[Concepts] Requires Expressions

Implement support for C++2a requires-expressions.

Re-commit after compilation failure on some platforms due to alignment issues with PointerIntPair.

Differential Revision: https://reviews.llvm.org/D50360

(cherry picked from commit a0f50d731639350c7a79f140f026c27a18215531)

12 files changed, 975 insertions, 74 deletions

diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 2cd158a8b43..f8da1cb89b9 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -1261,7 +1261,8 @@ DeclContext *Sema::getFunctionLevelDeclContext() {
   DeclContext *DC = CurContext;
 
   while (true) {
-    if (isa<BlockDecl>(DC) || isa<EnumDecl>(DC) || isa<CapturedDecl>(DC)) {
+    if (isa<BlockDecl>(DC) || isa<EnumDecl>(DC) || isa<CapturedDecl>(DC) ||
+        isa<RequiresExprBodyDecl>(DC)) {
       DC = DC->getParent();
     } else if (isa<CXXMethodDecl>(DC) &&
                cast<CXXMethodDecl>(DC)->getOverloadedOperator() == OO_Call &&
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 018ac2d7dc9..93e5b4511da 100755
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -17,7 +17,10 @@
 #include "clang/Sema/SemaDiagnostic.h"
 #include "clang/Sema/TemplateDeduction.h"
 #include "clang/Sema/Template.h"
-#include "clang/AST/ExprCXX.h"
+#include "clang/Sema/Overload.h"
+#include "clang/Sema/Initialization.h"
+#include "clang/Sema/SemaInternal.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/Basic/OperatorPrecedence.h"
 #include "llvm/ADT/DenseMap.h"
@@ -336,6 +339,118 @@ bool Sema::EnsureTemplateArgumentListConstraints(
   return false;
 }
 
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::ExprRequirement *Req,
+                                           bool First) {
+  assert(!Req->isSatisfied()
+         && "Diagnose() can only be used on an unsatisfied requirement");
+  switch (Req->getSatisfactionStatus()) {
+    case concepts::ExprRequirement::SS_Dependent:
+      llvm_unreachable("Diagnosing a dependent requirement");
+      break;
+    case concepts::ExprRequirement::SS_ExprSubstitutionFailure: {
+      auto *SubstDiag = Req->getExprSubstitutionDiagnostic();
+      if (!SubstDiag->DiagMessage.empty())
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_expr_substitution_error)
+               << (int)First << SubstDiag->SubstitutedEntity
+               << SubstDiag->DiagMessage;
+      else
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_expr_unknown_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity;
+      break;
+    }
+    case concepts::ExprRequirement::SS_NoexceptNotMet:
+      S.Diag(Req->getNoexceptLoc(),
+             diag::note_expr_requirement_noexcept_not_met)
+          << (int)First << Req->getExpr();
+      break;
+    case concepts::ExprRequirement::SS_TypeRequirementSubstitutionFailure: {
+      auto *SubstDiag =
+          Req->getReturnTypeRequirement().getSubstitutionDiagnostic();
+      if (!SubstDiag->DiagMessage.empty())
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_type_requirement_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity
+            << SubstDiag->DiagMessage;
+      else
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_type_requirement_unknown_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity;
+      break;
+    }
+    case concepts::ExprRequirement::SS_ConstraintsNotSatisfied: {
+      ConceptSpecializationExpr *ConstraintExpr =
+          Req->getReturnTypeRequirementSubstitutedConstraintExpr();
+      if (ConstraintExpr->getTemplateArgsAsWritten()->NumTemplateArgs == 1)
+        // A simple case - expr type is the type being constrained and the concept
+        // was not provided arguments.
+        S.Diag(ConstraintExpr->getBeginLoc(),
+               diag::note_expr_requirement_constraints_not_satisfied_simple)
+            << (int)First << S.BuildDecltypeType(Req->getExpr(),
+                                                 Req->getExpr()->getBeginLoc())
+            << ConstraintExpr->getNamedConcept();
+      else
+        S.Diag(ConstraintExpr->getBeginLoc(),
+               diag::note_expr_requirement_constraints_not_satisfied)
+            << (int)First << ConstraintExpr;
+      S.DiagnoseUnsatisfiedConstraint(ConstraintExpr->getSatisfaction());
+      break;
+    }
+    case concepts::ExprRequirement::SS_Satisfied:
+      llvm_unreachable("We checked this above");
+  }
+}
+
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::TypeRequirement *Req,
+                                           bool First) {
+  assert(!Req->isSatisfied()
+         && "Diagnose() can only be used on an unsatisfied requirement");
+  switch (Req->getSatisfactionStatus()) {
+  case concepts::TypeRequirement::SS_Dependent:
+    llvm_unreachable("Diagnosing a dependent requirement");
+    return;
+  case concepts::TypeRequirement::SS_SubstitutionFailure: {
+    auto *SubstDiag = Req->getSubstitutionDiagnostic();
+    if (!SubstDiag->DiagMessage.empty())
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_type_requirement_substitution_error) << (int)First
+          << SubstDiag->SubstitutedEntity << SubstDiag->DiagMessage;
+    else
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_type_requirement_unknown_substitution_error)
+          << (int)First << SubstDiag->SubstitutedEntity;
+    return;
+  }
+  default:
+    llvm_unreachable("Unknown satisfaction status");
+    return;
+  }
+}
+
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::NestedRequirement *Req,
+                                           bool First) {
+  if (Req->isSubstitutionFailure()) {
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag =
+        Req->getSubstitutionDiagnostic();
+    if (!SubstDiag->DiagMessage.empty())
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_nested_requirement_substitution_error)
+             << (int)First << SubstDiag->SubstitutedEntity
+             << SubstDiag->DiagMessage;
+    else
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_nested_requirement_unknown_substitution_error)
+          << (int)First << SubstDiag->SubstitutedEntity;
+    return;
+  }
+  S.DiagnoseUnsatisfiedConstraint(Req->getConstraintSatisfaction(), First);
+}
+
+
 static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
                                                         Expr *SubstExpr,
                                                         bool First = true) {
@@ -412,6 +527,19 @@ static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
     }
     S.DiagnoseUnsatisfiedConstraint(CSE->getSatisfaction());
     return;
+  } else if (auto *RE = dyn_cast<RequiresExpr>(SubstExpr)) {
+    for (concepts::Requirement *Req : RE->getRequirements())
+      if (!Req->isDependent() && !Req->isSatisfied()) {
+        if (auto *E = dyn_cast<concepts::ExprRequirement>(Req))
+          diagnoseUnsatisfiedRequirement(S, E, First);
+        else if (auto *T = dyn_cast<concepts::TypeRequirement>(Req))
+          diagnoseUnsatisfiedRequirement(S, T, First);
+        else
+          diagnoseUnsatisfiedRequirement(
+              S, cast<concepts::NestedRequirement>(Req), First);
+        break;
+      }
+    return;
   }
 
   S.Diag(SubstExpr->getSourceRange().getBegin(),
@@ -434,11 +562,11 @@ static void diagnoseUnsatisfiedConstraintExpr(
       Record.template get<Expr *>(), First);
 }
 
