Index: include/clang/AST/ASTConcept.h
===================================================================
--- /dev/null
+++ include/clang/AST/ASTConcept.h
@@ -0,0 +1,80 @@
+//===--- ASTConcept.h - Concepts Related AST Data Structures ----*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// \brief This file provides AST data structures related to concepts.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_ASTCONCEPT_H
+#define LLVM_CLANG_AST_ASTCONCEPT_H
+#include "clang/AST/Expr.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/PointerUnion.h"
+#include "llvm/ADT/SmallVector.h"
+#include <string>
+#include <utility>
+namespace clang {
+
+/// \brief The result of a constraint satisfaction check, containing the
+/// necessary information to diagnose an unsatisfied constraint.
+struct ConstraintSatisfaction {
+  using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
+  using Detail = llvm::PointerUnion<Expr *, SubstitutionDiagnostic *>;
+
+  bool IsSatisfied = false;
+
+  /// \brief Pairs of unsatisfied atomic constraint expressions along with the
+  /// substituted constraint expr, if the template arguments could be
+  /// substituted into them, or a diagnostic if substitution resulted in an
+  /// invalid expression.
+  llvm::SmallVector<std::pair<const Expr *, Detail>, 4> Details;
+
+  // This can leak if used in an AST node, use ASTConstraintSatisfaction
+  // instead.
+  void *operator new(size_t bytes, ASTContext &C) = delete;
+};
+
+/// Pairs of unsatisfied atomic constraint expressions along with the
+/// substituted constraint expr, if the template arguments could be
+/// substituted into them, or a diagnostic if substitution resulted in
+/// an invalid expression.
+using UnsatisfiedConstraintRecord =
+    std::pair<const Expr *,
+              llvm::PointerUnion<Expr *,
+                                 std::pair<SourceLocation, StringRef> *>>;
+
+/// \brief The result of a constraint satisfaction check, containing the
+/// necessary information to diagnose an unsatisfied constraint.
+///
+/// This is safe to store in an AST node, as opposed to ConstraintSatisfaction.
+struct ASTConstraintSatisfaction final :
+    llvm::TrailingObjects<ASTConstraintSatisfaction,
+                          UnsatisfiedConstraintRecord> {
+  std::size_t NumRecords;
+  bool IsSatisfied : 1;
+
+  const UnsatisfiedConstraintRecord *begin() const {
+    return getTrailingObjects<UnsatisfiedConstraintRecord>();
+  }
+
+  const UnsatisfiedConstraintRecord *end() const {
+    return getTrailingObjects<UnsatisfiedConstraintRecord>() + NumRecords;
+  }
+
+  ASTConstraintSatisfaction(const ASTContext &C,
+                            const ConstraintSatisfaction &Satisfaction);
+
+  static ASTConstraintSatisfaction *
+  Create(const ASTContext &C, const ConstraintSatisfaction &Satisfaction);
+};
+
+} // clang
+
+#endif // LLVM_CLANG_AST_ASTCONCEPT_H
\ No newline at end of file
Index: include/clang/AST/ExprCXX.h
===================================================================
--- include/clang/AST/ExprCXX.h
+++ include/clang/AST/ExprCXX.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_AST_EXPRCXX_H
 #define LLVM_CLANG_AST_EXPRCXX_H
 
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
@@ -4757,6 +4758,10 @@
                                     TemplateArgument> {
   friend class ASTStmtReader;
   friend TrailingObjects;
+public:
+  using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
+
+protected:
 
   // \brief The optional nested name specifier used when naming the concept.
   NestedNameSpecifierLoc NestedNameSpec;
@@ -4774,11 +4779,8 @@
   /// through a UsingShadowDecl.
   NamedDecl *FoundDecl;
 
-  /// \brief The concept named, and whether or not the concept with the given
-  /// arguments was satisfied when the expression was created.
-  /// If any of the template arguments are dependent (this expr would then be
-  /// isValueDependent()), this bit is to be ignored.
-  llvm::PointerIntPair<ConceptDecl *, 1, bool> NamedConcept;
+  /// \brief The concept named.
+  ConceptDecl *NamedConcept;
 
   /// \brief The template argument list source info used to specialize the
   /// concept.
@@ -4788,13 +4790,18 @@
   /// converted template arguments.
   unsigned NumTemplateArgs;
 
+  /// \brief Information about the satisfaction of the named concept with the
+  /// given arguments. If this expression is value dependent, this is to be
+  /// ignored.
+  ASTConstraintSatisfaction *Satisfaction;
+
   ConceptSpecializationExpr(ASTContext &C, NestedNameSpecifierLoc NNS,
                             SourceLocation TemplateKWLoc,
                             SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
                             ConceptDecl *NamedConcept,
                             const ASTTemplateArgumentListInfo *ArgsAsWritten,
                             ArrayRef<TemplateArgument> ConvertedArgs,
-                            bool IsSatisfied);
+                            const ConstraintSatisfaction &Satisfaction);
 
   ConceptSpecializationExpr(EmptyShell Empty, unsigned NumTemplateArgs);
 
@@ -4805,7 +4812,8 @@
          SourceLocation TemplateKWLoc, SourceLocation ConceptNameLoc,
          NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
          const ASTTemplateArgumentListInfo *ArgsAsWritten,
-         ArrayRef<TemplateArgument> ConvertedArgs, bool IsSatisfied);
+         ArrayRef<TemplateArgument> ConvertedArgs,
+         const ConstraintSatisfaction &Satisfaction);
 
   static ConceptSpecializationExpr *
   Create(ASTContext &C, EmptyShell Empty, unsigned NumTemplateArgs);
@@ -4819,7 +4827,7 @@
   }
 
   ConceptDecl *getNamedConcept() const {
-    return NamedConcept.getPointer();
+    return NamedConcept;
   }
 
   ArrayRef<TemplateArgument> getTemplateArguments() const {
@@ -4836,12 +4844,21 @@
                             ArrayRef<TemplateArgument> Converted);
 
   /// \brief Whether or not the concept with the given arguments was satisfied
-  /// when the expression was created. This method assumes that the expression
-  /// is not dependent!
+  /// when the expression was created.
+  /// The expression must not be dependent.
   bool isSatisfied() const {
     assert(!isValueDependent()
            && "isSatisfied called on a dependent ConceptSpecializationExpr");
-    return NamedConcept.getInt();
+    return Satisfaction->IsSatisfied;
+  }
+
+  /// \brief Get elaborated satisfaction info about the template arguments'
+  /// satisfaction of the named concept.
+  /// The expression must not be dependent.
+  const ASTConstraintSatisfaction &getSatisfaction() const {
+    assert(!isValueDependent()
+           && "getSatisfaction called on dependent ConceptSpecializationExpr");
+    return *Satisfaction;
   }
 
