Index: include/clang/AST/ASTNodeTraverser.h
===================================================================
--- include/clang/AST/ASTNodeTraverser.h
+++ include/clang/AST/ASTNodeTraverser.h
@@ -529,6 +529,11 @@
           D->defaultArgumentWasInherited() ? "inherited from" : "previous");
   }
 
+  void VisitConceptDecl(const ConceptDecl *D) {
+    dumpTemplateParameters(D->getTemplateParameters());
+    Visit(D->getConstraintExpr());
+  }
+
   void VisitUsingShadowDecl(const UsingShadowDecl *D) {
     if (auto *TD = dyn_cast<TypeDecl>(D->getUnderlyingDecl()))
       Visit(TD->getTypeForDecl());
Index: include/clang/AST/DeclTemplate.h
===================================================================
--- include/clang/AST/DeclTemplate.h
+++ include/clang/AST/DeclTemplate.h
@@ -3090,6 +3090,42 @@
   static bool classofKind(Kind K) { return K == VarTemplate; }
 };
 
+// \brief Declaration of a C++2a concept.
+class ConceptDecl : public TemplateDecl, public Mergeable<ConceptDecl> {
+protected:
+  Expr *ConstraintExpr;
+
+  ConceptDecl(DeclContext *DC,
+              SourceLocation L, DeclarationName Name,
+              TemplateParameterList *Params,
+              Expr *ConstraintExpr)
+      : TemplateDecl(nullptr, Concept, DC, L, Name, Params),
+        ConstraintExpr(ConstraintExpr) {};
+public:
+  static ConceptDecl *Create(ASTContext &C, DeclContext *DC,
+                             SourceLocation L, DeclarationName Name,
+                             TemplateParameterList *Params,
+                             Expr *ConstraintExpr);
+  static ConceptDecl *CreateDeserialized(ASTContext &C, unsigned ID);
+
+  Expr *getConstraintExpr() const {
+    return ConstraintExpr;
+  }
+
+  SourceRange getSourceRange() const override LLVM_READONLY {
+    return SourceRange(getTemplateParameters()->getTemplateLoc(),
+                       ConstraintExpr->getEndLoc());
+  }
+
+  // Implement isa/cast/dyncast/etc.
+  static bool classof(const Decl *D) { return classofKind(D->getKind()); }
+  static bool classofKind(Kind K) { return K == Concept; }
+
+  friend class ASTReader;
+  friend class ASTDeclReader;
+  friend class ASTDeclWriter;
+};
+
 inline NamedDecl *getAsNamedDecl(TemplateParameter P) {
   if (auto *PD = P.dyn_cast<TemplateTypeParmDecl *>())
     return PD;
Index: include/clang/AST/RecursiveASTVisitor.h
===================================================================
--- include/clang/AST/RecursiveASTVisitor.h
+++ include/clang/AST/RecursiveASTVisitor.h
@@ -1800,6 +1800,11 @@
   TRY_TO(TraverseTemplateParameterListHelper(D->getTemplateParameters()));
 })
 
+DEF_TRAVERSE_DECL(ConceptDecl, {
+  TRY_TO(TraverseTemplateParameterListHelper(D->getTemplateParameters()));
+  TRY_TO(TraverseStmt(D->getConstraintExpr()));
+})
+
 DEF_TRAVERSE_DECL(UnresolvedUsingTypenameDecl, {
   // A dependent using declaration which was marked with 'typename'.
   //   template<class T> class A : public B<T> { using typename B<T>::foo; };
Index: include/clang/AST/TextNodeDumper.h
===================================================================
--- include/clang/AST/TextNodeDumper.h
+++ include/clang/AST/TextNodeDumper.h
@@ -347,6 +347,7 @@
   void VisitObjCPropertyDecl(const ObjCPropertyDecl *D);
   void VisitObjCPropertyImplDecl(const ObjCPropertyImplDecl *D);
   void VisitBlockDecl(const BlockDecl *D);
+  void VisitConceptDecl(const ConceptDecl *D);
 };
 
 } // namespace clang
Index: include/clang/Basic/DeclNodes.td
===================================================================
--- include/clang/Basic/DeclNodes.td
+++ include/clang/Basic/DeclNodes.td
@@ -70,6 +70,7 @@
       def TypeAliasTemplate : DDecl<RedeclarableTemplate>;
     def TemplateTemplateParm : DDecl<Template>;
     def BuiltinTemplate : DDecl<Template>;
+    def Concept : DDecl<Template>;
   def Using : DDecl<Named>;
   def UsingPack : DDecl<Named>;
   def UsingShadow : DDecl<Named>;
Index: include/clang/Basic/DiagnosticParseKinds.td
===================================================================
--- include/clang/Basic/DiagnosticParseKinds.td
+++ include/clang/Basic/DiagnosticParseKinds.td
@@ -684,7 +684,7 @@
 def err_template_spec_syntax_non_template : Error<
   "identifier followed by '<' indicates a class template specialization but "
   "%0 %select{does not refer to a template|refers to a function template|"
-  "<unused>|refers to a variable template|<unused>}1">;
+  "<unused>|refers to a variable template|<unused>|refers to a concept}1">;
 def err_id_after_template_in_nested_name_spec : Error<
   "expected template name after 'template' keyword in nested name specifier">;
 def err_unexpected_template_in_unqualified_id : Error<
@@ -1287,4 +1287,12 @@
   "'co_await' modifier can only be applied to range-based for loop">;
 }
 
