer_sequence_negative_length : Error<
+  "integer sequences must have non-negative sequence length">;
+def err_integer_sequence_integral_element_type : Error<
+  "integer sequences must have integral element type">;
+
 // Objective-C++
 def err_objc_decls_may_only_appear_in_global_scope : Error<
   "Objective-C declarations may only appear in global scope">;
diff --git a/include/clang/Serialization/ASTBitCodes.h b/include/clang/Serialization/ASTBitCodes.h
index 7af3c62855..a37bafdbe0 100644
--- a/include/clang/Serialization/ASTBitCodes.h
+++ b/include/clang/Serialization/ASTBitCodes.h
@@ -982,13 +982,16 @@ namespace clang {
 
       /// \brief The extern "C" context.
       PREDEF_DECL_EXTERN_C_CONTEXT_ID = 12,
+
+      /// \brief The internal '__make_integer_seq' template.
+      PREDEF_DECL_MAKE_INTEGER_SEQ_ID = 13,
     };
 
     /// \brief The number of declaration IDs that are predefined.
     ///
     /// For more information about predefined declarations, see the
     /// \c PredefinedDeclIDs type and the PREDEF_DECL_*_ID constants.
-    const unsigned int NUM_PREDEF_DECL_IDS = 13;
+    const unsigned int NUM_PREDEF_DECL_IDS = 14;
 
     /// \brief Record code for a list of local redeclarations of a declaration.
     const unsigned int LOCAL_REDECLARATIONS = 50;
diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index 6e3fcd87f1..0c30df0913 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -744,7 +744,7 @@ ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
       ucontext_tDecl(nullptr), BlockDescriptorType(nullptr),
       BlockDescriptorExtendedType(nullptr), cudaConfigureCallDecl(nullptr),
       FirstLocalImport(), LastLocalImport(), ExternCContext(nullptr),
-      SourceMgr(SM), LangOpts(LOpts),
+      MakeIntegerSeqDecl(nullptr), SourceMgr(SM), LangOpts(LOpts),
       SanitizerBL(new SanitizerBlacklist(LangOpts.SanitizerBlacklistFiles, SM)),
       AddrSpaceMap(nullptr), Target(nullptr), AuxTarget(nullptr),
       PrintingPolicy(LOpts), Idents(idents), Selectors(sels),
@@ -913,6 +913,24 @@ ExternCContextDecl *ASTContext::getExternCContextDecl() const {
   return ExternCContext;
 }
 
+BuiltinTemplateDecl *
+ASTContext::buildBuiltinTemplateDecl(BuiltinTemplateKind BTK,
+                                     const IdentifierInfo *II) const {
+  auto *BuiltinTemplate = BuiltinTemplateDecl::Create(*this, TUDecl, II, BTK);
+  BuiltinTemplate->setImplicit();
+  TUDecl->addDecl(BuiltinTemplate);
+
+  return BuiltinTemplate;
+}
+
+BuiltinTemplateDecl *
+ASTContext::getMakeIntegerSeqDecl() const {
+  if (!MakeIntegerSeqDecl)
+    MakeIntegerSeqDecl = buildBuiltinTemplateDecl(BTK__make_integer_seq,
+                                                  getMakeIntegerSeqName());
+  return MakeIntegerSeqDecl;
+}
+
 RecordDecl *ASTContext::buildImplicitRecord(StringRef Name,
                                             RecordDecl::TagKind TK) const {
   SourceLocation Loc;
diff --git a/lib/AST/ASTDumper.cpp b/lib/AST/ASTDumper.cpp
index 0c9c14ba30..d5e5372f01 100644
--- a/lib/AST/ASTDumper.cpp
+++ b/lib/AST/ASTDumper.cpp
@@ -21,6 +21,7 @@
 #include "clang/AST/DeclVisitor.h"
 #include "clang/AST/StmtVisitor.h"
 #include "clang/AST/TypeVisitor.h"
+#include "clang/Basic/Builtins.h"
 #include "clang/Basic/Module.h"
 #include "clang/Basic/SourceManager.h"
 #include "llvm/Support/raw_ostream.h"
@@ -447,6 +448,7 @@ namespace  {
         const ClassTemplatePartialSpecializationDecl *D);
     void VisitClassScopeFunctionSpecializationDecl(
         const ClassScopeFunctionSpecializationDecl *D);
+    void VisitBuiltinTemplateDecl(const BuiltinTemplateDecl *D);
     void VisitVarTemplateDecl(const VarTemplateDecl *D);
     void VisitVarTemplateSpecializationDecl(
         const VarTemplateSpecializationDecl *D);
@@ -1333,6 +1335,11 @@ void ASTDumper::VisitVarTemplateDecl(const VarTemplateDecl *D) {
   VisitTemplateDecl(D, false);
 }
 