   SourceLocation getConceptNameLoc() const { return ConceptNameLoc; }
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2473,6 +2473,26 @@
   "expression">;
 def err_non_bool_atomic_constraint : Error<
   "atomic constraint must be of type 'bool' (found %0)">;
+def err_template_arg_list_constraints_not_satisfied : Error<
+  "constraints not satisfied for %select{class template|function template|variable template|alias template|"
+  "template template parameter|template}0 %1%2">;
+def note_constraints_not_satisfied : Note<
+  "constraints not satisfied">;
+def note_substituted_constraint_expr_is_ill_formed : Note<
+  "because substituted constraint expression is ill-formed%0">;
+def note_atomic_constraint_evaluated_to_false : Note<
+  "%select{and |because }0'%1' evaluated to false">;
+def note_concept_specialization_constraint_evaluated_to_false : Note<
+  "%select{and |because }0'%1' evaluated to false">;
+def note_single_arg_concept_specialization_constraint_evaluated_to_false : Note<
+  "%select{and |because }0%1 does not satisfy %2">;
+def note_atomic_constraint_evaluated_to_false_elaborated : Note<
+  "%select{and |because }0'%1' (%2 %3 %4) evaluated to false">;
+def err_could_not_normalize_ill_formed_constraint : Error<
+  "required expansion of concept specialization %0 failed, substituted "
+  "expression would be illegal">;
+def note_could_not_normalize_ill_formed_constraint_reason : Note<
+  "because: %0">;
 
 def err_template_different_requires_clause : Error<
   "requires clause differs in template redeclaration">;
@@ -3699,6 +3719,8 @@
 def note_ovl_candidate_explicit_arg_mismatch_named : Note<
     "candidate template ignored: invalid explicitly-specified argument "
     "for template parameter %0">;
+def note_ovl_candidate_unsatisfied_constraints : Note<
+    "candidate template ignored: constraints not satisfied%0">;
 def note_ovl_candidate_explicit_arg_mismatch_unnamed : Note<
     "candidate template ignored: invalid explicitly-specified argument "
     "for %ordinal0 template parameter">;
@@ -4382,6 +4404,14 @@
   "while checking a default template argument used here">;
 def note_concept_specialization_here : Note<
   "while checking the satisfaction of concept '%0' requested here">;
+def note_checking_constraints_for_template_id_here : Note<
+  "while checking constraint satisfaction for template '%0' required here">;
+def note_checking_constraints_for_var_spec_id_here : Note<
+  "while checking constraint satisfaction for variable template "
+  "partial specialization '%0' required here">;
+def note_checking_constraints_for_class_spec_id_here : Note<
+  "while checking constraint satisfaction for class template partial "
+  "specialization '%0' required here">;
 def note_constraint_substitution_here : Note<
   "while substituting template arguments into constraint expression here">;
 def note_instantiation_contexts_suppressed : Note<
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_SEMA_SEMA_H
 #define LLVM_CLANG_SEMA_SEMA_H
 
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/Availability.h"
 #include "clang/AST/ComparisonCategories.h"
@@ -5898,10 +5899,45 @@
   /// A diagnostic is emitted if it is not, and false is returned.
   bool CheckConstraintExpression(Expr *CE);
 
-  bool CalculateConstraintSatisfaction(ConceptDecl *NamedConcept,
-                                         MultiLevelTemplateArgumentList &MLTAL,
-                                         Expr *ConstraintExpr,
-                                         bool &IsSatisfied);
+  /// \brief Check whether the given list of constraint expressions are
+  /// satisfied (as if in a 'conjunction') given template arguments.
+  /// \param ConstraintExprs a list of constraint expressions, treated as if
+  /// they were 'AND'ed together.
+  /// \param TemplateArgs the list of template arguments to substitute into the
+  /// constraint expression.
+  /// \param TemplateIDRange The source range of the template id that
+  /// caused the constraints check.
+  /// \param Satisfaction if true is returned, will contain details of the
+  /// satisfaction, with enough information to diagnose an unsatisfied
+  /// expression.
+  /// \returns true if an error occurred and satisfaction could not be checked,
+  /// false otherwise.
+  bool CheckConstraintSatisfaction(TemplateDecl *Template,
+                                   ArrayRef<const Expr *> ConstraintExprs,
+                                   ArrayRef<TemplateArgument> TemplateArgs,
+                                   SourceRange TemplateIDRange,
+                                   ConstraintSatisfaction &Satisfaction);
+
+  bool CheckConstraintSatisfaction(ClassTemplatePartialSpecializationDecl *TD,
+                                   ArrayRef<const Expr *> ConstraintExprs,
+                                   ArrayRef<TemplateArgument> TemplateArgs,
+                                   SourceRange TemplateIDRange,
+                                   ConstraintSatisfaction &Satisfaction);
+
+  bool CheckConstraintSatisfaction(VarTemplatePartialSpecializationDecl *TD,
+                                   ArrayRef<const Expr *> ConstraintExprs,
+                                   ArrayRef<TemplateArgument> TemplateArgs,
+                                   SourceRange TemplateIDRange,
+                                   ConstraintSatisfaction &Satisfaction);
+
+  /// \brief Check whether the given non-dependent constraint expression is
+  /// satisfied. Returns false and updates Satisfaction with the satisfaction
+  /// verdict if successful, emits a diagnostic and returns true if an error
+  /// occured and satisfaction could not be determined.
+  ///
+  /// \returns true if an error occurred, false otherwise.
+  bool CheckConstraintSatisfaction(const Expr *ConstraintExpr,
+                                   ConstraintSatisfaction &Satisfaction);
 
   /// Check that the associated constraints of a template declaration match the
   /// associated constraints of an older declaration of which it is a
@@ -5909,6 +5945,38 @@
   bool CheckRedeclarationConstraintMatch(TemplateParameterList *Old,
                                          TemplateParameterList *New);
 
