Index: include/clang/AST/DeclTemplate.h
===================================================================
--- include/clang/AST/DeclTemplate.h
+++ include/clang/AST/DeclTemplate.h
@@ -46,7 +46,8 @@
 /// \brief Stores a list of template parameters for a TemplateDecl and its
 /// derived classes.
 class TemplateParameterList final
-    : private llvm::TrailingObjects<TemplateParameterList, NamedDecl *> {
+    : private llvm::TrailingObjects<TemplateParameterList, NamedDecl *,
+                                    Expr *> {
 
   /// The location of the 'template' keyword.
   SourceLocation TemplateLoc;
@@ -56,26 +57,35 @@
 
   /// The number of template parameters in this template
   /// parameter list.
-  unsigned NumParams : 31;
+  unsigned NumParams : 30;
 
   /// Whether this template parameter list contains an unexpanded parameter
   /// pack.
   unsigned ContainsUnexpandedParameterPack : 1;
 
+  /// Whether this template parameter list has an associated requires-clause
+  unsigned HasRequiresClause : 1;
+
 protected:
   size_t numTrailingObjects(OverloadToken<NamedDecl *>) const {
     return NumParams;
   }
 
+  size_t numTrailingObjects(OverloadToken<Expr *>) const {
+    return HasRequiresClause;
+  }
+
   TemplateParameterList(SourceLocation TemplateLoc, SourceLocation LAngleLoc,
-                        ArrayRef<NamedDecl *> Params, SourceLocation RAngleLoc);
+                        ArrayRef<NamedDecl *> Params, SourceLocation RAngleLoc,
+                        Expr *RequiresClause);
 
 public:
   static TemplateParameterList *Create(const ASTContext &C,
                                        SourceLocation TemplateLoc,
                                        SourceLocation LAngleLoc,
                                        ArrayRef<NamedDecl *> Params,
-                                       SourceLocation RAngleLoc);
+                                       SourceLocation RAngleLoc,
+                                       Expr *RequiresClause);
 
   /// \brief Iterates through the template parameters in this list.
   typedef NamedDecl** iterator;
@@ -127,6 +137,16 @@
     return ContainsUnexpandedParameterPack;
   }
 
+  /// \brief The constraint-expression of the associated requires-clause.
+  Expr *getRequiresClause() {
+    return HasRequiresClause ? *getTrailingObjects<Expr *>() : nullptr;
+  }
+
+  /// \brief The constraint-expression of the associated requires-clause.
+  const Expr *getRequiresClause() const {
+    return HasRequiresClause ? *getTrailingObjects<Expr *>() : nullptr;
+  }
+
   SourceLocation getTemplateLoc() const { return TemplateLoc; }
   SourceLocation getLAngleLoc() const { return LAngleLoc; }
   SourceLocation getRAngleLoc() const { return RAngleLoc; }
@@ -136,36 +156,33 @@
   }
 
   friend TrailingObjects;
-  template <size_t N> friend class FixedSizeTemplateParameterListStorage;
+
+  template <size_t N, bool HasRequiresClause>
+  friend class FixedSizeTemplateParameterListStorage;
 };
 
-/// \brief Stores a list of template parameters for a TemplateDecl and its
-/// derived classes. Suitable for creating on the stack.
-template <size_t N> class FixedSizeTemplateParameterListStorage {
-  // This is kinda ugly: TemplateParameterList usually gets allocated
-  // in a block of memory with NamedDecls appended to it. Here, to get
-  // it stack allocated, we include the params as a separate
-  // variable. After allocation, the TemplateParameterList object
-  // treats them as part of itself.
-  TemplateParameterList List;
-  NamedDecl *Params[N];
+/// \brief Stores a list of template parameters and the associated
+/// requires-clause (if any) for a TemplateDecl and its derived classes.
+/// Suitable for creating on the stack.
+template <size_t N, bool HasRequiresClause>
+class FixedSizeTemplateParameterListStorage
+    : public TemplateParameterList::FixedSizeStorageOwner {
+  typename TemplateParameterList::FixedSizeStorage<
+      NamedDecl *, Expr *>::with_counts<
+      N, HasRequiresClause ? 1u : 0u
+      >::type storage;
 
 public:
   FixedSizeTemplateParameterListStorage(SourceLocation TemplateLoc,
                                         SourceLocation LAngleLoc,
                                         ArrayRef<NamedDecl *> Params,
-                                        SourceLocation RAngleLoc)
-      : List(TemplateLoc, LAngleLoc, Params, RAngleLoc) {
-    // Because we're doing an evil layout hack above, have some
-    // asserts, just to double-check everything is laid out like
-    // expected.
-    assert(sizeof(*this) ==
-               TemplateParameterList::totalSizeToAlloc<NamedDecl *>(N) &&
-           "Object layout not as expected");
-    assert(this->Params == List.getTrailingObjects<NamedDecl *>() &&
-           "Object layout not as expected");
-  }
-  TemplateParameterList *get() { return &List; }
+                                        SourceLocation RAngleLoc,
+                                        Expr *RequiresClause)
+      : FixedSizeStorageOwner(
+            (assert(N == Params.size()),
+             assert(HasRequiresClause == static_cast<bool>(RequiresClause)),
+             new (static_cast<void *>(&storage)) TemplateParameterList(
+                 TemplateLoc, LAngleLoc, Params, RAngleLoc, RequiresClause))) {}
 };
 
