diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -1605,7 +1605,8 @@
                                    QualType Wrapped);
 
   QualType getSubstTemplateTypeParmType(const TemplateTypeParmType *Replaced,
-                                        QualType Replacement) const;
+                                        QualType Replacement,
+                                        Optional<unsigned> PackIndex) const;
   QualType getSubstTemplateTypeParmPackType(
                                           const TemplateTypeParmType *Replaced,
                                             const TemplateArgument &ArgPack);
diff --git a/clang/include/clang/AST/JSONNodeDumper.h b/clang/include/clang/AST/JSONNodeDumper.h
--- a/clang/include/clang/AST/JSONNodeDumper.h
+++ b/clang/include/clang/AST/JSONNodeDumper.h
@@ -219,6 +219,7 @@
   void VisitUnaryTransformType(const UnaryTransformType *UTT);
   void VisitTagType(const TagType *TT);
   void VisitTemplateTypeParmType(const TemplateTypeParmType *TTPT);
+  void VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *STTPT);
   void VisitAutoType(const AutoType *AT);
   void VisitTemplateSpecializationType(const TemplateSpecializationType *TST);
   void VisitInjectedClassNameType(const InjectedClassNameType *ICNT);
diff --git a/clang/include/clang/AST/TextNodeDumper.h b/clang/include/clang/AST/TextNodeDumper.h
--- a/clang/include/clang/AST/TextNodeDumper.h
+++ b/clang/include/clang/AST/TextNodeDumper.h
@@ -316,6 +316,7 @@
   void VisitUnaryTransformType(const UnaryTransformType *T);
   void VisitTagType(const TagType *T);
   void VisitTemplateTypeParmType(const TemplateTypeParmType *T);
+  void VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *T);
   void VisitAutoType(const AutoType *T);
   void VisitDeducedTemplateSpecializationType(
       const DeducedTemplateSpecializationType *T);
diff --git a/clang/include/clang/AST/Type.h b/clang/include/clang/AST/Type.h
--- a/clang/include/clang/AST/Type.h
+++ b/clang/include/clang/AST/Type.h
@@ -1734,6 +1734,17 @@
     unsigned NumArgs;
   };
 
+  class SubstTemplateTypeParmTypeBitfields {
+    friend class SubstTemplateTypeParmType;
+
+    unsigned : NumTypeBits;
+
+    /// Represents the index within a pack if this represents a substitution
+    /// from a pack expansion. Positive non-zero number represents the index + 1,
+    /// zero means this is not substituted from an expansion.
+    unsigned PackIndex;
+  };
+
   class SubstTemplateTypeParmPackTypeBitfields {
     friend class SubstTemplateTypeParmPackType;
 
@@ -1816,6 +1827,7 @@
     ElaboratedTypeBitfields ElaboratedTypeBits;
     VectorTypeBitfields VectorTypeBits;
     SubstTemplateTypeParmPackTypeBitfields SubstTemplateTypeParmPackTypeBits;
+    SubstTemplateTypeParmTypeBitfields SubstTemplateTypeParmTypeBits;
     TemplateSpecializationTypeBitfields TemplateSpecializationTypeBits;
     DependentTemplateSpecializationTypeBitfields
       DependentTemplateSpecializationTypeBits;
@@ -4918,9 +4930,12 @@
   // The original type parameter.
   const TemplateTypeParmType *Replaced;
 
-  SubstTemplateTypeParmType(const TemplateTypeParmType *Param, QualType Canon)
+  SubstTemplateTypeParmType(const TemplateTypeParmType *Param, QualType Canon,
+                            Optional<unsigned> PackIndex)
       : Type(SubstTemplateTypeParm, Canon, Canon->getDependence()),
-        Replaced(Param) {}
+        Replaced(Param) {
+    SubstTemplateTypeParmTypeBits.PackIndex = PackIndex ? *PackIndex + 1 : 0;
+  }
 
 public:
   /// Gets the template parameter that was substituted for.
@@ -4934,18 +4949,25 @@
     return getCanonicalTypeInternal();
   }
 
+  Optional<unsigned> getPackIndex() const {
+    if (SubstTemplateTypeParmTypeBits.PackIndex == 0U)
+      return None;
+    return SubstTemplateTypeParmTypeBits.PackIndex - 1U;
+  }
+
   bool isSugared() const { return true; }
   QualType desugar() const { return getReplacementType(); }
 
   void Profile(llvm::FoldingSetNodeID &ID) {
-    Profile(ID, getReplacedParameter(), getReplacementType());
+    Profile(ID, getReplacedParameter(), getReplacementType(), getPackIndex());
   }
 
   static void Profile(llvm::FoldingSetNodeID &ID,
                       const TemplateTypeParmType *Replaced,
-                      QualType Replacement) {
+                      QualType Replacement, Optional<unsigned> PackIndex) {
     ID.AddPointer(Replaced);
     ID.AddPointer(Replacement.getAsOpaquePtr());
+    ID.AddInteger(PackIndex ? *PackIndex - 1U : 0U);
   }
 