+let CategoryName = "Concepts Issue" in {
+def err_concept_definition_not_identifier : Error<
+  "name defined in concept definition must be an identifier">;
+def ext_concept_legacy_bool_keyword : ExtWarn<
+  "ISO C++2a does not permit the 'bool' keyword after 'concept'">,
+  InGroup<DiagGroup<"concepts-ts-compat">>;
+}
+
 } // end of Parser diagnostics
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2010,8 +2010,8 @@
   "%select{'auto'|'decltype(auto)'|'__auto_type'|"
   "use of "
   "%select{class template|function template|variable template|alias template|"
-  "template template parameter|template}2 %3 requires template arguments; "
-  "argument deduction}0 not allowed "
+  "template template parameter|concept|template}2 %3 requires template "
+  "arguments; argument deduction}0 not allowed "
   "%select{in function prototype"
   "|in non-static struct member|in struct member"
   "|in non-static union member|in union member"
@@ -2100,8 +2100,8 @@
   "deduced class template specialization type">;
 def err_deduced_non_class_template_specialization_type : Error<
   "%select{<error>|function template|variable template|alias template|"
-  "template template parameter|template}0 %1 requires template arguments; "
-  "argument deduction only allowed for class templates">;
+  "template template parameter|concept|template}0 %1 requires template "
+  "arguments; argument deduction only allowed for class templates">;
 def err_deduced_class_template_ctor_ambiguous : Error<
   "ambiguous deduction for template arguments of %0">;
 def err_deduced_class_template_ctor_no_viable : Error<
@@ -2128,7 +2128,7 @@
 def err_deduction_guide_name_not_class_template : Error<
   "cannot specify deduction guide for "
   "%select{<error>|function template|variable template|alias template|"
-  "template template parameter|dependent template name}0 %1">;
+  "template template parameter|concept|dependent template name}0 %1">;
 def err_deduction_guide_wrong_scope : Error<
   "deduction guide must be declared in the same scope as template %q0">;
 def err_deduction_guide_defines_function : Error<
@@ -2456,32 +2456,20 @@
 def note_private_extern : Note<
   "use __attribute__((visibility(\"hidden\"))) attribute instead">;
 
-// C++ Concepts TS
-def err_concept_wrong_decl_kind : Error<
-  "'concept' can only appear on the definition of a function template or variable template">;
-def err_concept_decls_may_only_appear_in_namespace_scope : Error<
-  "concept declarations may only appear in namespace scope">;
-def err_function_concept_not_defined : Error<
-  "function concept declaration must be a definition">;
-def err_var_concept_not_initialized : Error<
-  "variable concept declaration must be initialized">;
-def err_function_concept_exception_spec : Error<
-  "function concept cannot have exception specification">;
-def err_concept_decl_invalid_specifiers : Error<
-  "%select{variable|function}0 concept cannot be declared "
-  "'%select{thread_local|inline|friend|constexpr}1'">;
-def err_function_concept_with_params : Error<
-  "function concept cannot have any parameters">;
-def err_function_concept_bool_ret : Error<
-  "declared return type of function concept must be 'bool'">;
-def err_variable_concept_bool_decl : Error<
-  "declared type of variable concept must be 'bool'">;
-def err_concept_specified_specialization : Error<
-  "'concept' cannot be applied on an "
-  "%select{explicit instantiation|explicit specialization|partial specialization}0">;
-def err_concept_specialized : Error<
-  "%select{function|variable}0 concept cannot be "
-  "%select{explicitly instantiated|explicitly specialized|partially specialized}1">;
+// C++ Concepts
+def err_concept_initialized_with_non_bool_type : Error<
+  "constraint expression must be of type 'bool' but is of type %0">;
+def err_concept_decls_may_only_appear_in_global_namespace_scope : Error<
+  "concept declarations may only appear in global or namespace scope">;
+def err_concept_no_parameters : Error<
+  "concept template parameter list must have at least one parameter; explicit "
+  "specialization of concepts is not allowed">;
+def err_concept_extra_headers : Error<
+  "extraneous template parameter list in concept definition">;
+def err_concept_no_associated_constraints : Error<
+  "concept cannot have associated constraints">;
+def err_concept_not_implemented : Error<
+  "sorry, unimplemented concepts feature %0 used">;
 