-void Sema::DiagnoseUnsatisfiedConstraint(
-    const ConstraintSatisfaction& Satisfaction) {
+void
+Sema::DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction& Satisfaction,
+                                    bool First) {
   assert(!Satisfaction.IsSatisfied &&
          "Attempted to diagnose a satisfied constraint");
-  bool First = true;
   for (auto &Pair : Satisfaction.Details) {
     diagnoseUnsatisfiedConstraintExpr(*this, Pair.first, Pair.second, First);
     First = false;
@@ -446,10 +574,10 @@ void Sema::DiagnoseUnsatisfiedConstraint(
 }
 
 void Sema::DiagnoseUnsatisfiedConstraint(
-    const ASTConstraintSatisfaction &Satisfaction) {
+    const ASTConstraintSatisfaction &Satisfaction,
+    bool First) {
   assert(!Satisfaction.IsSatisfied &&
          "Attempted to diagnose a satisfied constraint");
-  bool First = true;
   for (auto &Pair : Satisfaction) {
     diagnoseUnsatisfiedConstraintExpr(*this, Pair.first, Pair.second, First);
     First = false;
@@ -826,3 +954,67 @@ bool Sema::MaybeEmitAmbiguousAtomicConstraintsDiagnostic(NamedDecl *D1,
       << AmbiguousAtomic2->getSourceRange();
   return true;
 }
+
+concepts::ExprRequirement::ExprRequirement(
+    Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+    ReturnTypeRequirement Req, SatisfactionStatus Status,
+    ConceptSpecializationExpr *SubstitutedConstraintExpr) :
+    Requirement(IsSimple ? RK_Simple : RK_Compound, Status == SS_Dependent,
+                Status == SS_Dependent &&
+                (E->containsUnexpandedParameterPack() ||
+                 Req.containsUnexpandedParameterPack()),
+                Status == SS_Satisfied), Value(E), NoexceptLoc(NoexceptLoc),
+    TypeReq(Req), SubstitutedConstraintExpr(SubstitutedConstraintExpr),
+    Status(Status) {
+  assert((!IsSimple || (Req.isEmpty() && NoexceptLoc.isInvalid())) &&
+         "Simple requirement must not have a return type requirement or a "
+         "noexcept specification");
+  assert((Status > SS_TypeRequirementSubstitutionFailure && Req.isTypeConstraint()) ==
+         (SubstitutedConstraintExpr != nullptr));
+}
+
+concepts::ExprRequirement::ExprRequirement(
+    SubstitutionDiagnostic *ExprSubstDiag, bool IsSimple,
+    SourceLocation NoexceptLoc, ReturnTypeRequirement Req) :
+    Requirement(IsSimple ? RK_Simple : RK_Compound, Req.isDependent(),
+                Req.containsUnexpandedParameterPack(), /*IsSatisfied=*/false),
+    Value(ExprSubstDiag), NoexceptLoc(NoexceptLoc), TypeReq(Req),
+    Status(SS_ExprSubstitutionFailure) {
+  assert((!IsSimple || (Req.isEmpty() && NoexceptLoc.isInvalid())) &&
+         "Simple requirement must not have a return type requirement or a "
+         "noexcept specification");
+}
+
+concepts::ExprRequirement::ReturnTypeRequirement::
+ReturnTypeRequirement(TemplateParameterList *TPL) :
+    TypeConstraintInfo(TPL, 0) {
+  assert(TPL->size() == 1);
+  const TypeConstraint *TC =
+      cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint();
+  assert(TC &&
+         "TPL must have a template type parameter with a type constraint");
+  auto *Constraint =
+      cast_or_null<ConceptSpecializationExpr>(
+          TC->getImmediatelyDeclaredConstraint());
+  bool Dependent = false;
+  if (Constraint->getTemplateArgsAsWritten()) {
+    for (auto &ArgLoc :
+         Constraint->getTemplateArgsAsWritten()->arguments().drop_front(1)) {
+      if (ArgLoc.getArgument().isDependent()) {
+        Dependent = true;
+        break;
+      }
+    }
+  }
+  TypeConstraintInfo.setInt(Dependent ? 1 : 0);
+}
+
+concepts::TypeRequirement::TypeRequirement(TypeSourceInfo *T) :
+    Requirement(RK_Type, T->getType()->isDependentType(),
+                T->getType()->containsUnexpandedParameterPack(),
+                // We reach this ctor with either dependent types (in which
+                // IsSatisfied doesn't matter) or with non-dependent type in
+                // which the existence of the type indicates satisfaction.
+                /*IsSatisfied=*/true
+                ), Value(T),
+    Status(T->getType()->isDependentType() ? SS_Dependent : SS_Satisfied) {}
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 507e4a6cd43..372f3d15859 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -6468,6 +6468,8 @@ static bool shouldConsiderLinkage(const VarDecl *VD) {
     return true;
   if (DC->isRecord())
     return false;
+  if (isa<RequiresExprBodyDecl>(DC))
+    return false;
   llvm_unreachable("Unexpected context");
 }
 
diff --git a/clang/lib/Sema/SemaExceptionSpec.cpp b/clang/lib/Sema/SemaExceptionSpec.cpp
index 5aedbe7644e..193eaa3e01f 100644
--- a/clang/lib/Sema/SemaExceptionSpec.cpp
+++ b/clang/lib/Sema/SemaExceptionSpec.cpp
@@ -1386,6 +1386,7 @@ CanThrowResult Sema::canThrow(const Stmt *S) {
   case Expr::StringLiteralClass:
   case Expr::SourceLocExprClass:
   case Expr::ConceptSpecializationExprClass:
+  case Expr::RequiresExprClass:
     // These expressions can never throw.
     return CT_Cannot;
 
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 5f4071924d3..ea4b93ee6a5 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -350,6 +350,17 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, ArrayRef<SourceLocation> Locs,
     }
   }
 
+  if (isa<ParmVarDecl>(D) && isa<RequiresExprBodyDecl>(D->getDeclContext()) &&
+      !isUnevaluatedContext()) {
+    // C++ [expr.prim.req.nested] p3
+    //   A local parameter shall only appear as an unevaluated operand
+    //   (Clause 8) within the constraint-expression.
+    Diag(Loc, diag::err_requires_expr_parameter_referenced_in_evaluated_context)
+        << D;
+    Diag(D->getLocation(), diag::note_entity_declared_at) << D;
+    return true;
+  }
+
   return false;
 }
 
@@ -1904,7 +1915,7 @@ Sema::BuildDeclRefExpr(ValueDecl *D, QualType Ty, ExprValueKind VK,
   bool RefersToCapturedVariable =
       isa<VarDecl>(D) &&
       NeedToCaptureVariable(cast<VarDecl>(D), NameInfo.getLoc());
-
+  
   DeclRefExpr *E = DeclRefExpr::Create(
       Context, NNS, TemplateKWLoc, D, RefersToCapturedVariable, NameInfo, Ty,
       VK, FoundD, TemplateArgs, getNonOdrUseReasonInCurrentContext(D));
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 938420d85c6..192c237b6c1 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -11,6 +11,7 @@
 ///
 //===----------------------------------------------------------------------===//
 