+  /// \brief Ensure that the given template arguments satisfy the constraints
+  /// associated with the given template, emitting a diagnostic if they do not.
+  ///
+  /// \param Template The template to which the template arguments are being
+  /// provided.
+  ///
+  /// \param TemplateArgs The converted, canonicalized template arguments.
+  ///
+  /// \param TemplateIDRange The source range of the template id that
+  /// caused the constraints check.
+  ///
+  /// \returns true if the constrains are not satisfied or could not be checked
+  /// for satisfaction, false if the constraints are satisfied.
+  bool EnsureTemplateArgumentListConstraints(TemplateDecl *Template,
+                                       ArrayRef<TemplateArgument> TemplateArgs,
+                                             SourceRange TemplateIDRange);
+
+  /// \brief Emit diagnostics explaining why a constraint expression was deemed
+  /// unsatisfied.
+  void
+  DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction& Satisfaction);
+
+  /// \brief Emit diagnostics explaining why a constraint expression was deemed
+  /// unsatisfied.
+  void
+  DiagnoseUnsatisfiedConstraint(const ASTConstraintSatisfaction& Satisfaction);
+
+  /// \brief Emit diagnostics explaining why a constraint expression was deemed
+  /// unsatisfied because it was ill-formed.
+  void DiagnoseUnsatisfiedIllFormedConstraint(SourceLocation DiagnosticLocation,
+                                              StringRef Diagnostic);
+
   // ParseObjCStringLiteral - Parse Objective-C string literals.
   ExprResult ParseObjCStringLiteral(SourceLocation *AtLocs,
                                     ArrayRef<Expr *> Strings);
@@ -6707,13 +6775,19 @@
   /// contain the converted forms of the template arguments as written.
   /// Otherwise, \p TemplateArgs will not be modified.
   ///
+  /// \param InstantiationDependent If provided, and no error occured, will
+  /// receive true if the arguments' match to the given template is
+  /// instantiation dependant - e.g. the arguments contain a pack expansion
+  /// into a non pack parameter.
+  ///
   /// \returns true if an error occurred, false otherwise.
   bool CheckTemplateArgumentList(TemplateDecl *Template,
                                  SourceLocation TemplateLoc,
                                  TemplateArgumentListInfo &TemplateArgs,
                                  bool PartialTemplateArgs,
                                  SmallVectorImpl<TemplateArgument> &Converted,
-                                 bool UpdateArgsWithConversions = true);
+                                 bool UpdateArgsWithConversions = true,
+                                 bool *InstantiationDependent = nullptr);
 
   bool CheckTemplateTypeArgument(TemplateTypeParmDecl *Param,
                                  TemplateArgumentLoc &Arg,
@@ -7255,6 +7329,9 @@
     TDK_InvalidExplicitArguments,
     /// Checking non-dependent argument conversions failed.
     TDK_NonDependentConversionFailure,
+    /// The deduced arguments did not satisfy the constraints associated
+    /// with the template.
+    TDK_ConstraintsNotSatisfied,
     /// Deduction failed; that's all we know.
     TDK_MiscellaneousDeductionFailure,
     /// CUDA Target attributes do not match.
@@ -7762,7 +7839,7 @@
     /// constrained entity (a concept declaration or a template with associated
     /// constraints).
     InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
-                          ConstraintsCheck, TemplateDecl *Template,
+                          ConstraintsCheck, NamedDecl *Template,
                           ArrayRef<TemplateArgument> TemplateArgs,
                           SourceRange InstantiationRange);
 
@@ -7771,7 +7848,7 @@
     /// with a template declaration or as part of the satisfaction check of a
     /// concept.
     InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
-                          ConstraintSubstitution, TemplateDecl *Template,
+                          ConstraintSubstitution, NamedDecl *Template,
                           sema::TemplateDeductionInfo &DeductionInfo,
                           SourceRange InstantiationRange);
 
Index: include/clang/Sema/TemplateDeduction.h
===================================================================
--- include/clang/Sema/TemplateDeduction.h
+++ include/clang/Sema/TemplateDeduction.h
@@ -14,6 +14,8 @@
 #ifndef LLVM_CLANG_SEMA_TEMPLATEDEDUCTION_H
 #define LLVM_CLANG_SEMA_TEMPLATEDEDUCTION_H
 
+#include "clang/Sema/Ownership.h"
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/DeclAccessPair.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/TemplateBase.h"
@@ -218,6 +220,10 @@
   ///
   /// FIXME: This should be kept internal to SemaTemplateDeduction.
   SmallVector<DeducedPack *, 8> PendingDeducedPacks;
+
+  /// \brief The constraint satisfaction details resulting from the associated
+  /// constraints satisfaction tests.
+  ConstraintSatisfaction AssociatedConstraintsSatisfaction;
 };
 
 } // namespace sema
Index: lib/AST/ASTConcept.cpp
===================================================================
--- /dev/null
+++ lib/AST/ASTConcept.cpp
@@ -0,0 +1,56 @@
+//===--- ASTConcept.cpp - Concepts Related AST Data Structures --*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// \brief This file defines AST data structures related to concepts.
+///
+//===----------------------------------------------------------------------===//
+
+#include "clang/AST/ASTConcept.h"
+#include "clang/AST/ASTContext.h"
+using namespace clang;
+
+ASTConstraintSatisfaction::ASTConstraintSatisfaction(const ASTContext &C,
+    const ConstraintSatisfaction &Satisfaction):
+    NumRecords{Satisfaction.Details.size()},
+    IsSatisfied{Satisfaction.IsSatisfied} {
+  for (unsigned I = 0; I < NumRecords; ++I) {
+    auto &Detail = Satisfaction.Details[I];
+    if (Detail.second.is<Expr *>())
+      new (getTrailingObjects<UnsatisfiedConstraintRecord>() + I)
+         UnsatisfiedConstraintRecord{Detail.first,
+                                     UnsatisfiedConstraintRecord::second_type(
+                                         Detail.second.get<Expr *>())};
+    else {
+      auto &SubstitutionDiagnostic =
+          *Detail.second.get<std::pair<SourceLocation, std::string> *>();
+      unsigned MessageSize = SubstitutionDiagnostic.second.size();
+      char *Mem = new (C) char[MessageSize + 1];
+      memcpy(Mem, SubstitutionDiagnostic.second.c_str(), MessageSize);
+      Mem[MessageSize + 1] = '\0';
+      auto *NewSubstDiag = new (C) std::pair<SourceLocation, StringRef>(
+          SubstitutionDiagnostic.first, StringRef(Mem));
+      new (getTrailingObjects<UnsatisfiedConstraintRecord>() + I)
+         UnsatisfiedConstraintRecord{Detail.first,
+                                     UnsatisfiedConstraintRecord::second_type(
+                                         NewSubstDiag)};
+    }
+  }
+}
+
+
+ASTConstraintSatisfaction *
+ASTConstraintSatisfaction::Create(const ASTContext &C,
+                                  const ConstraintSatisfaction &Satisfaction) {
+  std::size_t size =
+      totalSizeToAlloc<UnsatisfiedConstraintRecord>(
+          Satisfaction.Details.size());
+  void *Mem = C.Allocate(size, alignof(ASTConstraintSatisfaction));
+  return new (Mem) ASTConstraintSatisfaction(C, Satisfaction);
+}
Index: lib/AST/ASTContext.cpp
===================================================================
--- lib/AST/ASTContext.cpp
+++ lib/AST/ASTContext.cpp
@@ -13,6 +13,7 @@
 #include "clang/AST/ASTContext.h"
 #include "CXXABI.h"
 #include "clang/AST/APValue.h"
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/ASTTypeTraits.h"
 #include "clang/AST/Attr.h"
Index: lib/AST/CMakeLists.txt
===================================================================
--- lib/AST/CMakeLists.txt
+++ lib/AST/CMakeLists.txt
@@ -6,6 +6,7 @@
 