   static bool classof(const Type *T) {
diff --git a/clang/include/clang/AST/TypeProperties.td b/clang/include/clang/AST/TypeProperties.td
--- a/clang/include/clang/AST/TypeProperties.td
+++ b/clang/include/clang/AST/TypeProperties.td
@@ -734,12 +734,15 @@
   def : Property<"replacementType", QualType> {
     let Read = [{ node->getReplacementType() }];
   }
+  def : Property<"PackIndex", Optional<UInt32>> {
+    let Read = [{ node->getPackIndex() }];
+  }
 
   def : Creator<[{
     // The call to getCanonicalType here existed in ASTReader.cpp, too.
     return ctx.getSubstTemplateTypeParmType(
         cast<TemplateTypeParmType>(replacedParameter),
-        ctx.getCanonicalType(replacementType));
+        ctx.getCanonicalType(replacementType), PackIndex);
   }]>;
 }
 
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -4736,19 +4736,20 @@
 /// Retrieve a substitution-result type.
 QualType
 ASTContext::getSubstTemplateTypeParmType(const TemplateTypeParmType *Parm,
-                                         QualType Replacement) const {
+                                         QualType Replacement,
+                                         Optional<unsigned> PackIndex) const {
   assert(Replacement.isCanonical()
          && "replacement types must always be canonical");
 
   llvm::FoldingSetNodeID ID;
-  SubstTemplateTypeParmType::Profile(ID, Parm, Replacement);
+  SubstTemplateTypeParmType::Profile(ID, Parm, Replacement, PackIndex);
   void *InsertPos = nullptr;
   SubstTemplateTypeParmType *SubstParm
     = SubstTemplateTypeParmTypes.FindNodeOrInsertPos(ID, InsertPos);
 
   if (!SubstParm) {
     SubstParm = new (*this, TypeAlignment)
-      SubstTemplateTypeParmType(Parm, Replacement);
+        SubstTemplateTypeParmType(Parm, Replacement, PackIndex);
     Types.push_back(SubstParm);
     SubstTemplateTypeParmTypes.InsertNode(SubstParm, InsertPos);
   }
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -1529,7 +1529,8 @@
     return ToReplacementTypeOrErr.takeError();
 
   return Importer.getToContext().getSubstTemplateTypeParmType(
-      *ReplacedOrErr, ToReplacementTypeOrErr->getCanonicalType());
+      *ReplacedOrErr, ToReplacementTypeOrErr->getCanonicalType(),
+      T->getPackIndex());
 }
 
 ExpectedType ASTNodeImporter::VisitSubstTemplateTypeParmPackType(
diff --git a/clang/lib/AST/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
--- a/clang/lib/AST/ASTStructuralEquivalence.cpp
+++ b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -1062,6 +1062,8 @@
     if (!IsStructurallyEquivalent(Context, Subst1->getReplacementType(),
                                   Subst2->getReplacementType()))
       return false;
+    if (Subst1->getPackIndex() != Subst2->getPackIndex())
+      return false;
     break;
   }
 
diff --git a/clang/lib/AST/JSONNodeDumper.cpp b/clang/lib/AST/JSONNodeDumper.cpp
--- a/clang/lib/AST/JSONNodeDumper.cpp
+++ b/clang/lib/AST/JSONNodeDumper.cpp
@@ -680,6 +680,12 @@
   JOS.attribute("decl", createBareDeclRef(TTPT->getDecl()));
 }
 
+void JSONNodeDumper::VisitSubstTemplateTypeParmType(
+    const SubstTemplateTypeParmType *STTPT) {
+  if (auto PackIndex = STTPT->getPackIndex())
+    JOS.attribute("pack_index", *PackIndex);
+}
+
 void JSONNodeDumper::VisitAutoType(const AutoType *AT) {
   JOS.attribute("undeduced", !AT->isDeduced());
   switch (AT->getKeyword()) {
diff --git a/clang/lib/AST/TextNodeDumper.cpp b/clang/lib/AST/TextNodeDumper.cpp
--- a/clang/lib/AST/TextNodeDumper.cpp
+++ b/clang/lib/AST/TextNodeDumper.cpp
@@ -1568,6 +1568,12 @@
   dumpDeclRef(T->getDecl());
 }
 
+void TextNodeDumper::VisitSubstTemplateTypeParmType(
+    const SubstTemplateTypeParmType *T) {
+  if (auto PackIndex = T->getPackIndex())
+    OS << " pack_index " << *PackIndex;
+}
+
 void TextNodeDumper::VisitAutoType(const AutoType *T) {
   if (T->isDecltypeAuto())
     OS << " decltype(auto)";
diff --git a/clang/lib/AST/Type.cpp b/clang/lib/AST/Type.cpp
--- a/clang/lib/AST/Type.cpp
+++ b/clang/lib/AST/Type.cpp
@@ -1167,7 +1167,7 @@
       return QualType(T, 0);
 
     return Ctx.getSubstTemplateTypeParmType(T->getReplacedParameter(),
-                                            replacementType);
+                                            replacementType, T->getPackIndex());
   }
 
   // FIXME: Non-trivial to implement, but important for C++
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1815,6 +1815,7 @@
       return NewT;
     }
 