 def err_template_different_associated_constraints : Error<
   "associated constraints differ in template redeclaration">;
@@ -4012,11 +4000,12 @@
 def err_template_missing_args : Error<
   "use of "
   "%select{class template|function template|variable template|alias template|"
-  "template template parameter|template}0 %1 requires template arguments">;
+  "template template parameter|concept|template}0 %1 requires template "
+  "arguments">;
 def err_template_arg_list_different_arity : Error<
   "%select{too few|too many}0 template arguments for "
   "%select{class template|function template|variable template|alias template|"
-  "template template parameter|template}1 %2">;
+  "template template parameter|concept|template}1 %2">;
 def note_template_decl_here : Note<"template is declared here">;
 def err_template_arg_must_be_type : Error<
   "template argument for template type parameter must be a type">;
Index: include/clang/Basic/TemplateKinds.h
===================================================================
--- include/clang/Basic/TemplateKinds.h
+++ include/clang/Basic/TemplateKinds.h
@@ -48,9 +48,9 @@
   /// anyway. In C++20, this is mandatory in order to parse ADL-only function
   /// template specialization calls.
   TNK_Undeclared_template,
+  /// The name refers to a concept.
+  TNK_Concept_template,
 };
 
 }
 #endif
-
-
Index: include/clang/Parse/Parser.h
===================================================================
--- include/clang/Parse/Parser.h
+++ include/clang/Parse/Parser.h
@@ -3024,6 +3024,10 @@
                                    SourceLocation &DeclEnd,
                                    ParsedAttributes &AccessAttrs,
                                    AccessSpecifier AS = AS_none);
+  // C++2a: Template, concept definition [temp]
+  Decl *
+  ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
+                         SourceLocation &DeclEnd);
 
   //===--------------------------------------------------------------------===//
   // Modules
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -1973,6 +1973,7 @@
     VarTemplate,
     AliasTemplate,
     TemplateTemplateParam,
+    Concept,
     DependentTemplate
   };
   TemplateNameKindForDiagnostics
@@ -6551,6 +6552,13 @@
                                 SourceLocation TemplateLoc,
                                 const TemplateArgumentListInfo *TemplateArgs);
 
+  ExprResult
+  CheckConceptTemplateId(const CXXScopeSpec &SS,
+                         const DeclarationNameInfo &NameInfo,
+                         ConceptDecl *Template,
+                         SourceLocation TemplateLoc,
+                         const TemplateArgumentListInfo *TemplateArgs);
+
   void diagnoseMissingTemplateArguments(TemplateName Name, SourceLocation Loc);
 
   ExprResult BuildTemplateIdExpr(const CXXScopeSpec &SS,
@@ -6818,6 +6826,11 @@
                                   const TemplateArgument *Args,
                                   unsigned NumArgs);
 
+  // Concepts
+  Decl *ActOnConceptDefinition(
+      Scope *S, MultiTemplateParamsArg TemplateParameterLists,
+      IdentifierInfo *Name, SourceLocation NameLoc, Expr *ConstraintExpr);
+
   //===--------------------------------------------------------------------===//
   // C++ Variadic Templates (C++0x [temp.variadic])
   //===--------------------------------------------------------------------===//
Index: include/clang/Serialization/ASTBitCodes.h
===================================================================
--- include/clang/Serialization/ASTBitCodes.h
+++ include/clang/Serialization/ASTBitCodes.h
@@ -1489,7 +1489,10 @@
       /// A TypeAliasTemplateDecl record.
       DECL_TYPE_ALIAS_TEMPLATE,
 
-      /// A StaticAssertDecl record.
+      /// \brief A ConceptDecl record.
+      DECL_CONCEPT,
+
+      /// \brief A StaticAssertDecl record.
       DECL_STATIC_ASSERT,
 
       /// A record containing CXXBaseSpecifiers.
Index: lib/AST/ASTStructuralEquivalence.cpp
===================================================================
--- lib/AST/ASTStructuralEquivalence.cpp
+++ lib/AST/ASTStructuralEquivalence.cpp
@@ -1512,6 +1512,18 @@
                                   D2->getTemplatedDecl()->getType());
 }
 