 add_clang_library(clangAST
   APValue.cpp
+  ASTConcept.cpp
   ASTConsumer.cpp
   ASTContext.cpp
   ASTDiagnostic.cpp
Index: lib/AST/Decl.cpp
===================================================================
--- lib/AST/Decl.cpp
+++ lib/AST/Decl.cpp
@@ -4536,7 +4536,7 @@
 }
 
 void LabelDecl::setMSAsmLabel(StringRef Name) {
-  char *Buffer = new (getASTContext(), 1) char[Name.size() + 1];
+char *Buffer = new (getASTContext(), 1) char[Name.size() + 1];
   memcpy(Buffer, Name.data(), Name.size());
   Buffer[Name.size()] = '\0';
   MSAsmName = Buffer;
Index: lib/AST/ExprCXX.cpp
===================================================================
--- lib/AST/ExprCXX.cpp
+++ lib/AST/ExprCXX.cpp
@@ -1673,7 +1673,8 @@
     NestedNameSpecifierLoc NNS, SourceLocation TemplateKWLoc,
     SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
     ConceptDecl *NamedConcept, const ASTTemplateArgumentListInfo *ArgsAsWritten,
-    ArrayRef<TemplateArgument> ConvertedArgs, bool IsSatisfied)
+    ArrayRef<TemplateArgument> ConvertedArgs,
+    const ConstraintSatisfaction &Satisfaction)
     : Expr(ConceptSpecializationExprClass, C.BoolTy, VK_RValue, OK_Ordinary,
            /*TypeDependent=*/false,
            // All the flags below are set in setTemplateArguments.
@@ -1681,8 +1682,8 @@
            /*ContainsUnexpandedParameterPacks=*/false),
       NestedNameSpec(NNS), TemplateKWLoc(TemplateKWLoc),
       ConceptNameLoc(ConceptNameLoc), FoundDecl(FoundDecl),
-      NamedConcept(NamedConcept, IsSatisfied),
-      NumTemplateArgs(ConvertedArgs.size()) {
+      NamedConcept(NamedConcept), NumTemplateArgs(ConvertedArgs.size()),
+      Satisfaction(ASTConstraintSatisfaction::Create(C, Satisfaction)) {
   setTemplateArguments(ArgsAsWritten, ConvertedArgs);
 }
 
@@ -1732,13 +1733,13 @@
                                   ConceptDecl *NamedConcept,
                                const ASTTemplateArgumentListInfo *ArgsAsWritten,
                                   ArrayRef<TemplateArgument> ConvertedArgs,
-                                  bool IsSatisfied) {
+                                  const ConstraintSatisfaction &Satisfaction) {
   void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
                                 ConvertedArgs.size()));
   return new (Buffer) ConceptSpecializationExpr(C, NNS, TemplateKWLoc,
                                                 ConceptNameLoc, FoundDecl,
                                                 NamedConcept, ArgsAsWritten,
-                                                ConvertedArgs, IsSatisfied);
+                                                ConvertedArgs, Satisfaction);
 }
 
 ConceptSpecializationExpr *
Index: lib/AST/ItaniumMangle.cpp
===================================================================
--- lib/AST/ItaniumMangle.cpp
+++ lib/AST/ItaniumMangle.cpp
@@ -966,7 +966,7 @@
     assert(!AdditionalAbiTags &&
            "template template param cannot have abi tags");
     mangleTemplateParameter(TTP->getIndex());
-  } else if (isa<BuiltinTemplateDecl>(ND)) {
+  } else if (isa<BuiltinTemplateDecl>(ND) || isa<ConceptDecl>(ND)) {
     mangleUnscopedName(ND, AdditionalAbiTags);
   } else {
     mangleUnscopedName(ND->getTemplatedDecl(), AdditionalAbiTags);
@@ -1855,7 +1855,7 @@
     mangleTemplateParameter(TTP->getIndex());
   } else {
     manglePrefix(getEffectiveDeclContext(ND), NoFunction);
-    if (isa<BuiltinTemplateDecl>(ND))
+    if (isa<BuiltinTemplateDecl>(ND) || isa<ConceptDecl>(ND))
       mangleUnqualifiedName(ND, nullptr);
     else
       mangleUnqualifiedName(ND->getTemplatedDecl(), nullptr);
@@ -3614,7 +3614,6 @@
   case Expr::ConvertVectorExprClass:
   case Expr::StmtExprClass:
   case Expr::TypeTraitExprClass:
-  case Expr::ConceptSpecializationExprClass:
   case Expr::ArrayTypeTraitExprClass:
   case Expr::ExpressionTraitExprClass:
   case Expr::VAArgExprClass:
@@ -4124,6 +4123,18 @@
     mangleExpression(cast<ParenExpr>(E)->getSubExpr(), Arity);
     break;
 
+
+  case Expr::ConceptSpecializationExprClass: {
+    //  <expr-primary> ::= L <mangled-name> E # external name
+    Out << "L_Z";
+    auto *CSE = cast<ConceptSpecializationExpr>(E);
+    mangleTemplateName(CSE->getNamedConcept(),
+                       CSE->getTemplateArguments().data(),
+                       CSE->getTemplateArguments().size());
+    Out << 'E';
+    break;
+  }
+
   case Expr::DeclRefExprClass:
     mangleDeclRefExpr(cast<DeclRefExpr>(E)->getDecl());
     break;
Index: lib/Sema/SemaConcept.cpp
===================================================================
--- lib/Sema/SemaConcept.cpp
+++ lib/Sema/SemaConcept.cpp
@@ -12,10 +12,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Sema/Sema.h"
+#include "clang/Sema/SemaInternal.h"
 #include "clang/Sema/SemaDiagnostic.h"
 #include "clang/Sema/TemplateDeduction.h"
 #include "clang/Sema/Template.h"
 #include "clang/AST/ExprCXX.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/PointerUnion.h"
 using namespace clang;
 using namespace sema;
 