+void ASTDumper::VisitBuiltinTemplateDecl(const BuiltinTemplateDecl *D) {
+  dumpName(D);
+  dumpTemplateParameters(D->getTemplateParameters());
+}
+
 void ASTDumper::VisitVarTemplateSpecializationDecl(
     const VarTemplateSpecializationDecl *D) {
   dumpTemplateArgumentList(D->getTemplateArgs());
diff --git a/lib/AST/DeclBase.cpp b/lib/AST/DeclBase.cpp
index 406dff6899..fd6df024bb 100644
--- a/lib/AST/DeclBase.cpp
+++ b/lib/AST/DeclBase.cpp
@@ -639,6 +639,7 @@ unsigned Decl::getIdentifierNamespaceForKind(Kind DeclKind) {
     case ExternCContext:
 
     case UsingDirective:
+    case BuiltinTemplate:
     case ClassTemplateSpecialization:
     case ClassTemplatePartialSpecialization:
     case ClassScopeFunctionSpecialization:
diff --git a/lib/AST/DeclTemplate.cpp b/lib/AST/DeclTemplate.cpp
index dd3baef893..18a30b2f1c 100644
--- a/lib/AST/DeclTemplate.cpp
+++ b/lib/AST/DeclTemplate.cpp
@@ -18,6 +18,7 @@
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/TypeLoc.h"
+#include "clang/Basic/Builtins.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "llvm/ADT/STLExtras.h"
 #include <memory>
@@ -1191,3 +1192,69 @@ VarTemplatePartialSpecializationDecl::CreateDeserialized(ASTContext &C,
                                                          unsigned ID) {
   return new (C, ID) VarTemplatePartialSpecializationDecl(C);
 }
+
+static TemplateParameterList *
+createMakeIntegerSeqParameterList(const ASTContext &C, DeclContext *DC) {
+  // typename T
+  auto *T = TemplateTypeParmDecl::Create(
+      C, DC, SourceLocation(), SourceLocation(), /*Depth=*/1, /*Position=*/0,
+      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/false);
+  T->setImplicit(true);
+
+  // T ...Ints
+  TypeSourceInfo *TI =
+      C.getTrivialTypeSourceInfo(QualType(T->getTypeForDecl(), 0));
+  auto *N = NonTypeTemplateParmDecl::Create(
+      C, DC, SourceLocation(), SourceLocation(), /*Depth=*/0, /*Position=*/1,
+      /*Id=*/nullptr, TI->getType(), /*ParameterPack=*/true, TI);
+  N->setImplicit(true);
+
+  // <typename T, T ...Ints>
+  NamedDecl *P[2] = {T, N};
+  auto *TPL = TemplateParameterList::Create(
+      C, SourceLocation(), SourceLocation(), P, 2, SourceLocation());
+
+  // template <typename T, ...Ints> class IntSeq
+  auto *TemplateTemplateParm = TemplateTemplateParmDecl::Create(
+      C, DC, SourceLocation(), /*Depth=*/0, /*Position=*/0,
+      /*ParameterPack=*/false, /*Id=*/nullptr, TPL);
+  TemplateTemplateParm->setImplicit(true);
+
+  // typename T
+  auto *TemplateTypeParm = TemplateTypeParmDecl::Create(
+      C, DC, SourceLocation(), SourceLocation(), /*Depth=*/0, /*Position=*/1,
+      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/false);
+  TemplateTypeParm->setImplicit(true);
+
+  // T N
+  TypeSourceInfo *TInfo = C.getTrivialTypeSourceInfo(
+      QualType(TemplateTypeParm->getTypeForDecl(), 0));
+  auto *NonTypeTemplateParm = NonTypeTemplateParmDecl::Create(
+      C, DC, SourceLocation(), SourceLocation(), /*Depth=*/0, /*Position=*/2,
+      /*Id=*/nullptr, TInfo->getType(), /*ParameterPack=*/false, TInfo);
+  NamedDecl *Params[] = {TemplateTemplateParm, TemplateTypeParm,
+                         NonTypeTemplateParm};
+
+  // template <template <typename T, T ...Ints> class IntSeq, typename T, T N>
+  return TemplateParameterList::Create(C, SourceLocation(), SourceLocation(),
+                                       Params, 3, SourceLocation());
+}
+
+static TemplateParameterList *createBuiltinTemplateParameterList(
+    const ASTContext &C, DeclContext *DC, BuiltinTemplateKind BTK) {
+  switch (BTK) {
+  case BTK__make_integer_seq:
+    return createMakeIntegerSeqParameterList(C, DC);
+  }
+
+  llvm_unreachable("unhandled BuiltinTemplateKind!");
+}
+
+void BuiltinTemplateDecl::anchor() {}
+
+BuiltinTemplateDecl::BuiltinTemplateDecl(const ASTContext &C, DeclContext *DC,
+                                         DeclarationName Name,
+                                         BuiltinTemplateKind BTK)
+    : TemplateDecl(BuiltinTemplate, DC, SourceLocation(), Name,
+                   createBuiltinTemplateParameterList(C, DC, BTK)),
+      BTK(BTK) {}
diff --git a/lib/CodeGen/CGDecl.cpp b/lib/CodeGen/CGDecl.cpp
index fe80c8058b..35c7dfba06 100644
--- a/lib/CodeGen/CGDecl.cpp
+++ b/lib/CodeGen/CGDecl.cpp
@@ -35,6 +35,7 @@ using namespace CodeGen;
 