+#include "clang/Sema/Template.h"
 #include "clang/Sema/SemaInternal.h"
 #include "TreeTransform.h"
 #include "TypeLocBuilder.h"
@@ -8331,3 +8332,215 @@ Sema::CheckMicrosoftIfExistsSymbol(Scope *S, SourceLocation KeywordLoc,
 
   return CheckMicrosoftIfExistsSymbol(S, SS, TargetNameInfo);
 }
+
+concepts::Requirement *Sema::ActOnSimpleRequirement(Expr *E) {
+  return BuildExprRequirement(E, /*IsSimple=*/true,
+                              /*NoexceptLoc=*/SourceLocation(),
+                              /*ReturnTypeRequirement=*/{});
+}
+
+concepts::Requirement *
+Sema::ActOnTypeRequirement(SourceLocation TypenameKWLoc, CXXScopeSpec &SS,
+                           SourceLocation NameLoc, IdentifierInfo *TypeName,
+                           TemplateIdAnnotation *TemplateId) {
+  assert(((!TypeName && TemplateId) || (TypeName && !TemplateId)) &&
+         "Exactly one of TypeName and TemplateId must be specified.");
+  TypeSourceInfo *TSI = nullptr;
+  if (TypeName) {
+    QualType T = CheckTypenameType(ETK_Typename, TypenameKWLoc,
+                                   SS.getWithLocInContext(Context), *TypeName,
+                                   NameLoc, &TSI, /*DeducedTypeContext=*/false);
+    if (T.isNull())
+      return nullptr;
+  } else {
+    ASTTemplateArgsPtr ArgsPtr(TemplateId->getTemplateArgs(),
+                               TemplateId->NumArgs);
+    TypeResult T = ActOnTypenameType(CurScope, TypenameKWLoc, SS,
+                                     TemplateId->TemplateKWLoc,
+                                     TemplateId->Template, TemplateId->Name,
+                                     TemplateId->TemplateNameLoc,
+                                     TemplateId->LAngleLoc, ArgsPtr,
+                                     TemplateId->RAngleLoc);
+    if (T.isInvalid())
+      return nullptr;
+    if (GetTypeFromParser(T.get(), &TSI).isNull())
+      return nullptr;
+  }
+  return BuildTypeRequirement(TSI);
+}
+
+concepts::Requirement *
+Sema::ActOnCompoundRequirement(Expr *E, SourceLocation NoexceptLoc) {
+  return BuildExprRequirement(E, /*IsSimple=*/false, NoexceptLoc,
+                              /*ReturnTypeRequirement=*/{});
+}
+
+concepts::Requirement *
+Sema::ActOnCompoundRequirement(
+    Expr *E, SourceLocation NoexceptLoc, CXXScopeSpec &SS,
+    TemplateIdAnnotation *TypeConstraint, unsigned Depth) {
+  // C++2a [expr.prim.req.compound] p1.3.3
+  //   [..] the expression is deduced against an invented function template
+  //   F [...] F is a void function template with a single type template
+  //   parameter T declared with the constrained-parameter. Form a new
+  //   cv-qualifier-seq cv by taking the union of const and volatile specifiers
+  //   around the constrained-parameter. F has a single parameter whose
+  //   type-specifier is cv T followed by the abstract-declarator. [...]
+  //
+  // The cv part is done in the calling function - we get the concept with
+  // arguments and the abstract declarator with the correct CV qualification and
+  // have to synthesize T and the single parameter of F.
+  auto &II = Context.Idents.get("expr-type");
+  auto *TParam = TemplateTypeParmDecl::Create(Context, CurContext,
+                                              SourceLocation(),
+                                              SourceLocation(), Depth,
+                                              /*Index=*/0, &II,
+                                              /*Typename=*/true,
+                                              /*ParameterPack=*/false,
+                                              /*HasTypeConstraint=*/true);
+
+  if (ActOnTypeConstraint(SS, TypeConstraint, TParam,
+                          /*EllpsisLoc=*/SourceLocation()))
+    // Just produce a requirement with no type requirements.
+    return BuildExprRequirement(E, /*IsSimple=*/false, NoexceptLoc, {});
+
+  auto *TPL = TemplateParameterList::Create(Context, SourceLocation(),
+                                            SourceLocation(),
+                                            ArrayRef<NamedDecl *>(TParam),
+                                            SourceLocation(),
+                                            /*RequiresClause=*/nullptr);
+  return BuildExprRequirement(
+      E, /*IsSimple=*/false, NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement(TPL));
+}
+
+concepts::ExprRequirement *
+Sema::BuildExprRequirement(
+    Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+    concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement) {
+  auto Status = concepts::ExprRequirement::SS_Satisfied;
+  ConceptSpecializationExpr *SubstitutedConstraintExpr = nullptr;
+  if (E->isInstantiationDependent() || ReturnTypeRequirement.isDependent())
+    Status = concepts::ExprRequirement::SS_Dependent;
+  else if (NoexceptLoc.isValid() && canThrow(E) == CanThrowResult::CT_Can)
+    Status = concepts::ExprRequirement::SS_NoexceptNotMet;
+  else if (ReturnTypeRequirement.isSubstitutionFailure())
+    Status = concepts::ExprRequirement::SS_TypeRequirementSubstitutionFailure;
+  else if (ReturnTypeRequirement.isTypeConstraint()) {
+    // C++2a [expr.prim.req]p1.3.3
+    //     The immediately-declared constraint ([temp]) of decltype((E)) shall
+    //     be satisfied.
+    TemplateParameterList *TPL =
+        ReturnTypeRequirement.getTypeConstraintTemplateParameterList();
+    QualType MatchedType =
+        BuildDecltypeType(E, E->getBeginLoc()).getCanonicalType();
+    llvm::SmallVector<TemplateArgument, 1> Args;
+    Args.push_back(TemplateArgument(MatchedType));
+    TemplateArgumentList TAL(TemplateArgumentList::OnStack, Args);
+    MultiLevelTemplateArgumentList MLTAL(TAL);
+    for (unsigned I = 0; I < TPL->getDepth(); ++I)
+      MLTAL.addOuterRetainedLevel();
+    Expr *IDC =
+        cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint()
+            ->getImmediatelyDeclaredConstraint();
+    ExprResult Constraint = SubstExpr(IDC, MLTAL);
+    assert(!Constraint.isInvalid() &&
+           "Substitution cannot fail as it is simply putting a type template "
+           "argument into a concept specialization expression's parameter.");
+
+    SubstitutedConstraintExpr =
+        cast<ConceptSpecializationExpr>(Constraint.get());
+    if (!SubstitutedConstraintExpr->isSatisfied())
+      Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
+  }
+  return new (Context) concepts::ExprRequirement(E, IsSimple, NoexceptLoc,
+                                                 ReturnTypeRequirement, Status,
+                                                 SubstitutedConstraintExpr);
+}
+
+concepts::ExprRequirement *
+Sema::BuildExprRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *ExprSubstitutionDiagnostic,
+    bool IsSimple, SourceLocation NoexceptLoc,
+    concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement) {
+  return new (Context) concepts::ExprRequirement(ExprSubstitutionDiagnostic,
+                                                 IsSimple, NoexceptLoc,
+                                                 ReturnTypeRequirement);
+}
+
+concepts::TypeRequirement *
+Sema::BuildTypeRequirement(TypeSourceInfo *Type) {
+  return new (Context) concepts::TypeRequirement(Type);
+}
+
+concepts::TypeRequirement *
+Sema::BuildTypeRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+  return new (Context) concepts::TypeRequirement(SubstDiag);
+}
+
+concepts::Requirement *Sema::ActOnNestedRequirement(Expr *Constraint) {
+  return BuildNestedRequirement(Constraint);
+}
+
+concepts::NestedRequirement *
+Sema::BuildNestedRequirement(Expr *Constraint) {
+  ConstraintSatisfaction Satisfaction;
+  if (!Constraint->isInstantiationDependent() &&
+      CheckConstraintSatisfaction(Constraint, Satisfaction))
+    return nullptr;
+  return new (Context) concepts::NestedRequirement(Context, Constraint,
+                                                   Satisfaction);
+}
+
+concepts::NestedRequirement *
+Sema::BuildNestedRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+  return new (Context) concepts::NestedRequirement(SubstDiag);
+}
+
+RequiresExprBodyDecl *
+Sema::ActOnStartRequiresExpr(SourceLocation RequiresKWLoc,
+                             ArrayRef<ParmVarDecl *> LocalParameters,
+                             Scope *BodyScope) {
+  assert(BodyScope);
+
+  RequiresExprBodyDecl *Body = RequiresExprBodyDecl::Create(Context, CurContext,
+                                                            RequiresKWLoc);
+
+  PushDeclContext(BodyScope, Body);
+
+  for (ParmVarDecl *Param : LocalParameters) {
+    if (Param->hasDefaultArg())
+      // C++2a [expr.prim.req] p4
+      //     [...] A local parameter of a requires-expression shall not have a
+      //     default argument. [...]
+      Diag(Param->getDefaultArgRange().getBegin(),
+           diag::err_requires_expr_local_parameter_default_argument);
+    // Ignore default argument and move on
+
+    Param->setDeclContext(Body);
+    // If this has an identifier, add it to the scope stack.
+    if (Param->getIdentifier()) {
+      CheckShadow(BodyScope, Param);
+      PushOnScopeChains(Param, BodyScope);
+    }
+  }
+  return Body;
+}
+
+void Sema::ActOnFinishRequiresExpr() {
+  assert(CurContext && "DeclContext imbalance!");
+  CurContext = CurContext->getLexicalParent();
+  assert(CurContext && "Popped translation unit!");
+}
+
+ExprResult
+Sema::ActOnRequiresExpr(SourceLocation RequiresKWLoc,
+                        RequiresExprBodyDecl *Body,
+                        ArrayRef<ParmVarDecl *> LocalParameters,
+                        ArrayRef<concepts::Requirement *> Requirements,
+                        SourceLocation ClosingBraceLoc) {
+  return RequiresExpr::Create(Context, RequiresKWLoc, Body, LocalParameters,
+                              Requirements, ClosingBraceLoc);
+}
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 0ed51de0cc1..8d96404a5c2 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -1575,7 +1575,9 @@ llvm::DenseSet<Module*> &Sema::getLookupModules() {
   unsigned N = CodeSynthesisContexts.size();
   for (unsigned I = CodeSynthesisContextLookupModules.size();
        I != N; ++I) {
-    Module *M = getDefiningModule(*this, CodeSynthesisContexts[I].Entity);
+    Module *M = CodeSynthesisContexts[I].Entity ?
+                getDefiningModule(*this, CodeSynthesisContexts[I].Entity) :
+                nullptr;
     if (M && !LookupModulesCache.insert(M).second)
       M = nullptr;
     CodeSynthesisContextLookupModules.push_back(M);
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 8a50a9e1538..661a66246a5 100755
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -10010,24 +10010,12 @@ Sema::ActOnTypenameType(Scope *S, SourceLocation TypenameLoc,
       << FixItHint::CreateRemoval(TypenameLoc);
 
   NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
+  TypeSourceInfo *TSI = nullptr;
   QualType T = CheckTypenameType(TypenameLoc.isValid()? ETK_Typename : ETK_None,
-                                 TypenameLoc, QualifierLoc, II, IdLoc);
+                                 TypenameLoc, QualifierLoc, II, IdLoc, &TSI,
+                                 /*DeducedTSTContext=*/true);
   if (T.isNull())
     return true;
-
-  TypeSourceInfo *TSI = Context.CreateTypeSourceInfo(T);
-  if (isa<DependentNameType>(T)) {
-    DependentNameTypeLoc TL = TSI->getTypeLoc().castAs<DependentNameTypeLoc>();
-    TL.setElaboratedKeywordLoc(TypenameLoc);
-    TL.setQualifierLoc(QualifierLoc);
-    TL.setNameLoc(IdLoc);
-  } else {
-    ElaboratedTypeLoc TL = TSI->getTypeLoc().castAs<ElaboratedTypeLoc>();
-    TL.setElaboratedKeywordLoc(TypenameLoc);
-    TL.setQualifierLoc(QualifierLoc);
-    TL.getNamedTypeLoc().castAs<TypeSpecTypeLoc>().setNameLoc(IdLoc);
-  }
-
   return CreateParsedType(T, TSI);
 }
 
@@ -10164,6 +10152,35 @@ static bool isEnableIf(NestedNameSpecifierLoc NNS, const IdentifierInfo &II,
   return true;
 }
 
+QualType
+Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
+                        SourceLocation KeywordLoc,
+                        NestedNameSpecifierLoc QualifierLoc,
+                        const IdentifierInfo &II,
+                        SourceLocation IILoc,
+                        TypeSourceInfo **TSI,
+                        bool DeducedTSTContext) {
+  QualType T = CheckTypenameType(Keyword, KeywordLoc, QualifierLoc, II, IILoc,
+                                 DeducedTSTContext);
+  if (T.isNull())
+    return QualType();
+
+  *TSI = Context.CreateTypeSourceInfo(T);
+  if (isa<DependentNameType>(T)) {
+    DependentNameTypeLoc TL =
+        (*TSI)->getTypeLoc().castAs<DependentNameTypeLoc>();
+    TL.setElaboratedKeywordLoc(KeywordLoc);
+    TL.setQualifierLoc(QualifierLoc);
+    TL.setNameLoc(IILoc);
+  } else {
+    ElaboratedTypeLoc TL = (*TSI)->getTypeLoc().castAs<ElaboratedTypeLoc>();
+    TL.setElaboratedKeywordLoc(KeywordLoc);
+    TL.setQualifierLoc(QualifierLoc);
+    TL.getNamedTypeLoc().castAs<TypeSpecTypeLoc>().setNameLoc(IILoc);
+  }
+  return T;
+}
+
 /// Build the type that describes a C++ typename specifier,
 /// e.g., "typename T::type".
 QualType
@@ -10171,32 +10188,38 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
                         SourceLocation KeywordLoc,
                         NestedNameSpecifierLoc QualifierLoc,
                         const IdentifierInfo &II,
-                        SourceLocation IILoc) {
+                        SourceLocation IILoc, bool DeducedTSTContext) {
   CXXScopeSpec SS;
   SS.Adopt(QualifierLoc);
 
-  DeclContext *Ctx = computeDeclContext(SS);
-  if (!Ctx) {
-    // If the nested-name-specifier is dependent and couldn't be
-    // resolved to a type, build a typename type.
-    assert(QualifierLoc.getNestedNameSpecifier()->isDependent());
-    return Context.getDependentNameType(Keyword,
-                                        QualifierLoc.getNestedNameSpecifier(),
-                                        &II);
+  DeclContext *Ctx = nullptr;
+  if (QualifierLoc) {
+    Ctx = computeDeclContext(SS);
+    if (!Ctx) {
+      // If the nested-name-specifier is dependent and couldn't be
+      // resolved to a type, build a typename type.
+      assert(QualifierLoc.getNestedNameSpecifier()->isDependent());
+      return Context.getDependentNameType(Keyword,
+                                          QualifierLoc.getNestedNameSpecifier(),
+                                          &II);
+    }
+
+    // If the nested-name-specifier refers to the current instantiation,
+    // the "typename" keyword itself is superfluous. In C++03, the
+    // program is actually ill-formed. However, DR 382 (in C++0x CD1)
+    // allows such extraneous "typename" keywords, and we retroactively
+    // apply this DR to C++03 code with only a warning. In any case we continue.
+
+    if (RequireCompleteDeclContext(SS, Ctx))
+      return QualType();
   }
 