@@ -43,80 +46,363 @@
   return true;
 }
 
-bool
-Sema::CalculateConstraintSatisfaction(ConceptDecl *NamedConcept,
-                                      MultiLevelTemplateArgumentList &MLTAL,
-                                      Expr *ConstraintExpr,
-                                      bool &IsSatisfied) {
+template <typename AtomicEvaluator>
+static bool
+calculateConstraintSatisfaction(Sema &S, const Expr *ConstraintExpr,
+                                ConstraintSatisfaction &Satisfaction,
+                                AtomicEvaluator &&Evaluator) {
   if (auto *BO = dyn_cast<BinaryOperator>(ConstraintExpr)) {
-    if (BO->getOpcode() == BO_LAnd) {
-      if (CalculateConstraintSatisfaction(NamedConcept, MLTAL, BO->getLHS(),
-                                          IsSatisfied))
+    if (BO->getOpcode() == BO_LAnd || BO->getOpcode() == BO_LOr) {
+      if (calculateConstraintSatisfaction(S, BO->getLHS(), Satisfaction,
+                                          Evaluator))
         return true;
-      if (!IsSatisfied)
+
+      bool IsLHSSatisfied = Satisfaction.IsSatisfied;
+
+      if (BO->getOpcode() == BO_LOr && IsLHSSatisfied)
+        // [temp.constr.op] p3
+        //    A disjunction is a constraint taking two operands. To determine if
+        //    a disjunction is satisfied, the satisfaction of the first operand
+        //    is checked. If that is satisfied, the disjunction is satisfied.
+        //    Otherwise, the disjunction is satisfied if and only if the second
+        //    operand is satisfied.
         return false;
-      return CalculateConstraintSatisfaction(NamedConcept, MLTAL, BO->getRHS(),
-                                             IsSatisfied);
-    } else if (BO->getOpcode() == BO_LOr) {
-      if (CalculateConstraintSatisfaction(NamedConcept, MLTAL, BO->getLHS(),
-                                          IsSatisfied))
-        return true;
-      if (IsSatisfied)
+
+      if (BO->getOpcode() == BO_LAnd && !IsLHSSatisfied)
+        // [temp.constr.op] p2
+        //    A conjunction is a constraint taking two operands. To determine if
+        //    a conjunction is satisfied, the satisfaction of the first operand
+        //    is checked. If that is not satisfied, the conjunction is not
+        //    satisfied. Otherwise, the conjunction is satisfied if and only if
+        //    the second operand is satisfied.
         return false;
-      return CalculateConstraintSatisfaction(NamedConcept, MLTAL, BO->getRHS(),
-                                             IsSatisfied);
+
+      return calculateConstraintSatisfaction(S, BO->getRHS(), Satisfaction,
+          std::forward<AtomicEvaluator>(Evaluator));
     }
   } else if (auto *PO = dyn_cast<ParenExpr>(ConstraintExpr))
-    return CalculateConstraintSatisfaction(NamedConcept, MLTAL,
-                                           PO->getSubExpr(), IsSatisfied);
+    return calculateConstraintSatisfaction(S, PO->getSubExpr(), Satisfaction,
+        std::forward<AtomicEvaluator>(Evaluator));
   else if (auto *C = dyn_cast<ExprWithCleanups>(ConstraintExpr))
-    return CalculateConstraintSatisfaction(NamedConcept, MLTAL, C->getSubExpr(),
-                                           IsSatisfied);
+    return calculateConstraintSatisfaction(S, C->getSubExpr(), Satisfaction,
+        std::forward<AtomicEvaluator>(Evaluator));
 
-  EnterExpressionEvaluationContext ConstantEvaluated(
-      *this, Sema::ExpressionEvaluationContext::ConstantEvaluated);
-
-  // Atomic constraint - substitute arguments and check satisfaction.
-  ExprResult E;
-  {
-    TemplateDeductionInfo Info(ConstraintExpr->getBeginLoc());
-    InstantiatingTemplate Inst(*this, ConstraintExpr->getBeginLoc(),
-                               InstantiatingTemplate::ConstraintSubstitution{},
-                               NamedConcept, Info,
-                               ConstraintExpr->getSourceRange());
-    if (Inst.isInvalid())
-      return true;
-    // We do not want error diagnostics escaping here.
-    Sema::SFINAETrap Trap(*this);
+  // An atomic constraint expression
+  ExprResult SubstitutedAtomicExpr = Evaluator(ConstraintExpr);
 
-    E = SubstExpr(ConstraintExpr, MLTAL);
-    if (E.isInvalid() || Trap.hasErrorOccurred()) {
-      // C++2a [temp.constr.atomic]p1
-      //   ...If substitution results in an invalid type or expression, the
-      //   constraint is not satisfied.
-      IsSatisfied = false;
-      return false;
-    }
-  }
-
-  if (!CheckConstraintExpression(E.get()))
+  if (SubstitutedAtomicExpr.isInvalid())
     return true;
 
+  if (!SubstitutedAtomicExpr.isUsable())
+    // Evaluator has decided satisfaction without yielding an expression.
+    return false;
+
+  EnterExpressionEvaluationContext ConstantEvaluated(
+      S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
   SmallVector<PartialDiagnosticAt, 2> EvaluationDiags;
   Expr::EvalResult EvalResult;
   EvalResult.Diag = &EvaluationDiags;
-  if (!E.get()->EvaluateAsRValue(EvalResult, Context)) {
-    // C++2a [temp.constr.atomic]p1
-    //   ...E shall be a constant expression of type bool.
-    Diag(E.get()->getBeginLoc(),
-         diag::err_non_constant_constraint_expression)
-        << E.get()->getSourceRange();
+  if (!SubstitutedAtomicExpr.get()->EvaluateAsRValue(EvalResult, S.Context)) {
+      // C++2a [temp.constr.atomic]p1
+      //   ...E shall be a constant expression of type bool.
+    S.Diag(SubstitutedAtomicExpr.get()->getBeginLoc(),
+           diag::err_non_constant_constraint_expression)
+        << SubstitutedAtomicExpr.get()->getSourceRange();
     for (const PartialDiagnosticAt &PDiag : EvaluationDiags)
-      Diag(PDiag.first, PDiag.second);
+      S.Diag(PDiag.first, PDiag.second);
     return true;
   }
 