 /// \brief A template argument list.
@@ -355,6 +372,11 @@
     return TemplateParams;
   }
 
+  /// Get the constraint-expression from the associated requires-clause (if any)
+  const Expr *getRequiresClause() const {
+    return TemplateParams ? TemplateParams->getRequiresClause() : nullptr;
+  }
+
   /// Get the underlying, templated declaration.
   NamedDecl *getTemplatedDecl() const { return TemplatedDecl.getPointer(); }
 
Index: lib/AST/ASTContext.cpp
===================================================================
--- lib/AST/ASTContext.cpp
+++ lib/AST/ASTContext.cpp
@@ -653,6 +653,10 @@
                                            cast<TemplateTemplateParmDecl>(*P)));
   }
 
+  assert(!TTP->getRequiresClause() &&
+         "Unexpected requires-clause on template template-parameter");
+  constexpr Expr *const CanonRequiresClause = nullptr;
+
   TemplateTemplateParmDecl *CanonTTP
     = TemplateTemplateParmDecl::Create(*this, getTranslationUnitDecl(), 
                                        SourceLocation(), TTP->getDepth(),
@@ -662,7 +666,8 @@
                          TemplateParameterList::Create(*this, SourceLocation(),
                                                        SourceLocation(),
                                                        CanonParams,
-                                                       SourceLocation()));
+                                                       SourceLocation(),
+                                                       CanonRequiresClause));
 
   // Get the new insert position for the node we care about.
   Canonical = CanonTemplateTemplateParms.FindNodeOrInsertPos(ID, InsertPos);
Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -2245,11 +2245,22 @@
     ToParams.push_back(cast<NamedDecl>(To));
   }
   
+  Expr *ToRequiresClause;
+  if (Expr *const R = Params->getRequiresClause()) {
+    ToRequiresClause = Importer.Import(R);
+    if (!ToRequiresClause)
+      return nullptr;
+  }
+  else {
+    ToRequiresClause = nullptr;
+  }
+
   return TemplateParameterList::Create(Importer.getToContext(),
                                        Importer.Import(Params->getTemplateLoc()),
                                        Importer.Import(Params->getLAngleLoc()),
                                        ToParams,
-                                       Importer.Import(Params->getRAngleLoc()));
+                                       Importer.Import(Params->getRAngleLoc()),
+                                       ToRequiresClause);
 }
 
 TemplateArgument 
Index: lib/AST/DeclTemplate.cpp
===================================================================
--- lib/AST/DeclTemplate.cpp
+++ lib/AST/DeclTemplate.cpp
@@ -31,9 +31,11 @@
 TemplateParameterList::TemplateParameterList(SourceLocation TemplateLoc,
                                              SourceLocation LAngleLoc,
                                              ArrayRef<NamedDecl *> Params,
-                                             SourceLocation RAngleLoc)
+                                             SourceLocation RAngleLoc,
+                                             Expr *RequiresClause)
   : TemplateLoc(TemplateLoc), LAngleLoc(LAngleLoc), RAngleLoc(RAngleLoc),
-    NumParams(Params.size()), ContainsUnexpandedParameterPack(false) {
+    NumParams(Params.size()), ContainsUnexpandedParameterPack(false),
+    HasRequiresClause(static_cast<bool>(RequiresClause)) {
   assert(this->NumParams == NumParams && "Too many template parameters");
   for (unsigned Idx = 0; Idx < NumParams; ++Idx) {
     NamedDecl *P = Params[Idx];
@@ -52,15 +54,21 @@
       // template parameter list does too.
     }
   }
+  if (RequiresClause) {
+    *getTrailingObjects<Expr *>() = RequiresClause;
+  }
 }
 
-TemplateParameterList *TemplateParameterList::Create(
-    const ASTContext &C, SourceLocation TemplateLoc, SourceLocation LAngleLoc,
-    ArrayRef<NamedDecl *> Params, SourceLocation RAngleLoc) {
-  void *Mem = C.Allocate(totalSizeToAlloc<NamedDecl *>(Params.size()),
+TemplateParameterList *
+TemplateParameterList::Create(const ASTContext &C, SourceLocation TemplateLoc,
+                              SourceLocation LAngleLoc,
+                              ArrayRef<NamedDecl *> Params,
+                              SourceLocation RAngleLoc, Expr *RequiresClause) {
+  void *Mem = C.Allocate(totalSizeToAlloc<NamedDecl *, Expr *>(
+                             Params.size(), RequiresClause ? 1u : 0u),
                          llvm::alignOf<TemplateParameterList>());
   return new (Mem) TemplateParameterList(TemplateLoc, LAngleLoc, Params,
-                                         RAngleLoc);
+                                         RAngleLoc, RequiresClause);
 }
 
 unsigned TemplateParameterList::getMinRequiredArguments() const {
@@ -1211,7 +1219,7 @@
   // <typename T, T ...Ints>
   NamedDecl *P[2] = {T, N};
   auto *TPL = TemplateParameterList::Create(
-      C, SourceLocation(), SourceLocation(), P, SourceLocation());
+      C, SourceLocation(), SourceLocation(), P, SourceLocation(), nullptr);
 