 void CodeGenFunction::EmitDecl(const Decl &D) {
   switch (D.getKind()) {
+  case Decl::BuiltinTemplate:
   case Decl::TranslationUnit:
   case Decl::ExternCContext:
   case Decl::Namespace:
diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index 0ba34ea8ef..66cfada9f4 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -627,6 +627,11 @@ static bool LookupBuiltin(Sema &S, LookupResult &R) {
         R.addDecl(S.getASTContext().getFloat128StubType());
         return true;
       }
+      if (S.getLangOpts().CPlusPlus && NameKind == Sema::LookupOrdinaryName &&
+          II == S.getASTContext().getMakeIntegerSeqName()) {
+        R.addDecl(S.getASTContext().getMakeIntegerSeqDecl());
+        return true;
+      }
 
       // If this is a builtin on this (or all) targets, create the decl.
       if (unsigned BuiltinID = II->getBuiltinID()) {
diff --git a/lib/Sema/SemaTemplate.cpp b/lib/Sema/SemaTemplate.cpp
index de89ce15ff..8821088baa 100644
--- a/lib/Sema/SemaTemplate.cpp
+++ b/lib/Sema/SemaTemplate.cpp
@@ -18,6 +18,7 @@
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/TypeVisitor.h"
+#include "clang/Basic/Builtins.h"
 #include "clang/Basic/LangOptions.h"
 #include "clang/Basic/PartialDiagnostic.h"
 #include "clang/Basic/TargetInfo.h"
@@ -208,7 +209,8 @@ TemplateNameKind Sema::isTemplateName(Scope *S,
       R.suppressDiagnostics();
     } else {
       assert(isa<ClassTemplateDecl>(TD) || isa<TemplateTemplateParmDecl>(TD) ||
-             isa<TypeAliasTemplateDecl>(TD) || isa<VarTemplateDecl>(TD));
+             isa<TypeAliasTemplateDecl>(TD) || isa<VarTemplateDecl>(TD) ||
+             isa<BuiltinTemplateDecl>(TD));
       TemplateKind =
           isa<VarTemplateDecl>(TD) ? TNK_Var_template : TNK_Type_template;
     }
@@ -2017,6 +2019,58 @@ void Sema::NoteAllFoundTemplates(TemplateName Name) {
   }
 }
 