+    Optional<unsigned> PackIndex;
     if (T->isParameterPack()) {
       assert(Arg.getKind() == TemplateArgument::Pack &&
              "Missing argument pack");
@@ -1832,6 +1833,7 @@
       }
 
       Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
+      PackIndex = getSema().ArgumentPackSubstitutionIndex;
     }
 
     assert(Arg.getKind() == TemplateArgument::Type &&
@@ -1840,8 +1842,8 @@
     QualType Replacement = Arg.getAsType();
 
     // TODO: only do this uniquing once, at the start of instantiation.
-    QualType Result
-      = getSema().Context.getSubstTemplateTypeParmType(T, Replacement);
+    QualType Result = getSema().Context.getSubstTemplateTypeParmType(
+        T, Replacement, PackIndex);
     SubstTemplateTypeParmTypeLoc NewTL
       = TLB.push<SubstTemplateTypeParmTypeLoc>(Result);
     NewTL.setNameLoc(TL.getNameLoc());
@@ -1879,11 +1881,10 @@
 
   TemplateArgument Arg = TL.getTypePtr()->getArgumentPack();
   Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
-  QualType Result = Arg.getAsType();
 
-  Result = getSema().Context.getSubstTemplateTypeParmType(
-                                      TL.getTypePtr()->getReplacedParameter(),
-                                                          Result);
+  QualType Result = getSema().Context.getSubstTemplateTypeParmType(
+      TL.getTypePtr()->getReplacedParameter(), Arg.getAsType(),
+      getSema().ArgumentPackSubstitutionIndex);
   SubstTemplateTypeParmTypeLoc NewTL
     = TLB.push<SubstTemplateTypeParmTypeLoc>(Result);
   NewTL.setNameLoc(TL.getNameLoc());
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -4858,7 +4858,8 @@
         Replacement = SemaRef.Context.getQualifiedType(
             Replacement.getUnqualifiedType(), Qs);
         T = SemaRef.Context.getSubstTemplateTypeParmType(
-            SubstTypeParam->getReplacedParameter(), Replacement);
+            SubstTypeParam->getReplacedParameter(), Replacement,
+            SubstTypeParam->getPackIndex());
       } else if ((AutoTy = dyn_cast<AutoType>(T)) && AutoTy->isDeduced()) {
         // 'auto' types behave the same way as template parameters.
         QualType Deduced = AutoTy->getDeducedType();
@@ -6414,9 +6415,8 @@
 
   // Always canonicalize the replacement type.
   Replacement = SemaRef.Context.getCanonicalType(Replacement);
-  QualType Result
-    = SemaRef.Context.getSubstTemplateTypeParmType(T->getReplacedParameter(),
-                                                   Replacement);
+  QualType Result = SemaRef.Context.getSubstTemplateTypeParmType(
+      T->getReplacedParameter(), Replacement, T->getPackIndex());
 
   // Propagate type-source information.
   SubstTemplateTypeParmTypeLoc NewTL
diff --git a/clang/test/AST/ast-dump-template-decls.cpp b/clang/test/AST/ast-dump-template-decls.cpp
--- a/clang/test/AST/ast-dump-template-decls.cpp
+++ b/clang/test/AST/ast-dump-template-decls.cpp
@@ -136,13 +136,13 @@
 };
 using t1 = foo<int, short>::bind<char, float>;
 // CHECK:      TemplateSpecializationType 0x{{[^ ]*}} 'Y<char, float, int, short>' sugar Y
-// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'char' sugar
+// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'char' sugar pack_index 0
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'Bs' dependent contains_unexpanded_pack depth 0 index 0 pack
-// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'float' sugar
+// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'float' sugar pack_index 1
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'Bs' dependent contains_unexpanded_pack depth 0 index 0 pack
-// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'int' sugar
+// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'int' sugar pack_index 2
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'Bs' dependent contains_unexpanded_pack depth 0 index 0 pack
-// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'short' sugar
+// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'short' sugar pack_index 3
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'Bs' dependent contains_unexpanded_pack depth 0 index 0 pack
 
 template <typename... T> struct D {
@@ -152,13 +152,13 @@
 // CHECK:      TemplateSpecializationType 0x{{[^ ]*}} 'B<int, short>' sugar alias B
 // CHECK:      FunctionProtoType 0x{{[^ ]*}} 'int (int (*)(float, int), int (*)(char, short))' cdecl
 // CHECK:      FunctionProtoType 0x{{[^ ]*}} 'int (float, int)' cdecl
-// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'float' sugar
+// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'float' sugar pack_index 0
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'T' dependent contains_unexpanded_pack depth 0 index 0 pack
-// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'int' sugar
+// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'int' sugar pack_index 0
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'U' dependent contains_unexpanded_pack depth 1 index 0 pack
 // CHECK:      FunctionProtoType 0x{{[^ ]*}} 'int (char, short)' cdecl
-// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'char' sugar
+// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'char' sugar pack_index 1
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'T' dependent contains_unexpanded_pack depth 0 index 0 pack
-// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'short' sugar
+// CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'short' sugar pack_index 1
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'U' dependent contains_unexpanded_pack depth 1 index 0 pack
 } // namespace PR56099