   // template <typename T, ...Ints> class IntSeq
   auto *TemplateTemplateParm = TemplateTemplateParmDecl::Create(
@@ -1236,7 +1244,7 @@
 
   // template <template <typename T, T ...Ints> class IntSeq, typename T, T N>
   return TemplateParameterList::Create(C, SourceLocation(), SourceLocation(),
-                                       Params, SourceLocation());
+                                       Params, SourceLocation(), nullptr);
 }
 
 static TemplateParameterList *createBuiltinTemplateParameterList(
Index: lib/Sema/SemaLambda.cpp
===================================================================
--- lib/Sema/SemaLambda.cpp
+++ lib/Sema/SemaLambda.cpp
@@ -235,7 +235,7 @@
         /*Template kw loc*/ SourceLocation(), LAngleLoc,
         llvm::makeArrayRef((NamedDecl *const *)LSI->AutoTemplateParams.data(),
                            LSI->AutoTemplateParams.size()),
-        RAngleLoc);
+        RAngleLoc, nullptr);
   }
   return LSI->GLTemplateParameterList;
 }
Index: lib/Sema/SemaTemplate.cpp
===================================================================
--- lib/Sema/SemaTemplate.cpp
+++ lib/Sema/SemaTemplate.cpp
@@ -831,11 +831,10 @@
   if (ExportLoc.isValid())
     Diag(ExportLoc, diag::warn_template_export_unsupported);
 
-  // FIXME: store RequiresClause
   return TemplateParameterList::Create(
       Context, TemplateLoc, LAngleLoc,
       llvm::makeArrayRef((NamedDecl *const *)Params.data(), Params.size()),
-      RAngleLoc);
+      RAngleLoc, RequiresClause);
 }
 
 static void SetNestedNameSpecifier(TagDecl *T, const CXXScopeSpec &SS) {
@@ -1953,7 +1952,7 @@
       // Fabricate an empty template parameter list for the invented header.
       return TemplateParameterList::Create(Context, SourceLocation(),
                                            SourceLocation(), None,
-                                           SourceLocation());
+                                           SourceLocation(), nullptr);
     }
 
     return nullptr;
Index: lib/Sema/SemaTemplateDeduction.cpp
===================================================================
--- lib/Sema/SemaTemplateDeduction.cpp
+++ lib/Sema/SemaTemplateDeduction.cpp
@@ -4022,8 +4022,8 @@
                                  nullptr, false, false);
   QualType TemplArg = QualType(TemplParam->getTypeForDecl(), 0);
   NamedDecl *TemplParamPtr = TemplParam;
-  FixedSizeTemplateParameterListStorage<1> TemplateParamsSt(
-      Loc, Loc, TemplParamPtr, Loc);
+  FixedSizeTemplateParameterListStorage<1, false> TemplateParamsSt(
+      Loc, Loc, TemplParamPtr, Loc, nullptr);
 
   QualType FuncParam = SubstituteAutoTransform(*this, TemplArg).Apply(Type);
   assert(!FuncParam.isNull() &&
Index: lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2947,10 +2947,14 @@
   if (Invalid)
     return nullptr;
 
+  // Note: we substitute into associated constraints later
+  Expr *const UninstantiatedRequiresClause = L->getRequiresClause();
+
   TemplateParameterList *InstL
     = TemplateParameterList::Create(SemaRef.Context, L->getTemplateLoc(),
                                     L->getLAngleLoc(), Params,
-                                    L->getRAngleLoc());
+                                    L->getRAngleLoc(),
+                                    UninstantiatedRequiresClause);
   return InstL;
 }
 
Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -7892,9 +7892,10 @@
   while (NumParams--)
     Params.push_back(ReadDeclAs<NamedDecl>(F, Record, Idx));
 
+  // TODO: Concepts
   TemplateParameterList* TemplateParams =
     TemplateParameterList::Create(Context, TemplateLoc, LAngleLoc,
-                                  Params, RAngleLoc);
+                                  Params, RAngleLoc, nullptr);
   return TemplateParams;
 }
 
Index: lib/Serialization/ASTWriter.cpp
===================================================================
--- lib/Serialization/ASTWriter.cpp
+++ lib/Serialization/ASTWriter.cpp
@@ -5359,6 +5359,7 @@
   AddSourceLocation(TemplateParams->getTemplateLoc());
   AddSourceLocation(TemplateParams->getLAngleLoc());
   AddSourceLocation(TemplateParams->getRAngleLoc());
+  // TODO: Concepts
   Record->push_back(TemplateParams->size());
   for (const auto &P : *TemplateParams)
     AddDeclRef(P);