-  IsSatisfied = EvalResult.Val.getInt().getBoolValue();
+  Satisfaction.IsSatisfied = EvalResult.Val.getInt().getBoolValue();
+  if (!Satisfaction.IsSatisfied)
+    Satisfaction.Details.emplace_back(ConstraintExpr,
+                                      SubstitutedAtomicExpr.get());
+
+  return false;
+}
+
+template <typename TemplateDeclT>
+static bool calculateConstraintSatisfaction(
+    Sema &S, TemplateDeclT *Template, ArrayRef<TemplateArgument> TemplateArgs,
+    SourceLocation TemplateNameLoc, MultiLevelTemplateArgumentList &MLTAL,
+    const Expr *ConstraintExpr, ConstraintSatisfaction &Satisfaction) {
+  return calculateConstraintSatisfaction(
+      S, ConstraintExpr, Satisfaction, [&](const Expr *AtomicExpr) {
+        EnterExpressionEvaluationContext ConstantEvaluated(
+            S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+
+        // Atomic constraint - substitute arguments and check satisfaction.
+        ExprResult SubstitutedExpression;
+        {
+          TemplateDeductionInfo Info(TemplateNameLoc);
+          Sema::InstantiatingTemplate Inst(S, AtomicExpr->getBeginLoc(),
+              Sema::InstantiatingTemplate::ConstraintSubstitution{}, Template,
+              Info, AtomicExpr->getSourceRange());
+          if (Inst.isInvalid())
+            return ExprError();
+          // We do not want error diagnostics escaping here.
+          Sema::SFINAETrap Trap(S);
+          SubstitutedExpression = S.SubstExpr(const_cast<Expr *>(AtomicExpr),
+                                              MLTAL);
+          if (SubstitutedExpression.isInvalid() || Trap.hasErrorOccurred()) {
+            // C++2a [temp.constr.atomic]p1
+            //   ...If substitution results in an invalid type or expression, the
+            //   constraint is not satisfied.
+            if (!Trap.hasErrorOccurred())
+              // A non-SFINAE error has occured as a result of this
+              // substitution.
+              return ExprError();
+
+            PartialDiagnosticAt SubstDiag{SourceLocation(),
+                                          PartialDiagnostic::NullDiagnostic()};
+            Info.takeSFINAEDiagnostic(SubstDiag);
+            SmallString<128> DiagString;
+            DiagString = ": ";
+            SubstDiag.second.EmitToString(S.getDiagnostics(), DiagString);
+            Satisfaction.Details.emplace_back(
+                AtomicExpr,
+                new (S.Context) ConstraintSatisfaction::SubstitutionDiagnostic{
+                        SubstDiag.first,
+                        std::string(DiagString.begin(), DiagString.end())});
+            Satisfaction.IsSatisfied = false;
+            return ExprEmpty();
+          }
+        }
+
+        if (!S.CheckConstraintExpression(SubstitutedExpression.get()))
+          return ExprError();
+
+        return SubstitutedExpression;
+      });
+}
+
+template<typename TemplateDeclT>
+static bool CheckConstraintSatisfaction(Sema &S, TemplateDeclT *Template,
+                                        ArrayRef<const Expr *> ConstraintExprs,
+                                        ArrayRef<TemplateArgument> TemplateArgs,
+                                        SourceRange TemplateIDRange,
+                                        ConstraintSatisfaction &Satisfaction) {
+  if (ConstraintExprs.empty()) {
+    Satisfaction.IsSatisfied = true;
+    return false;
+  }
+
+  for (auto& Arg : TemplateArgs)
+    if (Arg.isInstantiationDependent()) {
+      // No need to check satisfaction for dependent constraint expressions.
+      Satisfaction.IsSatisfied = true;
+      return false;
+    }
+
+  Sema::InstantiatingTemplate Inst(S, TemplateIDRange.getBegin(),
+      Sema::InstantiatingTemplate::ConstraintsCheck{}, Template, TemplateArgs,
+      TemplateIDRange);
+  if (Inst.isInvalid())
+    return true;
+
+  MultiLevelTemplateArgumentList MLTAL;
+  MLTAL.addOuterTemplateArguments(TemplateArgs);
+
+  for (const Expr *ConstraintExpr : ConstraintExprs) {
+    if (calculateConstraintSatisfaction(S, Template, TemplateArgs,
+                                        TemplateIDRange.getBegin(), MLTAL,
+                                        ConstraintExpr, Satisfaction))
+      return true;
+    if (!Satisfaction.IsSatisfied)
+      // [temp.constr.op] p2
+      //   [...] To determine if a conjunction is satisfied, the satisfaction
+      //   of the first operand is checked. If that is not satisfied, the
+      //   conjunction is not satisfied. [...]
+      return false;
+  }
+  return false;
+}
+
+bool Sema::CheckConstraintSatisfaction(TemplateDecl *Template,
+                                       ArrayRef<const Expr *> ConstraintExprs,
+                                       ArrayRef<TemplateArgument> TemplateArgs,
+                                       SourceRange TemplateIDRange,
+                                       ConstraintSatisfaction &Satisfaction) {
+  return ::CheckConstraintSatisfaction(*this, Template, ConstraintExprs,
+                                       TemplateArgs, TemplateIDRange,
+                                       Satisfaction);
+}
 