-  // If the nested-name-specifier refers to the current instantiation,
-  // the "typename" keyword itself is superfluous. In C++03, the
-  // program is actually ill-formed. However, DR 382 (in C++0x CD1)
-  // allows such extraneous "typename" keywords, and we retroactively
-  // apply this DR to C++03 code with only a warning. In any case we continue.
-
-  if (RequireCompleteDeclContext(SS, Ctx))
-    return QualType();
-
   DeclarationName Name(&II);
   LookupResult Result(*this, Name, IILoc, LookupOrdinaryName);
-  LookupQualifiedName(Result, Ctx, SS);
+  if (Ctx)
+    LookupQualifiedName(Result, Ctx, SS);
+  else
+    LookupName(Result, CurScope);
   unsigned DiagID = 0;
   Decl *Referenced = nullptr;
   switch (Result.getResultKind()) {
@@ -10205,7 +10228,7 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
     // a more specific diagnostic.
     SourceRange CondRange;
     Expr *Cond = nullptr;
-    if (isEnableIf(QualifierLoc, II, CondRange, Cond)) {
+    if (Ctx && isEnableIf(QualifierLoc, II, CondRange, Cond)) {
       // If we have a condition, narrow it down to the specific failed
       // condition.
       if (Cond) {
@@ -10221,12 +10244,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
         return QualType();
       }
 
-      Diag(CondRange.getBegin(), diag::err_typename_nested_not_found_enable_if)
+      Diag(CondRange.getBegin(),
+           diag::err_typename_nested_not_found_enable_if)
           << Ctx << CondRange;
       return QualType();
     }
 
-    DiagID = diag::err_typename_nested_not_found;
+    DiagID = Ctx ? diag::err_typename_nested_not_found
+                 : diag::err_unknown_typename;
     break;
   }
 
@@ -10292,6 +10317,19 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
     //   is a placeholder for a deduced class type [...].
     if (getLangOpts().CPlusPlus17) {
       if (auto *TD = getAsTypeTemplateDecl(Result.getFoundDecl())) {
+        if (!DeducedTSTContext) {
+          QualType T(QualifierLoc
+                         ? QualifierLoc.getNestedNameSpecifier()->getAsType()
+                         : nullptr, 0);
+          if (!T.isNull())
+            Diag(IILoc, diag::err_dependent_deduced_tst)
+              << (int)getTemplateNameKindForDiagnostics(TemplateName(TD)) << T;
+          else
+            Diag(IILoc, diag::err_deduced_tst)
+              << (int)getTemplateNameKindForDiagnostics(TemplateName(TD));
+          Diag(TD->getLocation(), diag::note_template_decl_here);
+          return QualType();
+        }
         return Context.getElaboratedType(
             Keyword, QualifierLoc.getNestedNameSpecifier(),
             Context.getDeducedTemplateSpecializationType(TemplateName(TD),
@@ -10299,12 +10337,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
       }
     }
 
-    DiagID = diag::err_typename_nested_not_type;
+    DiagID = Ctx ? diag::err_typename_nested_not_type
+                 : diag::err_typename_not_type;
     Referenced = Result.getFoundDecl();
     break;
 
   case LookupResult::FoundOverloaded:
-    DiagID = diag::err_typename_nested_not_type;
+    DiagID = Ctx ? diag::err_typename_nested_not_type
+                 : diag::err_typename_not_type;
     Referenced = *Result.begin();
     break;
 
@@ -10316,9 +10356,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
   // type. Emit an appropriate diagnostic and return an error.
   SourceRange FullRange(KeywordLoc.isValid() ? KeywordLoc : SS.getBeginLoc(),
                         IILoc);
-  Diag(IILoc, DiagID) << FullRange << Name << Ctx;
+  if (Ctx)
+    Diag(IILoc, DiagID) << FullRange << Name << Ctx;
+  else
+    Diag(IILoc, DiagID) << FullRange << Name;
   if (Referenced)
-    Diag(Referenced->getLocation(), diag::note_typename_refers_here)
+    Diag(Referenced->getLocation(),
+         Ctx ? diag::note_typename_member_refers_here
+             : diag::note_typename_refers_here)
       << Name;
   return QualType();
 }
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index af41e231134..39bc28d6230 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -26,6 +26,7 @@
 #include "clang/Sema/Template.h"
 #include "clang/Sema/TemplateDeduction.h"
 #include "clang/Sema/TemplateInstCallback.h"
+#include "clang/Sema/SemaConcept.h"
 #include "llvm/Support/TimeProfiler.h"
 
 using namespace clang;
@@ -199,8 +200,10 @@ bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
   case DeducedTemplateArgumentSubstitution:
   case PriorTemplateArgumentSubstitution:
   case ConstraintsCheck:
+  case NestedRequirementConstraintsCheck:
     return true;
 
+  case RequirementInstantiation:
   case DefaultTemplateArgumentChecking:
   case DeclaringSpecialMember:
   case DeclaringImplicitEqualityComparison:
@@ -247,7 +250,7 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
     Inst.InstantiationRange = InstantiationRange;
     SemaRef.pushCodeSynthesisContext(Inst);
 
-    AlreadyInstantiating =
+    AlreadyInstantiating = !Inst.Entity ? false :
         !SemaRef.InstantiatingSpecializations
              .insert(std::make_pair(Inst.Entity->getCanonicalDecl(), Inst.Kind))
              .second;
@@ -366,6 +369,26 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
 
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
+    concepts::Requirement *Req, sema::TemplateDeductionInfo &DeductionInfo,
+    SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::RequirementInstantiation,
+          PointOfInstantiation, InstantiationRange, /*Entity=*/nullptr,
+          /*Template=*/nullptr, /*TemplateArgs=*/None, &DeductionInfo) {}
+
+
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    concepts::NestedRequirement *Req, ConstraintsCheck,
+    SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::NestedRequirementConstraintsCheck,
+          PointOfInstantiation, InstantiationRange, /*Entity=*/nullptr,
+          /*Template=*/nullptr, /*TemplateArgs=*/None) {}
+
+
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
     ConstraintsCheck, NamedDecl *Template,
     ArrayRef<TemplateArgument> TemplateArgs, SourceRange InstantiationRange)
     : InstantiatingTemplate(
@@ -446,8 +469,9 @@ void Sema::InstantiatingTemplate::Clear() {
   if (!Invalid) {
     if (!AlreadyInstantiating) {
       auto &Active = SemaRef.CodeSynthesisContexts.back();
-      SemaRef.InstantiatingSpecializations.erase(
-          std::make_pair(Active.Entity, Active.Kind));
+      if (Active.Entity)
+        SemaRef.InstantiatingSpecializations.erase(
+            std::make_pair(Active.Entity, Active.Kind));
     }
 
     atTemplateEnd(SemaRef.TemplateInstCallbacks, SemaRef,
@@ -684,6 +708,18 @@ void Sema::PrintInstantiationStack() {
         << Active->InstantiationRange;
       break;
 
+    case CodeSynthesisContext::RequirementInstantiation:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_template_requirement_instantiation_here)
+        << Active->InstantiationRange;
+      break;
+
+    case CodeSynthesisContext::NestedRequirementConstraintsCheck:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_nested_requirement_here)
+        << Active->InstantiationRange;
+      break;
+
     case CodeSynthesisContext::DeclaringSpecialMember:
       Diags.Report(Active->PointOfInstantiation,
                    diag::note_in_declaration_of_implicit_special_member)
@@ -788,6 +824,7 @@ Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
     case CodeSynthesisContext::ConstraintsCheck:
     case CodeSynthesisContext::ParameterMappingSubstitution:
     case CodeSynthesisContext::ConstraintNormalization:
+    case CodeSynthesisContext::NestedRequirementConstraintsCheck:
       // This is a template instantiation, so there is no SFINAE.
       return None;
 