+static QualType
+checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
+                           const SmallVectorImpl<TemplateArgument> &Converted,
+                           SourceLocation TemplateLoc,
+                           TemplateArgumentListInfo &TemplateArgs) {
+  ASTContext &Context = SemaRef.getASTContext();
+  switch (BTD->getBuiltinTemplateKind()) {
+  case BTK__make_integer_seq:
+    // Specializations of __make_integer_seq<S, T, N> are treated like
+    // S<T, 0, ..., N-1>.
+
+    // C++14 [inteseq.intseq]p1:
+    //   T shall be an integer type.
+    if (!Converted[1].getAsType()->isIntegralType(Context)) {
+      SemaRef.Diag(TemplateArgs[1].getLocation(),
+                   diag::err_integer_sequence_integral_element_type);
+      return QualType();
+    }
+
+    // C++14 [inteseq.make]p1:
+    //   If N is negative the program is ill-formed.
+    TemplateArgument NumArgsArg = Converted[2];
+    llvm::APSInt NumArgs = NumArgsArg.getAsIntegral();
+    if (NumArgs < 0) {
+      SemaRef.Diag(TemplateArgs[2].getLocation(),
+                   diag::err_integer_sequence_negative_length);
+      return QualType();
+    }
+
+    QualType ArgTy = NumArgsArg.getIntegralType();
+    TemplateArgumentListInfo SyntheticTemplateArgs;
+    // The type argument gets reused as the first template argument in the
+    // synthetic template argument list.
+    SyntheticTemplateArgs.addArgument(TemplateArgs[1]);
+    // Expand N into 0 ... N-1.
+    for (llvm::APSInt I(NumArgs.getBitWidth(), NumArgs.isUnsigned());
+         I < NumArgs; ++I) {
+      TemplateArgument TA(Context, I, ArgTy);
+      Expr *E = SemaRef.BuildExpressionFromIntegralTemplateArgument(
+                           TA, TemplateArgs[2].getLocation())
+                    .getAs<Expr>();
+      SyntheticTemplateArgs.addArgument(
+          TemplateArgumentLoc(TemplateArgument(E), E));
+    }
+    // The first template argument will be reused as the template decl that
+    // our synthetic template arguments will be applied to.
+    return SemaRef.CheckTemplateIdType(Converted[0].getAsTemplate(),
+                                       TemplateLoc, SyntheticTemplateArgs);
+  }
+  llvm_unreachable("unexpected BuiltinTemplateDecl!");
+}
+
 QualType Sema::CheckTemplateIdType(TemplateName Name,
                                    SourceLocation TemplateLoc,
                                    TemplateArgumentListInfo &TemplateArgs) {
@@ -2171,6 +2225,9 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     CanonType = Context.getTypeDeclType(Decl);
     assert(isa<RecordType>(CanonType) &&
            "type of non-dependent specialization is not a RecordType");
+  } else if (auto *BTD = dyn_cast<BuiltinTemplateDecl>(Template)) {
+    CanonType = checkBuiltinTemplateIdType(*this, BTD, Converted, TemplateLoc,
+                                           TemplateArgs);
   }
 
   // Build the fully-sugared type for this class template
diff --git a/lib/Sema/SemaTemplateInstantiateDecl.cpp b/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 1789855c40..f5f03d05e8 100644
--- a/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -922,6 +922,11 @@ Decl *TemplateDeclInstantiator::VisitEnumConstantDecl(EnumConstantDecl *D) {
   llvm_unreachable("EnumConstantDecls can only occur within EnumDecls.");
 }
 