+bool
+Sema::CheckConstraintSatisfaction(ClassTemplatePartialSpecializationDecl* Part,
+                                  ArrayRef<const Expr *> ConstraintExprs,
+                                  ArrayRef<TemplateArgument> TemplateArgs,
+                                  SourceRange TemplateIDRange,
+                                  ConstraintSatisfaction &Satisfaction) {
+  return ::CheckConstraintSatisfaction(*this, Part, ConstraintExprs,
+                                       TemplateArgs, TemplateIDRange,
+                                       Satisfaction);
+}
+
+bool
+Sema::CheckConstraintSatisfaction(VarTemplatePartialSpecializationDecl* Partial,
+                                  ArrayRef<const Expr *> ConstraintExprs,
+                                  ArrayRef<TemplateArgument> TemplateArgs,
+                                  SourceRange TemplateIDRange,
+                                  ConstraintSatisfaction &Satisfaction) {
+  return ::CheckConstraintSatisfaction(*this, Partial, ConstraintExprs,
+                                       TemplateArgs, TemplateIDRange,
+                                       Satisfaction);
+}
+
+bool Sema::CheckConstraintSatisfaction(const Expr *ConstraintExpr,
+                                       ConstraintSatisfaction &Satisfaction) {
+  return calculateConstraintSatisfaction(
+      *this, ConstraintExpr, Satisfaction,
+      [](const Expr *AtomicExpr) -> ExprResult {
+        return ExprResult(const_cast<Expr *>(AtomicExpr));
+      });
+}
+
+bool Sema::EnsureTemplateArgumentListConstraints(
+    TemplateDecl *TD, ArrayRef<TemplateArgument> TemplateArgs,
+    SourceRange TemplateIDRange) {
+  ConstraintSatisfaction Satisfaction;
+  llvm::SmallVector<const Expr *, 3> AssociatedConstraints;
+  TD->getAssociatedConstraints(AssociatedConstraints);
+  if (CheckConstraintSatisfaction(TD, AssociatedConstraints, TemplateArgs,
+                                  TemplateIDRange, Satisfaction))
+    return true;
+
+  if (!Satisfaction.IsSatisfied) {
+    SmallString<128> TemplateArgString;
+    TemplateArgString = " ";
+    TemplateArgString += getTemplateArgumentBindingsText(
+        TD->getTemplateParameters(), TemplateArgs.data(), TemplateArgs.size());
+
+    Diag(TemplateIDRange.getBegin(),
+         diag::err_template_arg_list_constraints_not_satisfied)
+        << (int)getTemplateNameKindForDiagnostics(TemplateName(TD)) << TD
+        << TemplateArgString << TemplateIDRange;
+    DiagnoseUnsatisfiedConstraint(Satisfaction);
+    return true;
+  }
   return false;
+}
+
+static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
+                                                        Expr *SubstExpr,
+                                                        bool First = true) {
+  if (BinaryOperator *BO = dyn_cast<BinaryOperator>(SubstExpr)) {
+    switch (BO->getOpcode()) {
+    // These two cases will in practice only be reached when using fold
+    // expressions with || and &&, since otherwise the || and && will have been
+    // broken down into atomic constraints during satisfaction checking.
+    case BO_LOr:
+      // Or evaluated to false - meaning both RHS and LHS evaluated to false.
+      diagnoseWellFormedUnsatisfiedConstraintExpr(S, BO->getLHS(), First);
+      diagnoseWellFormedUnsatisfiedConstraintExpr(S, BO->getRHS(),
+                                                  /*First=*/false);
+      return;
+    case BO_LAnd:
+      bool LHSSatisfied;
+      BO->getLHS()->EvaluateAsBooleanCondition(LHSSatisfied, S.Context);
+      if (LHSSatisfied) {
+        // LHS is true, so RHS must be false.
+        diagnoseWellFormedUnsatisfiedConstraintExpr(S, BO->getRHS(), First);
+        return;
+      }
+      // LHS is false
+      diagnoseWellFormedUnsatisfiedConstraintExpr(S, BO->getLHS(), First);
+
+      // RHS might also be false
+      bool RHSSatisfied;
+      BO->getRHS()->EvaluateAsBooleanCondition(RHSSatisfied, S.Context);
+      if (!RHSSatisfied)
+        diagnoseWellFormedUnsatisfiedConstraintExpr(S, BO->getRHS(),
+                                                    /*First=*/false);
+      return;
+    case BO_GE:
+    case BO_LE:
+    case BO_GT:
+    case BO_LT:
+    case BO_EQ:
+    case BO_NE:
+      if (BO->getLHS()->getType()->isIntegerType() &&
+          BO->getRHS()->getType()->isIntegerType()) {
+        Expr::EvalResult SimplifiedLHS;
+        Expr::EvalResult SimplifiedRHS;
+        BO->getLHS()->EvaluateAsInt(SimplifiedLHS, S.Context);
+        BO->getRHS()->EvaluateAsInt(SimplifiedRHS, S.Context);
+        if (!SimplifiedLHS.Diag && ! SimplifiedRHS.Diag) {
+          S.Diag(SubstExpr->getBeginLoc(),
+                 diag::note_atomic_constraint_evaluated_to_false_elaborated)
+              << (int)First << SubstExpr
+              << SimplifiedLHS.Val.getInt().toString(10)
+              << BinaryOperator::getOpcodeStr(BO->getOpcode())
+              << SimplifiedRHS.Val.getInt().toString(10);
+          return;
+        }
+      }
+      break;
+
+    default:
+      break;
+    }
+  } else if (ParenExpr *PE = dyn_cast<ParenExpr>(SubstExpr)) {
+    diagnoseWellFormedUnsatisfiedConstraintExpr(S, PE->getSubExpr(), First);
+    return;
+  } else if (auto *CSE = dyn_cast<ConceptSpecializationExpr>(SubstExpr)) {
+    if (CSE->getTemplateArgsAsWritten()->NumTemplateArgs == 1) {
+      S.Diag(
+          CSE->getSourceRange().getBegin(),
+          diag::
+          note_single_arg_concept_specialization_constraint_evaluated_to_false)
+          << (int)First
+          << CSE->getTemplateArgsAsWritten()->arguments()[0].getArgument()
+          << CSE->getNamedConcept();
+    } else {
+      S.Diag(SubstExpr->getSourceRange().getBegin(),
+             diag::note_concept_specialization_constraint_evaluated_to_false)
+          << (int)First << CSE;
+    }
+    S.DiagnoseUnsatisfiedConstraint(CSE->getSatisfaction());
+    return;
+  }
+
+  S.Diag(SubstExpr->getSourceRange().getBegin(),
+         diag::note_atomic_constraint_evaluated_to_false)
+      << (int)First << SubstExpr;
+}
+
+template<typename SubstitutionDiagnostic>
+static void diagnoseUnsatisfiedConstraintExpr(
+    Sema &S, const Expr *E,
+    const llvm::PointerUnion<Expr *, SubstitutionDiagnostic *> &Record,
+    bool First = true) {
+  if (auto *Diag = Record.template dyn_cast<SubstitutionDiagnostic *>()){
+    S.Diag(Diag->first, diag::note_substituted_constraint_expr_is_ill_formed)
+        << Diag->second;
+    return;
+  }
+
+  diagnoseWellFormedUnsatisfiedConstraintExpr(S,
+      Record.template get<Expr *>(), First);
+}
+
+void Sema::DiagnoseUnsatisfiedConstraint(
+    const ConstraintSatisfaction& Satisfaction) {
+  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;
+  }
+}
+
+void Sema::DiagnoseUnsatisfiedConstraint(
+    const ASTConstraintSatisfaction &Satisfaction) {
+  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;
+  }
 }
\ No newline at end of file
Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -14030,8 +14030,16 @@
       std::string InnerCondDescription;
       std::tie(InnerCond, InnerCondDescription) =
         findFailedBooleanCondition(Converted.get());
-      if (InnerCond && !isa<CXXBoolLiteralExpr>(InnerCond)
-                    && !isa<IntegerLiteral>(InnerCond)) {
+      if (InnerCond && isa<ConceptSpecializationExpr>(InnerCond)) {
+        // Drill down into concept specialization expressions to see why they
+        // weren't satisfied.
+        Diag(StaticAssertLoc, diag::err_static_assert_failed)
+          << !AssertMessage << Msg.str() << AssertExpr->getSourceRange();
+        ConstraintSatisfaction Satisfaction;
+        if (!CheckConstraintSatisfaction(InnerCond, Satisfaction))
+          DiagnoseUnsatisfiedConstraint(Satisfaction);
+      } else if (InnerCond && !isa<CXXBoolLiteralExpr>(InnerCond)
+                           && !isa<IntegerLiteral>(InnerCond)) {
         Diag(StaticAssertLoc, diag::err_static_assert_requirement_failed)
           << InnerCondDescription << !AssertMessage
           << Msg.str() << InnerCond->getSourceRange();
Index: lib/Sema/SemaOverload.cpp
===================================================================
--- lib/Sema/SemaOverload.cpp
+++ lib/Sema/SemaOverload.cpp
@@ -581,6 +581,10 @@
     TemplateArgumentList *TemplateArgs;
     unsigned CallArgIndex;
   };
+  struct CNSInfo {
+    TemplateArgumentList *TemplateArgs;
+    ConstraintSatisfaction Satisfaction;
+  };
 }
 