+static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
+                                     ConceptDecl *D1,
+                                     ConceptDecl *D2) {
+  // Check template parameters.
+  if (!IsTemplateDeclCommonStructurallyEquivalent(Context, D1, D2))
+    return false;
+
+  // Check the constraint expression.
+  return IsStructurallyEquivalent(Context, D1->getConstraintExpr(),
+                                  D2->getConstraintExpr());
+}
+
 static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
                                      FriendDecl *D1, FriendDecl *D2) {
   if ((D1->getFriendType() && D2->getFriendDecl()) ||
@@ -1771,6 +1783,14 @@
       // Class template/non-class-template mismatch.
       return false;
     }
+  } else if (auto *ConceptDecl1 = dyn_cast<ConceptDecl>(D1)) {
+    if (auto *ConceptDecl2 = dyn_cast<ConceptDecl>(D2)) {
+      if (!::IsStructurallyEquivalent(*this, ConceptDecl1, ConceptDecl2))
+        return false;
+    } else {
+      // Concept/non-concept mismatch.
+      return false;
+    }
   } else if (auto *TTP1 = dyn_cast<TemplateTypeParmDecl>(D1)) {
     if (auto *TTP2 = dyn_cast<TemplateTypeParmDecl>(D2)) {
       if (!::IsStructurallyEquivalent(*this, TTP1, TTP2))
Index: lib/AST/DeclBase.cpp
===================================================================
--- lib/AST/DeclBase.cpp
+++ lib/AST/DeclBase.cpp
@@ -710,6 +710,7 @@
     case Binding:
     case NonTypeTemplateParm:
     case VarTemplate:
+    case Concept:
       // These (C++-only) declarations are found by redeclaration lookup for
       // tag types, so we include them in the tag namespace.
       return IDNS_Ordinary | IDNS_Tag;
Index: lib/AST/DeclPrinter.cpp
===================================================================
--- lib/AST/DeclPrinter.cpp
+++ lib/AST/DeclPrinter.cpp
@@ -1123,8 +1123,13 @@
     if (TTP->isParameterPack())
       Out << "...";
     Out << D->getName();
-  } else {
-    Visit(D->getTemplatedDecl());
+  } else if (auto *TD = D->getTemplatedDecl())
+    Visit(TD);
+  else if (const auto *Concept = dyn_cast<ConceptDecl>(D)) {
+    Out << "concept " << Concept->getName() << " = " ;
+    Concept->getConstraintExpr()->printPretty(Out, nullptr, Policy,
+                                              Indentation);
+    Out << ";";
   }
 }
 
Index: lib/AST/DeclTemplate.cpp
===================================================================
--- lib/AST/DeclTemplate.cpp
+++ lib/AST/DeclTemplate.cpp
@@ -821,6 +821,26 @@
   }
 }
 
+//===----------------------------------------------------------------------===//
+// ConceptDecl Implementation
+//===----------------------------------------------------------------------===//
+ConceptDecl *ConceptDecl::Create(ASTContext &C, DeclContext *DC,
+                                 SourceLocation L, DeclarationName Name,
+                                 TemplateParameterList *Params,
+                                 Expr *ConstraintExpr) {
+  AdoptTemplateParameterList(Params, DC);
+  return new (C, DC) ConceptDecl(DC, L, Name, Params, ConstraintExpr);
+}
+
+ConceptDecl *ConceptDecl::CreateDeserialized(ASTContext &C,
+                                             unsigned ID) {
+  ConceptDecl *Result = new (C, ID) ConceptDecl(nullptr, SourceLocation(),
+                                                DeclarationName(),
+                                                nullptr, nullptr);
+
+  return Result;
+}
+
 //===----------------------------------------------------------------------===//
 // ClassTemplatePartialSpecializationDecl Implementation
 //===----------------------------------------------------------------------===//
Index: lib/AST/TextNodeDumper.cpp
===================================================================
--- lib/AST/TextNodeDumper.cpp
+++ lib/AST/TextNodeDumper.cpp
@@ -1936,3 +1936,7 @@
   if (D->capturesCXXThis())
     OS << " captures_this";
 }
+
+void TextNodeDumper::VisitConceptDecl(const ConceptDecl *D) {
+  dumpName(D);
+}
\ No newline at end of file
Index: lib/CodeGen/CGDecl.cpp
===================================================================
--- lib/CodeGen/CGDecl.cpp
+++ lib/CodeGen/CGDecl.cpp
@@ -108,6 +108,7 @@
   case Decl::OMPCapturedExpr:
   case Decl::OMPRequires:
   case Decl::Empty:
+  case Decl::Concept:
     // None of these decls require codegen support.
     return;
 
Index: lib/CodeGen/CodeGenModule.cpp
===================================================================
--- lib/CodeGen/CodeGenModule.cpp
+++ lib/CodeGen/CodeGenModule.cpp
@@ -5162,6 +5162,7 @@
   case Decl::UsingShadow:
   case Decl::ClassTemplate:
   case Decl::VarTemplate:
+  case Decl::Concept:
   case Decl::VarTemplatePartialSpecialization:
   case Decl::FunctionTemplate:
   case Decl::TypeAliasTemplate:
Index: lib/Index/IndexDecl.cpp
===================================================================
--- lib/Index/IndexDecl.cpp
+++ lib/Index/IndexDecl.cpp
@@ -652,10 +652,10 @@
   }
 
   static bool shouldIndexTemplateParameterDefaultValue(const NamedDecl *D) {
-    if (!D)
-      return false;
     // We want to index the template parameters only once when indexing the
     // canonical declaration.
+    if (!D)
+      return false;
     if (const auto *FD = dyn_cast<FunctionDecl>(D))
       return FD->getCanonicalDecl() == FD;
     else if (const auto *TD = dyn_cast<TagDecl>(D))
Index: lib/Parse/ParseTemplate.cpp
===================================================================
--- lib/Parse/ParseTemplate.cpp
+++ lib/Parse/ParseTemplate.cpp
@@ -49,6 +49,15 @@
 ///       template-declaration: [C++ temp]
 ///         'export'[opt] 'template' '<' template-parameter-list '>' declaration
 ///
+///       template-declaration: [C++2a]
+///         template-head declaration
+///         template-head concept-definition
+///
+///       TODO: requires-clause
+///       template-head: [C++2a]
+///         'template' '<' template-parameter-list '>'
+///             requires-clause[opt]
+///
 ///       explicit-specialization: [ C++ temp.expl.spec]
 ///         'template' '<' '>' declaration
 Decl *Parser::ParseTemplateDeclarationOrSpecialization(
@@ -142,6 +151,12 @@
   ParseScopeFlags TemplateScopeFlags(this, NewFlags, isSpecialization);
 