@@ -802,9 +839,10 @@ Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
     case CodeSynthesisContext::ExplicitTemplateArgumentSubstitution:
     case CodeSynthesisContext::DeducedTemplateArgumentSubstitution:
     case CodeSynthesisContext::ConstraintSubstitution:
-      // We're either substituting explicitly-specified template arguments
-      // or deduced template arguments or a constraint expression, so SFINAE
-      // applies.
+    case CodeSynthesisContext::RequirementInstantiation:
+      // We're either substituting explicitly-specified template arguments,
+      // deduced template arguments, a constraint expression or a requirement
+      // in a requires expression, so SFINAE applies.
       assert(Active->DeductionInfo && "Missing deduction info pointer");
       return Active->DeductionInfo;
 
@@ -1056,6 +1094,41 @@ namespace {
       return TreeTransform<TemplateInstantiator>::TransformLambdaExpr(E);
     }
 
+    ExprResult TransformRequiresExpr(RequiresExpr *E) {
+      LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
+      return TreeTransform<TemplateInstantiator>::TransformRequiresExpr(E);
+    }
+
+    bool TransformRequiresExprRequirements(
+        ArrayRef<concepts::Requirement *> Reqs,
+        SmallVectorImpl<concepts::Requirement *> &Transformed) {
+      bool SatisfactionDetermined = false;
+      for (concepts::Requirement *Req : Reqs) {
+        concepts::Requirement *TransReq = nullptr;
+        if (!SatisfactionDetermined) {
+          if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req))
+            TransReq = TransformTypeRequirement(TypeReq);
+          else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req))
+            TransReq = TransformExprRequirement(ExprReq);
+          else
+            TransReq = TransformNestedRequirement(
+                cast<concepts::NestedRequirement>(Req));
+          if (!TransReq)
+            return true;
+          if (!TransReq->isDependent() && !TransReq->isSatisfied())
+            // [expr.prim.req]p6
+            //   [...]  The substitution and semantic constraint checking
+            //   proceeds in lexical order and stops when a condition that
+            //   determines the result of the requires-expression is
+            //   encountered. [..]
+            SatisfactionDetermined = true;
+        } else
+          TransReq = Req;
+        Transformed.push_back(TransReq);
+      }
+      return false;
+    }
+
     TemplateParameterList *TransformTemplateParameterList(
                               TemplateParameterList *OrigTPL)  {
       if (!OrigTPL || !OrigTPL->size()) return OrigTPL;
@@ -1065,6 +1138,14 @@ namespace {
                         /* DeclContext *Owner */ Owner, TemplateArgs);
       return DeclInstantiator.SubstTemplateParams(OrigTPL);
     }
+
+    concepts::TypeRequirement *
+    TransformTypeRequirement(concepts::TypeRequirement *Req);
+    concepts::ExprRequirement *
+    TransformExprRequirement(concepts::ExprRequirement *Req);
+    concepts::NestedRequirement *
+    TransformNestedRequirement(concepts::NestedRequirement *Req);
+
   private:
     ExprResult transformNonTypeTemplateParmRef(NonTypeTemplateParmDecl *parm,
                                                SourceLocation loc,
@@ -1669,6 +1750,163 @@ TemplateInstantiator::TransformSubstTemplateTypeParmPackType(
   return Result;
 }
 
+template<typename EntityPrinter>
+static concepts::Requirement::SubstitutionDiagnostic *
+createSubstDiag(Sema &S, TemplateDeductionInfo &Info, EntityPrinter Printer) {
+  SmallString<128> Message;
+  SourceLocation ErrorLoc;
+  if (Info.hasSFINAEDiagnostic()) {
+    PartialDiagnosticAt PDA(SourceLocation(),
+                            PartialDiagnostic::NullDiagnostic{});
+    Info.takeSFINAEDiagnostic(PDA);
+    PDA.second.EmitToString(S.getDiagnostics(), Message);
+    ErrorLoc = PDA.first;
+  } else {
+    ErrorLoc = Info.getLocation();
+  }
+  char *MessageBuf = new (S.Context) char[Message.size()];
+  std::copy(Message.begin(), Message.end(), MessageBuf);
+  SmallString<128> Entity;
+  llvm::raw_svector_ostream OS(Entity);
+  Printer(OS);
+  char *EntityBuf = new (S.Context) char[Entity.size()];
+  std::copy(Entity.begin(), Entity.end(), EntityBuf);
+  return new (S.Context) concepts::Requirement::SubstitutionDiagnostic{
+      StringRef(EntityBuf, Entity.size()), ErrorLoc,
+      StringRef(MessageBuf, Message.size())};
+}
+
+concepts::TypeRequirement *
+TemplateInstantiator::TransformTypeRequirement(concepts::TypeRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+  if (Req->isSubstitutionFailure()) {
+    if (AlwaysRebuild())
+      return RebuildTypeRequirement(
+              Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+
+  Sema::SFINAETrap Trap(SemaRef);
+  TemplateDeductionInfo Info(Req->getType()->getTypeLoc().getBeginLoc());
+  Sema::InstantiatingTemplate TypeInst(SemaRef,
+      Req->getType()->getTypeLoc().getBeginLoc(), Req, Info,
+      Req->getType()->getTypeLoc().getSourceRange());
+  if (TypeInst.isInvalid())
+    return nullptr;
+  TypeSourceInfo *TransType = TransformType(Req->getType());
+  if (!TransType || Trap.hasErrorOccurred())
+    return RebuildTypeRequirement(createSubstDiag(SemaRef, Info,
+        [&] (llvm::raw_ostream& OS) {
+            Req->getType()->getType().print(OS, SemaRef.getPrintingPolicy());
+        }));
+  return RebuildTypeRequirement(TransType);
+}
+
+concepts::ExprRequirement *
+TemplateInstantiator::TransformExprRequirement(concepts::ExprRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+
+  Sema::SFINAETrap Trap(SemaRef);
+  TemplateDeductionInfo Info(Req->getExpr()->getBeginLoc());
+
+  llvm::PointerUnion<Expr *, concepts::Requirement::SubstitutionDiagnostic *>
+      TransExpr;
+  if (Req->isExprSubstitutionFailure())
+    TransExpr = Req->getExprSubstitutionDiagnostic();
+  else {
+    Sema::InstantiatingTemplate ExprInst(SemaRef, Req->getExpr()->getBeginLoc(),
+                                         Req, Info,
+                                         Req->getExpr()->getSourceRange());
+    if (ExprInst.isInvalid())
+      return nullptr;
+    ExprResult TransExprRes = TransformExpr(Req->getExpr());
+    if (TransExprRes.isInvalid() || Trap.hasErrorOccurred())
+      TransExpr = createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              Req->getExpr()->printPretty(OS, nullptr,
+                                          SemaRef.getPrintingPolicy());
+          });
+    else
+      TransExpr = TransExprRes.get();
+  }
+
+  llvm::Optional<concepts::ExprRequirement::ReturnTypeRequirement> TransRetReq;
+  const auto &RetReq = Req->getReturnTypeRequirement();
+  if (RetReq.isEmpty())
+    TransRetReq.emplace();
+  else if (RetReq.isSubstitutionFailure())
+    TransRetReq.emplace(RetReq.getSubstitutionDiagnostic());
+  else if (RetReq.isTypeConstraint()) {
+    TemplateParameterList *OrigTPL =
+        RetReq.getTypeConstraintTemplateParameterList();
+    Sema::InstantiatingTemplate TPLInst(SemaRef, OrigTPL->getTemplateLoc(),
+                                        Req, Info, OrigTPL->getSourceRange());
+    if (TPLInst.isInvalid())
+      return nullptr;
+    TemplateParameterList *TPL =
+        TransformTemplateParameterList(OrigTPL);
+    if (!TPL)
+      TransRetReq.emplace(createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              RetReq.getTypeConstraint()->getImmediatelyDeclaredConstraint()
+                  ->printPretty(OS, nullptr, SemaRef.getPrintingPolicy());
+          }));
+    else {
+      TPLInst.Clear();
+      TransRetReq.emplace(TPL);
+    }
+  }
+  assert(TransRetReq.hasValue() &&
+         "All code paths leading here must set TransRetReq");
+  if (Expr *E = TransExpr.dyn_cast<Expr *>())
+    return RebuildExprRequirement(E, Req->isSimple(), Req->getNoexceptLoc(),
+                                  std::move(*TransRetReq));
+  return RebuildExprRequirement(
+      TransExpr.get<concepts::Requirement::SubstitutionDiagnostic *>(),
+      Req->isSimple(), Req->getNoexceptLoc(), std::move(*TransRetReq));
+}
+
+concepts::NestedRequirement *
+TemplateInstantiator::TransformNestedRequirement(
+    concepts::NestedRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+  if (Req->isSubstitutionFailure()) {
+    if (AlwaysRebuild())
+      return RebuildNestedRequirement(
+          Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  Sema::InstantiatingTemplate ReqInst(SemaRef,
+      Req->getConstraintExpr()->getBeginLoc(), Req,
+      Sema::InstantiatingTemplate::ConstraintsCheck{},
+      Req->getConstraintExpr()->getSourceRange());
+
+  ExprResult TransConstraint;
+  TemplateDeductionInfo Info(Req->getConstraintExpr()->getBeginLoc());
+  {
+    EnterExpressionEvaluationContext ContextRAII(
+        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+    Sema::SFINAETrap Trap(SemaRef);
+    Sema::InstantiatingTemplate ConstrInst(SemaRef,
+        Req->getConstraintExpr()->getBeginLoc(), Req, Info,
+        Req->getConstraintExpr()->getSourceRange());
+    if (ConstrInst.isInvalid())
+      return nullptr;
+    TransConstraint = TransformExpr(Req->getConstraintExpr());
+    if (TransConstraint.isInvalid() || Trap.hasErrorOccurred())
+      return RebuildNestedRequirement(createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              Req->getConstraintExpr()->printPretty(OS, nullptr,
+                                                    SemaRef.getPrintingPolicy());
+          }));
+  }
+  return RebuildNestedRequirement(TransConstraint.get());
+}
+
+
 /// Perform substitution on the type T with a given set of template
 /// arguments.
 ///
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 64500d0a26d..a470cfc8744 100755
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3600,6 +3600,12 @@ Decl *TemplateDeclInstantiator::VisitConceptDecl(ConceptDecl *D) {
   llvm_unreachable("Concept definitions cannot reside inside a template");
 }
 