+Decl *
+TemplateDeclInstantiator::VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D) {
+  llvm_unreachable("BuiltinTemplateDecls cannot be instantiated.");
+}
+
 Decl *TemplateDeclInstantiator::VisitClassTemplateDecl(ClassTemplateDecl *D) {
   bool isFriend = (D->getFriendObjectKind() != Decl::FOK_None);
 
diff --git a/lib/Serialization/ASTCommon.cpp b/lib/Serialization/ASTCommon.cpp
index ec1a936684..2b78d74586 100644
--- a/lib/Serialization/ASTCommon.cpp
+++ b/lib/Serialization/ASTCommon.cpp
@@ -329,6 +329,7 @@ bool serialization::isRedeclarableDeclKind(unsigned Kind) {
   case Decl::ClassScopeFunctionSpecialization:
   case Decl::Import:
   case Decl::OMPThreadPrivate:
+  case Decl::BuiltinTemplate:
     return false;
 
   // These indirectly derive from Redeclarable<T> but are not actually
diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index c09e97c158..b7b56f68b8 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -6412,6 +6412,9 @@ static Decl *getPredefinedDecl(ASTContext &Context, PredefinedDeclIDs ID) {
 
   case PREDEF_DECL_EXTERN_C_CONTEXT_ID:
     return Context.getExternCContextDecl();
+
+  case PREDEF_DECL_MAKE_INTEGER_SEQ_ID:
+    return Context.getMakeIntegerSeqDecl();
   }
   llvm_unreachable("PredefinedDeclIDs unknown enum value");
 }
diff --git a/lib/Serialization/ASTReaderDecl.cpp b/lib/Serialization/ASTReaderDecl.cpp
index 50ef36ef63..317846c7a4 100644
--- a/lib/Serialization/ASTReaderDecl.cpp
+++ b/lib/Serialization/ASTReaderDecl.cpp
@@ -293,6 +293,7 @@ namespace clang {
     DeclID VisitTemplateDecl(TemplateDecl *D);
     RedeclarableResult VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D);
     void VisitClassTemplateDecl(ClassTemplateDecl *D);
+    void VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D);
     void VisitVarTemplateDecl(VarTemplateDecl *D);
     void VisitFunctionTemplateDecl(FunctionTemplateDecl *D);
     void VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D);
@@ -1856,6 +1857,10 @@ void ASTDeclReader::VisitClassTemplateDecl(ClassTemplateDecl *D) {
   }
 }
 
+void ASTDeclReader::VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D) {
+  llvm_unreachable("BuiltinTemplates are not serialized");
+}
+
 /// TODO: Unify with ClassTemplateDecl version?
 ///       May require unifying ClassTemplateDecl and
 ///        VarTemplateDecl beyond TemplateDecl...
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index c5a9755fed..8fc32eaaa9 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -4136,6 +4136,8 @@ uint64_t ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   RegisterPredefDecl(Context.BuiltinMSVaListDecl,
                      PREDEF_DECL_BUILTIN_MS_VA_LIST_ID);
   RegisterPredefDecl(Context.ExternCContext, PREDEF_DECL_EXTERN_C_CONTEXT_ID);
+  RegisterPredefDecl(Context.MakeIntegerSeqDecl,
+                     PREDEF_DECL_MAKE_INTEGER_SEQ_ID);
 