   // Parse the actual template declaration.
+  if (Tok.is(tok::kw_concept))
+    return ParseConceptDefinition(
+        ParsedTemplateInfo(&ParamLists, isSpecialization,
+                           LastParamListWasEmpty),
+        DeclEnd);
+
   return ParseSingleDeclarationAfterTemplate(
       Context,
       ParsedTemplateInfo(&ParamLists, isSpecialization, LastParamListWasEmpty),
@@ -315,6 +330,85 @@
   return ThisDecl;
 }
 
+/// \brief Parse a single declaration that declares a concept.
+///
+/// \param DeclEnd will receive the source location of the last token
+/// within this declaration.
+///
+/// \returns the new declaration.
+Decl *
+Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
+                               SourceLocation &DeclEnd) {
+  assert(TemplateInfo.Kind != ParsedTemplateInfo::NonTemplate &&
+         "Template information required");
+  assert(Tok.is(tok::kw_concept) &&
+         "ParseConceptDefinition must be called when at a 'concept' keyword");
+
+  ConsumeToken(); // Consume 'concept'
+
+  SourceLocation BoolKWLoc;
+  if (TryConsumeToken(tok::kw_bool, BoolKWLoc))
+    Diag(Tok.getLocation(), diag::ext_concept_legacy_bool_keyword) <<
+        FixItHint::CreateRemoval(SourceLocation(BoolKWLoc));
+
+  DiagnoseAndSkipCXX11Attributes();
+
+  CXXScopeSpec SS;
+  if (ParseOptionalCXXScopeSpecifier(SS, ParsedType(),
+      /*EnteringContext=*/false, /*MayBePseudoDestructor=*/nullptr,
+      /*IsTypename=*/false, /*LastII=*/nullptr, /*OnlyNamespace=*/true) ||
+      SS.isInvalid()) {
+    SkipUntil(tok::semi);
+    return nullptr;
+  }
+
+  if (SS.isNotEmpty())
+    Diag(SS.getBeginLoc(),
+         diag::err_concept_definition_not_identifier);
+
+  UnqualifiedId Result;
+  if (ParseUnqualifiedId(SS, /*EnteringContext=*/false,
+                         /*AllowDestructorName=*/false,
+                         /*AllowConstructorName=*/false,
+                         /*AllowDeductionGuide=*/false,
+                         /*ObjectType=*/ParsedType(), /*TemplateKWLoc=*/nullptr,
+                         Result)) {
+    SkipUntil(tok::semi);
+    return nullptr;
+  }
+
+  if (Result.getKind() != UnqualifiedIdKind::IK_Identifier) {
+    Diag(Result.getBeginLoc(), diag::err_concept_definition_not_identifier);
+    SkipUntil(tok::semi);
+    return nullptr;
+  }
+
+  IdentifierInfo *Id = Result.Identifier;
+  SourceLocation IdLoc = Result.getBeginLoc();
+
+  DiagnoseAndSkipCXX11Attributes();
+
+  if (!TryConsumeToken(tok::equal)) {
+    Diag(Tok.getLocation(), diag::err_expected) << tok::equal;
+    SkipUntil(tok::semi);
+    return nullptr;
+  }
+
+  ExprResult ConstraintExprResult =
+      Actions.CorrectDelayedTyposInExpr(ParseConstraintExpression());
+  if (ConstraintExprResult.isInvalid()) {
+    SkipUntil(tok::semi);
+    return nullptr;
+  }
+
+  DeclEnd = Tok.getLocation();
+  ExpectAndConsumeSemi(diag::err_expected_semi_declaration);
+  Expr *ConstraintExpr = ConstraintExprResult.get();
+  return Actions.ActOnConceptDefinition(getCurScope(),
+                                        *TemplateInfo.TemplateParams,
+                                        Id, IdLoc, ConstraintExpr);
+}
+
 /// ParseTemplateParameters - Parses a template-parameter-list enclosed in
 /// angle brackets. Depth is the depth of this template-parameter-list, which
 /// is the number of template headers directly enclosing this template header.
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -1189,6 +1189,8 @@
     return TemplateNameKindForDiagnostics::AliasTemplate;
   if (isa<TemplateTemplateParmDecl>(TD))
     return TemplateNameKindForDiagnostics::TemplateTemplateParam;
+  if (isa<ConceptDecl>(TD))
+    return TemplateNameKindForDiagnostics::Concept;
   return TemplateNameKindForDiagnostics::DependentTemplate;
 }
 
Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -9802,7 +9802,7 @@
     NonTemplateTarget = TargetTD->getTemplatedDecl();
 