+Decl *
+TemplateDeclInstantiator::VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D) {
+  return RequiresExprBodyDecl::Create(SemaRef.Context, D->getDeclContext(),
+                                      D->getBeginLoc());
+}
+
 Decl *TemplateDeclInstantiator::VisitDecl(Decl *D) {
   llvm_unreachable("Unexpected decl");
 }
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 3884fdae8fe..bafeed6e844 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -2994,6 +2994,9 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
     case DeclaratorContext::PrototypeContext:
       Error = 0;
       break;
+    case DeclaratorContext::RequiresExprContext:
+      Error = 21;
+      break;
     case DeclaratorContext::LambdaExprParameterContext:
       // In C++14, generic lambdas allow 'auto' in their parameters.
       if (!SemaRef.getLangOpts().CPlusPlus14 ||
@@ -3221,6 +3224,7 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
     case DeclaratorContext::ObjCParameterContext:
     case DeclaratorContext::ObjCResultContext:
     case DeclaratorContext::KNRTypeListContext:
+    case DeclaratorContext::RequiresExprContext:
       // C++ [dcl.fct]p6:
       //   Types shall not be defined in return or parameter types.
       DiagID = diag::err_type_defined_in_param_type;
@@ -4279,6 +4283,7 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TypeNameContext:
     case DeclaratorContext::FunctionalCastContext:
+    case DeclaratorContext::RequiresExprContext:
       // Don't infer in these contexts.
       break;
     }
@@ -5227,6 +5232,7 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
     switch (D.getContext()) {
     case DeclaratorContext::PrototypeContext:
     case DeclaratorContext::LambdaExprParameterContext:
+    case DeclaratorContext::RequiresExprContext:
       // C++0x [dcl.fct]p13:
       //   [...] When it is part of a parameter-declaration-clause, the
       //   parameter pack is a function parameter pack (14.5.3). The type T
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 3b827fbc950..1f725117383 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -19,6 +19,7 @@
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
@@ -509,6 +510,15 @@ public:
   DeclarationNameInfo
   TransformDeclarationNameInfo(const DeclarationNameInfo &NameInfo);
 
+  bool TransformRequiresExprRequirements(ArrayRef<concepts::Requirement *> Reqs,
+      llvm::SmallVectorImpl<concepts::Requirement *> &Transformed);
+  concepts::TypeRequirement *
+  TransformTypeRequirement(concepts::TypeRequirement *Req);
+  concepts::ExprRequirement *
+  TransformExprRequirement(concepts::ExprRequirement *Req);
+  concepts::NestedRequirement *
+  TransformNestedRequirement(concepts::NestedRequirement *Req);
+
   /// Transform the given template name.
   ///
   /// \param SS The nested-name-specifier that qualifies the template
@@ -1056,23 +1066,8 @@ public:
     }
 
     if (Keyword == ETK_None || Keyword == ETK_Typename) {
-      QualType T = SemaRef.CheckTypenameType(Keyword, KeywordLoc, QualifierLoc,
-                                             *Id, IdLoc);
-      // If a dependent name resolves to a deduced template specialization type,
-      // check that we're in one of the syntactic contexts permitting it.
-      if (!DeducedTSTContext) {
-        if (auto *Deduced = dyn_cast_or_null<DeducedTemplateSpecializationType>(
-                T.isNull() ? nullptr : T->getContainedDeducedType())) {
-          SemaRef.Diag(IdLoc, diag::err_dependent_deduced_tst)
-            << (int)SemaRef.getTemplateNameKindForDiagnostics(
-                   Deduced->getTemplateName())
-            << QualType(QualifierLoc.getNestedNameSpecifier()->getAsType(), 0);
-          if (auto *TD = Deduced->getTemplateName().getAsTemplateDecl())
-            SemaRef.Diag(TD->getLocation(), diag::note_template_decl_here);
-          return QualType();
-        }
-      }
-      return T;
+      return SemaRef.CheckTypenameType(Keyword, KeywordLoc, QualifierLoc,
+                                       *Id, IdLoc, DeducedTSTContext);
     }
 
     TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForKeyword(Keyword);
@@ -3078,7 +3073,56 @@ public:
     return Result;
   }
 