   // Build a record containing all of the tentative definitions in this file, in
   // TentativeDefinitions order.  Generally, this record will be empty for
diff --git a/test/PCH/make-integer-seq.cpp b/test/PCH/make-integer-seq.cpp
new file mode 100644
index 0000000000..3622c33449
--- /dev/null
+++ b/test/PCH/make-integer-seq.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -std=c++14 -x c++-header %s -emit-pch -o %t.pch
+// RUN: %clang_cc1 -std=c++14 -x c++ /dev/null -include-pch %t.pch
+
+template <class T, T... I>
+struct Seq {
+    static constexpr T PackSize = sizeof...(I);
+};
+
+template <typename T, T N>
+using MakeSeq = __make_integer_seq<Seq, T, N>;
+
+void fn1() {
+  MakeSeq<int, 3> x;
+}
diff --git a/test/SemaCXX/make_integer_seq.cpp b/test/SemaCXX/make_integer_seq.cpp
new file mode 100644
index 0000000000..f19bed2786
--- /dev/null
+++ b/test/SemaCXX/make_integer_seq.cpp
@@ -0,0 +1,47 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+
+template <class T, T... I>
+struct Seq {
+  static constexpr T PackSize = sizeof...(I);
+};
+
+template <typename T, T N>
+using MakeSeq = __make_integer_seq<Seq, T, N>;
+
+static_assert(__is_same(MakeSeq<int, 0>, Seq<int>), "");
+static_assert(__is_same(MakeSeq<int, 1>, Seq<int, 0>), "");
+static_assert(__is_same(MakeSeq<int, 2>, Seq<int, 0, 1>), "");
+static_assert(__is_same(MakeSeq<int, 3>, Seq<int, 0, 1, 2>), "");
+static_assert(__is_same(MakeSeq<int, 4>, Seq<int, 0, 1, 2, 3>), "");
+
+static_assert(__is_same(MakeSeq<unsigned int, 0U>, Seq<unsigned int>), "");
+static_assert(__is_same(MakeSeq<unsigned int, 1U>, Seq<unsigned int, 0U>), "");
+static_assert(__is_same(MakeSeq<unsigned int, 2U>, Seq<unsigned int, 0U, 1U>), "");
+static_assert(__is_same(MakeSeq<unsigned int, 3U>, Seq<unsigned int, 0U, 1U, 2U>), "");
+static_assert(__is_same(MakeSeq<unsigned int, 4U>, Seq<unsigned int, 0U, 1U, 2U, 3U>), "");
+
+static_assert(__is_same(MakeSeq<long long, 0LL>, Seq<long long>), "");
+static_assert(__is_same(MakeSeq<long long, 1LL>, Seq<long long, 0LL>), "");
+static_assert(__is_same(MakeSeq<long long, 2LL>, Seq<long long, 0LL, 1LL>), "");
+static_assert(__is_same(MakeSeq<long long, 3LL>, Seq<long long, 0LL, 1LL, 2LL>), "");
+static_assert(__is_same(MakeSeq<long long, 4LL>, Seq<long long, 0LL, 1LL, 2LL, 3LL>), "");
+
+static_assert(__is_same(MakeSeq<unsigned long long, 0ULL>, Seq<unsigned long long>), "");
+static_assert(__is_same(MakeSeq<unsigned long long, 1ULL>, Seq<unsigned long long, 0ULL>), "");
+static_assert(__is_same(MakeSeq<unsigned long long, 2ULL>, Seq<unsigned long long, 0ULL, 1ULL>), "");
+static_assert(__is_same(MakeSeq<unsigned long long, 3ULL>, Seq<unsigned long long, 0ULL, 1ULL, 2ULL>), "");
+static_assert(__is_same(MakeSeq<unsigned long long, 4ULL>, Seq<unsigned long long, 0ULL, 1ULL, 2ULL, 3ULL>), "");
+
+template <typename T, T N>
+using ErrorSeq = __make_integer_seq<Seq, T, N>; // expected-error{{must have non-negative sequence length}} \
+                                                   expected-error{{must have integral element type}}
+
+enum Color : int { Red,
+                   Green,
+                   Blue };
+using illformed1 = ErrorSeq<Color, Blue>; // expected-note{{in instantiation}}
+
+using illformed2 = ErrorSeq<int, -5>;
+
+template <typename T, T N> void f() {}
+__make_integer_seq<f, int, 0> x; // expected-error{{template template parameter must be a class template or type alias template}}
diff --git a/tools/libclang/CIndex.cpp b/tools/libclang/CIndex.cpp
index be53b82274..ac6f98de40 100644
--- a/tools/libclang/CIndex.cpp
+++ b/tools/libclang/CIndex.cpp
@@ -5110,6 +5110,7 @@ CXCursor clang_getCursorDefinition(CXCursor C) {
   case Decl::Import:
   case Decl::OMPThreadPrivate:
   case Decl::ObjCTypeParam:
+  case Decl::BuiltinTemplate:
     return C;
 
   // Declaration kinds that don't make any sense here, but are
-- 
2.50.1