   UsingShadowDecl *Shadow;
-  if (isa<CXXConstructorDecl>(NonTemplateTarget)) {
+  if (NonTemplateTarget && isa<CXXConstructorDecl>(NonTemplateTarget)) {
     bool IsVirtualBase =
         isVirtualDirectBase(cast<CXXRecordDecl>(CurContext),
                             UD->getQualifier()->getAsRecordDecl());
Index: lib/Sema/SemaLookup.cpp
===================================================================
--- lib/Sema/SemaLookup.cpp
+++ lib/Sema/SemaLookup.cpp
@@ -5168,7 +5168,8 @@
   if (ObjCProtocolDecl *PD = dyn_cast<ObjCProtocolDecl>(D))
     return PD->getDefinition();
   if (TemplateDecl *TD = dyn_cast<TemplateDecl>(D))
-    return getDefinitionToImport(TD->getTemplatedDecl());
+    if (NamedDecl *TTD = TD->getTemplatedDecl())
+      return getDefinitionToImport(TTD);
   return nullptr;
 }
 
Index: lib/Sema/SemaTemplate.cpp
===================================================================
--- lib/Sema/SemaTemplate.cpp
+++ lib/Sema/SemaTemplate.cpp
@@ -271,9 +271,11 @@
     } else {
       assert(isa<ClassTemplateDecl>(TD) || isa<TemplateTemplateParmDecl>(TD) ||
              isa<TypeAliasTemplateDecl>(TD) || isa<VarTemplateDecl>(TD) ||
-             isa<BuiltinTemplateDecl>(TD));
+             isa<BuiltinTemplateDecl>(TD) || isa<ConceptDecl>(TD));
       TemplateKind =
-          isa<VarTemplateDecl>(TD) ? TNK_Var_template : TNK_Type_template;
+          isa<VarTemplateDecl>(TD) ? TNK_Var_template :
+          isa<ConceptDecl>(TD) ? TNK_Concept_template :
+          TNK_Type_template;
     }
   }
 
@@ -3227,7 +3229,8 @@
 
   TemplateDecl *Template = Name.getAsTemplateDecl();
   if (!Template || isa<FunctionTemplateDecl>(Template) ||
-      isa<VarTemplateDecl>(Template)) {
+      isa<VarTemplateDecl>(Template) ||
+      isa<ConceptDecl>(Template)) {
     // We might have a substituted template template parameter pack. If so,
     // build a template specialization type for it.
     if (Name.getAsSubstTemplateTemplateParmPack())
@@ -4234,6 +4237,18 @@
   }
 }
 
+ExprResult
+Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
+                             const DeclarationNameInfo &NameInfo,
+                             ConceptDecl *Template,
+                             SourceLocation TemplateLoc,
+                             const TemplateArgumentListInfo *TemplateArgs) {
+  // TODO: Do concept specialization here.
+  Diag(NameInfo.getBeginLoc(), diag::err_concept_not_implemented) <<
+    "concept specialization";
+  return ExprError();
+}
+
 ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
                                      SourceLocation TemplateKWLoc,
                                      LookupResult &R,
@@ -4274,6 +4289,12 @@
                               TemplateKWLoc, TemplateArgs);
   }
 
+  if (R.getAsSingle<ConceptDecl>() && !AnyDependentArguments()) {
+    return CheckConceptTemplateId(SS, R.getLookupNameInfo(),
+                                  R.getAsSingle<ConceptDecl>(),
+                                  TemplateKWLoc, TemplateArgs);
+  }
+
   // We don't want lookup warnings at this point.
   R.suppressDiagnostics();
 