-    /// \brief Build a new Objective-C boxed expression.
+  /// \brief Build a new requires expression.
+  ///
+  /// By default, performs semantic analysis to build the new expression.
+  /// Subclasses may override this routine to provide different behavior.
+  ExprResult RebuildRequiresExpr(SourceLocation RequiresKWLoc,
+                                 RequiresExprBodyDecl *Body,
+                                 ArrayRef<ParmVarDecl *> LocalParameters,
+                                 ArrayRef<concepts::Requirement *> Requirements,
+                                 SourceLocation ClosingBraceLoc) {
+    return RequiresExpr::Create(SemaRef.Context, RequiresKWLoc, Body,
+                                LocalParameters, Requirements, ClosingBraceLoc);
+  }
+
+  concepts::TypeRequirement *
+  RebuildTypeRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+    return SemaRef.BuildTypeRequirement(SubstDiag);
+  }
+
+  concepts::TypeRequirement *RebuildTypeRequirement(TypeSourceInfo *T) {
+    return SemaRef.BuildTypeRequirement(T);
+  }
+
+  concepts::ExprRequirement *
+  RebuildExprRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag, bool IsSimple,
+      SourceLocation NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement Ret) {
+    return SemaRef.BuildExprRequirement(SubstDiag, IsSimple, NoexceptLoc,
+                                        std::move(Ret));
+  }
+
+  concepts::ExprRequirement *
+  RebuildExprRequirement(Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+                         concepts::ExprRequirement::ReturnTypeRequirement Ret) {
+    return SemaRef.BuildExprRequirement(E, IsSimple, NoexceptLoc,
+                                        std::move(Ret));
+  }
+
+  concepts::NestedRequirement *
+  RebuildNestedRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+    return SemaRef.BuildNestedRequirement(SubstDiag);
+  }
+
+  concepts::NestedRequirement *RebuildNestedRequirement(Expr *Constraint) {
+    return SemaRef.BuildNestedRequirement(Constraint);
+  }
+
+  /// \brief Build a new Objective-C boxed expression.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
@@ -11179,6 +11223,146 @@ TreeTransform<Derived>::TransformConceptSpecializationExpr(
       &TransArgs);
 }
 
+template<typename Derived>
+ExprResult
+TreeTransform<Derived>::TransformRequiresExpr(RequiresExpr *E) {
+  SmallVector<ParmVarDecl*, 4> TransParams;
+  SmallVector<QualType, 4> TransParamTypes;
+  Sema::ExtParameterInfoBuilder ExtParamInfos;
+
+  // C++2a [expr.prim.req]p2
+  // Expressions appearing within a requirement-body are unevaluated operands.
+  EnterExpressionEvaluationContext Ctx(
+      SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
+
+  RequiresExprBodyDecl *Body = RequiresExprBodyDecl::Create(
+      getSema().Context, E->getBody()->getDeclContext(),
+      E->getBody()->getBeginLoc());
+
+  Sema::ContextRAII SavedContext(getSema(), Body, /*NewThisContext*/false);
+
+  if (getDerived().TransformFunctionTypeParams(E->getRequiresKWLoc(),
+                                               E->getLocalParameters(),
+                                               /*ParamTypes=*/nullptr,
+                                               /*ParamInfos=*/nullptr,
+                                               TransParamTypes, &TransParams,
+                                               ExtParamInfos))
+    return ExprError();
+
+  for (ParmVarDecl *Param : TransParams)
+    Param->setDeclContext(Body);
+
+  SmallVector<concepts::Requirement *, 4> TransReqs;
+  if (getDerived().TransformRequiresExprRequirements(E->getRequirements(),
+                                                     TransReqs))
+    return ExprError();
+
+  for (concepts::Requirement *Req : TransReqs) {
+    if (auto *ER = dyn_cast<concepts::ExprRequirement>(Req)) {
+      if (ER->getReturnTypeRequirement().isTypeConstraint()) {
+        ER->getReturnTypeRequirement()
+                .getTypeConstraintTemplateParameterList()->getParam(0)
+                ->setDeclContext(Body);
+      }
+    }
+  }
+
+  return getDerived().RebuildRequiresExpr(E->getRequiresKWLoc(), Body,
+                                          TransParams, TransReqs,
+                                          E->getRBraceLoc());
+}
+
+template<typename Derived>
+bool TreeTransform<Derived>::TransformRequiresExprRequirements(
+    ArrayRef<concepts::Requirement *> Reqs,
+    SmallVectorImpl<concepts::Requirement *> &Transformed) {
+  for (concepts::Requirement *Req : Reqs) {
+    concepts::Requirement *TransReq = nullptr;
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req))
+      TransReq = getDerived().TransformTypeRequirement(TypeReq);
+    else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req))
+      TransReq = getDerived().TransformExprRequirement(ExprReq);
+    else
+      TransReq = getDerived().TransformNestedRequirement(
+                     cast<concepts::NestedRequirement>(Req));
+    if (!TransReq)
+      return true;
+    Transformed.push_back(TransReq);
+  }
+  return false;
+}
+
+template<typename Derived>
+concepts::TypeRequirement *
+TreeTransform<Derived>::TransformTypeRequirement(
+    concepts::TypeRequirement *Req) {
+  if (Req->isSubstitutionFailure()) {
+    if (getDerived().AlwaysRebuild())
+      return getDerived().RebuildTypeRequirement(
+              Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  TypeSourceInfo *TransType = getDerived().TransformType(Req->getType());
+  if (!TransType)
+    return nullptr;
+  return getDerived().RebuildTypeRequirement(TransType);
+}
+
+template<typename Derived>
+concepts::ExprRequirement *
+TreeTransform<Derived>::TransformExprRequirement(concepts::ExprRequirement *Req) {
+  llvm::PointerUnion<Expr *, concepts::Requirement::SubstitutionDiagnostic *> TransExpr;
+  if (Req->isExprSubstitutionFailure())
+    TransExpr = Req->getExprSubstitutionDiagnostic();
+  else {
+    ExprResult TransExprRes = getDerived().TransformExpr(Req->getExpr());
+    if (TransExprRes.isInvalid())
+      return nullptr;
+    TransExpr = TransExprRes.get();
+  }
+
+  llvm::Optional<concepts::ExprRequirement::ReturnTypeRequirement> TransRetReq;
+  const auto &RetReq = Req->getReturnTypeRequirement();
+  if (RetReq.isEmpty())
+    TransRetReq.emplace();
+  else if (RetReq.isSubstitutionFailure())
+    TransRetReq.emplace(RetReq.getSubstitutionDiagnostic());
+  else if (RetReq.isTypeConstraint()) {
+    TemplateParameterList *OrigTPL =
+        RetReq.getTypeConstraintTemplateParameterList();
+    TemplateParameterList *TPL =
+        getDerived().TransformTemplateParameterList(OrigTPL);
+    if (!TPL)
+      return nullptr;
+    TransRetReq.emplace(TPL);
+  }
+  assert(TransRetReq.hasValue() &&
+         "All code paths leading here must set TransRetReq");
+  if (Expr *E = TransExpr.dyn_cast<Expr *>())
+    return getDerived().RebuildExprRequirement(E, Req->isSimple(),
+                                               Req->getNoexceptLoc(),
+                                               std::move(*TransRetReq));
+  return getDerived().RebuildExprRequirement(
+      TransExpr.get<concepts::Requirement::SubstitutionDiagnostic *>(),
+      Req->isSimple(), Req->getNoexceptLoc(), std::move(*TransRetReq));
+}
+
+template<typename Derived>
+concepts::NestedRequirement *
+TreeTransform<Derived>::TransformNestedRequirement(
+    concepts::NestedRequirement *Req) {
+  if (Req->isSubstitutionFailure()) {
+    if (getDerived().AlwaysRebuild())
+      return getDerived().RebuildNestedRequirement(
+          Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  ExprResult TransConstraint =
+      getDerived().TransformExpr(Req->getConstraintExpr());
+  if (TransConstraint.isInvalid())
+    return nullptr;
+  return getDerived().RebuildNestedRequirement(TransConstraint.get());
+}
 
 template<typename Derived>
 ExprResult