@@ -7974,7 +7995,74 @@
   return NewDecl;
 }
 
-/// Strips various properties off an implicit instantiation
+Decl *Sema::ActOnConceptDefinition(Scope *S,
+                              MultiTemplateParamsArg TemplateParameterLists,
+                                   IdentifierInfo *Name, SourceLocation NameLoc,
+                                   Expr *ConstraintExpr) {
+  DeclContext *DC = CurContext;
+
+  if (!DC->getRedeclContext()->isFileContext()) {
+    Diag(NameLoc,
+      diag::err_concept_decls_may_only_appear_in_global_namespace_scope);
+    return nullptr;
+  }
+
+  if (TemplateParameterLists.size() > 1) {
+    Diag(NameLoc, diag::err_concept_extra_headers);
+    return nullptr;
+  }
+
+  if (TemplateParameterLists.front()->size() == 0) {
+    Diag(NameLoc, diag::err_concept_no_parameters);
+    return nullptr;
+  }
+
+  ConceptDecl *NewDecl = ConceptDecl::Create(Context, DC, NameLoc, Name,
+                                             TemplateParameterLists.front(),
+                                             ConstraintExpr);
+
+  if (!ConstraintExpr->isTypeDependent() &&
+      ConstraintExpr->getType() != Context.BoolTy) {
+    // C++2a [temp.constr.atomic]p3:
+    // E shall be a constant expression of type bool.
+    // TODO: Do this check for individual atomic constraints
+    // and not the constraint expression. Probably should do it in
+    // ParseConstraintExpression.
+    Diag(ConstraintExpr->getSourceRange().getBegin(),
+        diag::err_concept_initialized_with_non_bool_type)
+      << ConstraintExpr->getType();
+    NewDecl->setInvalidDecl();
+  }
+
+  if (NewDecl->getAssociatedConstraints()) {
+    // C++2a [temp.concept]p4:
+    // A concept shall not have associated constraints.
+    // TODO: Make a test once we have actual associated constraints.
+    Diag(NameLoc, diag::err_concept_no_associated_constraints);
+    NewDecl->setInvalidDecl();
+  }
+
+  // Check for conflicting previous declaration.
+  DeclarationNameInfo NameInfo(NewDecl->getDeclName(), NameLoc);
+  LookupResult Previous(*this, NameInfo, LookupOrdinaryName,
+                        ForVisibleRedeclaration);
+  LookupName(Previous, S);
+
+  FilterLookupForScope(Previous, DC, S, /*ConsiderLinkage=*/false,
+                       /*AllowInlineNamespace*/false);
+  if (!Previous.empty()) {
+    auto *Old = Previous.getRepresentativeDecl();
+    Diag(NameLoc, isa<ConceptDecl>(Old) ? diag::err_redefinition :
+         diag::err_redefinition_different_kind) << NewDecl->getDeclName();
+    Diag(Old->getLocation(), diag::note_previous_definition);
+  }
+
+  ActOnDocumentableDecl(NewDecl);
+  PushOnScopeChains(NewDecl, S);
+  return NewDecl;
+}
+
+/// \brief Strips various properties off an implicit instantiation
 /// that has just been explicitly specialized.
 static void StripImplicitInstantiation(NamedDecl *D) {
   D->dropAttr<DLLImportAttr>();
Index: lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3401,6 +3401,10 @@
   return nullptr;
 }
 
+Decl *TemplateDeclInstantiator::VisitConceptDecl(ConceptDecl *D) {
+  llvm_unreachable("Concept definitions cannot reside inside a template");
+}
+
 Decl *TemplateDeclInstantiator::VisitDecl(Decl *D) {
   llvm_unreachable("Unexpected decl");
 }
Index: lib/Serialization/ASTCommon.cpp
===================================================================
--- lib/Serialization/ASTCommon.cpp
+++ lib/Serialization/ASTCommon.cpp
@@ -395,6 +395,7 @@
   case Decl::BuiltinTemplate:
   case Decl::Decomposition:
   case Decl::Binding:
+  case Decl::Concept:
     return false;
 
   // These indirectly derive from Redeclarable<T> but are not actually
Index: lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- lib/Serialization/ASTReaderDecl.cpp
+++ lib/Serialization/ASTReaderDecl.cpp
@@ -382,6 +382,7 @@
     void VisitBindingDecl(BindingDecl *BD);
     void VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D);
     DeclID VisitTemplateDecl(TemplateDecl *D);
+    void VisitConceptDecl(ConceptDecl *D);
     RedeclarableResult VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D);
     void VisitClassTemplateDecl(ClassTemplateDecl *D);
     void VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D);
@@ -2089,6 +2090,12 @@
   return PatternID;
 }
 
+void ASTDeclReader::VisitConceptDecl(ConceptDecl *D) {
+  VisitTemplateDecl(D);
+  D->ConstraintExpr = Record.readExpr();
+  mergeMergeable(D);
+}
+
 ASTDeclReader::RedeclarableResult
 ASTDeclReader::VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D) {
   RedeclarableResult Redecl = VisitRedeclarable(D);
@@ -3829,6 +3836,9 @@
   case DECL_TYPE_ALIAS_TEMPLATE:
     D = TypeAliasTemplateDecl::CreateDeserialized(Context, ID);
     break;
+  case DECL_CONCEPT:
+    D = ConceptDecl::CreateDeserialized(Context, ID);
+    break;
   case DECL_STATIC_ASSERT:
     D = StaticAssertDecl::CreateDeserialized(Context, ID);
     break;
Index: lib/Serialization/ASTWriter.cpp
===================================================================
--- lib/Serialization/ASTWriter.cpp
+++ lib/Serialization/ASTWriter.cpp
@@ -1293,6 +1293,7 @@
   RECORD(DECL_TEMPLATE_TYPE_PARM);
   RECORD(DECL_NON_TYPE_TEMPLATE_PARM);
   RECORD(DECL_TEMPLATE_TEMPLATE_PARM);
+  RECORD(DECL_CONCEPT);
   RECORD(DECL_TYPE_ALIAS_TEMPLATE);
   RECORD(DECL_STATIC_ASSERT);
   RECORD(DECL_CXX_BASE_SPECIFIERS);
Index: lib/Serialization/ASTWriterDecl.cpp
===================================================================
--- lib/Serialization/ASTWriterDecl.cpp
+++ lib/Serialization/ASTWriterDecl.cpp
@@ -102,6 +102,7 @@
     void VisitBindingDecl(BindingDecl *D);
     void VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D);
     void VisitTemplateDecl(TemplateDecl *D);
+    void VisitConceptDecl(ConceptDecl *D);
     void VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D);
     void VisitClassTemplateDecl(ClassTemplateDecl *D);
     void VisitVarTemplateDecl(VarTemplateDecl *D);
@@ -1432,6 +1433,12 @@
   Record.AddTemplateParameterList(D->getTemplateParameters());
 }
 
+void ASTDeclWriter::VisitConceptDecl(ConceptDecl *D) {
+  VisitTemplateDecl(D);
+  Record.AddStmt(D->getConstraintExpr());
+  Code = serialization::DECL_CONCEPT;
+}
+
 void ASTDeclWriter::VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D) {
   VisitRedeclarable(D);
 
Index: test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp
===================================================================
--- /dev/null
+++ test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp
@@ -0,0 +1,4 @@
+// RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -verify %s
+
+template<typename T> concept C = true;
+static_assert(C<int>); // expected-error{{sorry, unimplemented concepts feature concept specialization used}}
Index: test/Parser/cxx-concept-declaration.cpp
===================================================================
--- test/Parser/cxx-concept-declaration.cpp
+++ /dev/null
@@ -1,7 +0,0 @@
-
-// Support parsing of concepts
-// Disabled for now.
-// expected-no-diagnostics
-
-// RUN:  %clang_cc1 -std=c++14 -fconcepts-ts -x c++ -verify %s
-// template<typename T> concept C1 = true;
Index: test/Parser/cxx2a-concept-declaration.cpp
===================================================================
--- /dev/null
+++ test/Parser/cxx2a-concept-declaration.cpp
@@ -0,0 +1,73 @@
+// Support parsing of concepts
+
+// RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -verify %s
+template<typename T> concept C1 = true; // expected-note 2{{previous}}
+
+template<typename T> concept C1 = true; // expected-error{{redefinition}}
+
+template<concept T> concept D1 = true;
+// expected-error@-1{{expected template parameter}}
+// expected-error@-2{{concept template parameter list must have at least one parameter; explicit specialization of concepts is not allowed}}
+
+template<template<typename> concept T> concept D2 = true;
+// expected-error@-1{{expected identifier}}
+// expected-error@-2{{template template parameter requires 'class' after the parameter list}}
+// expected-error@-3{{concept template parameter list must have at least one parameter; explicit specialization of concepts is not allowed}}
+
+template<typename T> concept C2 = 0.f; // expected-error {{constraint expression must be of type 'bool' but is of type 'float'}}
+
+struct S1 {
+  template<typename T> concept C1 = true; // expected-error {{concept declarations may only appear in global or namespace scope}}
+};
+
+extern "C++" {
+  template<typename T> concept C1 = true; // expected-error{{redefinition}}
+}
+
+template<typename A>
+template<typename B>
+concept C4 = true; // expected-error {{extraneous template parameter list in concept definition}}
+
+template<typename T> concept C5 = true; // expected-note {{previous}} expected-note {{previous}}
+int C5; // expected-error {{redefinition}}
+struct C5 {}; // expected-error {{redefinition}}
+
+struct C6 {}; // expected-note{{previous definition is here}}
+template<typename T> concept C6 = true;
+// expected-error@-1{{redefinition of 'C6' as different kind of symbol}}
+
+// TODO: Add test to prevent explicit specialization, partial specialization
+// and explicit instantiation of concepts.
+
+template<typename T, T v>
+struct integral_constant { static constexpr T value = v; };
+
+namespace N {
+  template<typename T> concept C7 = true;
+}
+using N::C7;
+
+template <bool word> concept C8 = integral_constant<bool, wor>::value;
+// expected-error@-1{{use of undeclared identifier 'wor'; did you mean 'word'?}}
+// expected-note@-2{{'word' declared here}}
+
+template<typename T> concept bool C9 = true;
+// expected-warning@-1{{ISO C++2a does not permit the 'bool' keyword after 'concept'}}
+
+template<> concept C10 = false;
+// expected-error@-1{{concept template parameter list must have at least one parameter; explicit specialization of concepts is not allowed}}
+
+template<> concept C9<int> = false;
+// expected-error@-1{{name defined in concept definition must be an identifier}}
+
+template<typename T> concept N::C11 = false;
+// expected-error@-1{{name defined in concept definition must be an identifier}}
+
+class A { };
+// expected-note@-1{{'A' declared here}}
+
+template<typename T> concept A::C12 = false;
+// expected-error@-1{{expected namespace name}}
+
+template<typename T> concept operator int = false;
+// expected-error@-1{{name defined in concept definition must be an identifier}}
Index: tools/libclang/CIndex.cpp
===================================================================
--- tools/libclang/CIndex.cpp
+++ tools/libclang/CIndex.cpp
@@ -6269,6 +6269,7 @@
   case Decl::PragmaComment:
   case Decl::PragmaDetectMismatch:
   case Decl::UsingPack:
+  case Decl::Concept:
     return C;
 
   // Declaration kinds that don't make any sense here, but are