 /// Convert from Sema's representation of template deduction information
@@ -651,6 +655,14 @@
     }
     break;
 
+  case Sema::TDK_ConstraintsNotSatisfied: {
+    CNSInfo *Saved = new (Context) CNSInfo;
+    Saved->TemplateArgs = Info.take();
+    Saved->Satisfaction = Info.AssociatedConstraintsSatisfaction;
+    Result.Data = Saved;
+    break;
+  }
+
   case Sema::TDK_Success:
   case Sema::TDK_NonDependentConversionFailure:
     llvm_unreachable("not a deduction failure");
@@ -691,6 +703,15 @@
     }
     break;
 
+  case Sema::TDK_ConstraintsNotSatisfied:
+    // FIXME: Destroy the template argument list?
+    Data = nullptr;
+    if (PartialDiagnosticAt *Diag = getSFINAEDiagnostic()) {
+      Diag->~PartialDiagnosticAt();
+      HasDiagnostic = false;
+    }
+    break;
+
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
     break;
@@ -716,6 +737,7 @@
   case Sema::TDK_NonDeducedMismatch:
   case Sema::TDK_CUDATargetMismatch:
   case Sema::TDK_NonDependentConversionFailure:
+  case Sema::TDK_ConstraintsNotSatisfied:
     return TemplateParameter();
 
   case Sema::TDK_Incomplete:
@@ -759,6 +781,9 @@
   case Sema::TDK_SubstitutionFailure:
     return static_cast<TemplateArgumentList*>(Data);
 
+  case Sema::TDK_ConstraintsNotSatisfied:
+    return static_cast<CNSInfo*>(Data)->TemplateArgs;
+
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
     break;
@@ -779,6 +804,7 @@
   case Sema::TDK_SubstitutionFailure:
   case Sema::TDK_CUDATargetMismatch:
   case Sema::TDK_NonDependentConversionFailure:
+  case Sema::TDK_ConstraintsNotSatisfied:
     return nullptr;
 
   case Sema::TDK_IncompletePack:
@@ -810,6 +836,7 @@
   case Sema::TDK_SubstitutionFailure:
   case Sema::TDK_CUDATargetMismatch:
   case Sema::TDK_NonDependentConversionFailure:
+  case Sema::TDK_ConstraintsNotSatisfied:
     return nullptr;
 
   case Sema::TDK_Inconsistent:
@@ -1226,6 +1253,8 @@
     return NewTarget != OldTarget;
   }
 
+  // TODO: Concepts: Check function trailing requires clauses here.
+
   // The signatures match; this is not an overload.
   return false;
 }
@@ -10143,6 +10172,21 @@
     MaybeEmitInheritedConstructorNote(S, Found);
     return;
 
+  case Sema::TDK_ConstraintsNotSatisfied: {
+    // Format the template argument list into the argument string.
+    SmallString<128> TemplateArgString;
+    TemplateArgumentList *Args = DeductionFailure.getTemplateArgumentList();
+    TemplateArgString = " ";
+    TemplateArgString += S.getTemplateArgumentBindingsText(
+        getDescribedTemplate(Templated)->getTemplateParameters(), *Args);
+    S.Diag(Templated->getLocation(),
+           diag::note_ovl_candidate_unsatisfied_constraints)
+        << TemplateArgString;
+
+    S.DiagnoseUnsatisfiedConstraint(
+        static_cast<CNSInfo*>(DeductionFailure.Data)->Satisfaction);
+    return;
+  }
   case Sema::TDK_TooManyArguments:
   case Sema::TDK_TooFewArguments:
     DiagnoseArityMismatch(S, Found, Templated, NumArgs);
@@ -10599,15 +10643,18 @@
   case Sema::TDK_CUDATargetMismatch:
     return 3;
 
-  case Sema::TDK_InstantiationDepth:
+  case Sema::TDK_ConstraintsNotSatisfied:
     return 4;
 
-  case Sema::TDK_InvalidExplicitArguments:
+  case Sema::TDK_InstantiationDepth:
     return 5;
 
+  case Sema::TDK_InvalidExplicitArguments:
+    return 6;
+
   case Sema::TDK_TooManyArguments:
   case Sema::TDK_TooFewArguments:
-    return 6;
+    return 7;
   }
   llvm_unreachable("Unhandled deduction result");
 }
Index: lib/Sema/SemaTemplate.cpp
===================================================================
--- lib/Sema/SemaTemplate.cpp
+++ lib/Sema/SemaTemplate.cpp
@@ -3175,8 +3175,7 @@
 
   TemplateDecl *Template = Name.getAsTemplateDecl();
   if (!Template || isa<FunctionTemplateDecl>(Template) ||
-      isa<VarTemplateDecl>(Template) ||
-      isa<ConceptDecl>(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())
@@ -3190,9 +3189,17 @@
 
   // Check that the template argument list is well-formed for this
   // template.
+  bool InstantiationDependentMatch;
   SmallVector<TemplateArgument, 4> Converted;
   if (CheckTemplateArgumentList(Template, TemplateLoc, TemplateArgs,
-                                false, Converted))
+                                false, Converted,
+                                /*UpdateArgsWithConversion=*/true,
+                                &InstantiationDependentMatch))
+    return QualType();
+
+  if (!InstantiationDependentMatch
+      && EnsureTemplateArgumentListConstraints(Template, Converted,
+             SourceRange(TemplateLoc, TemplateArgs.getRAngleLoc())))
     return QualType();
 
   QualType CanonType;
@@ -3200,6 +3207,7 @@
   bool InstantiationDependent = false;
   if (TypeAliasTemplateDecl *AliasTemplate =
           dyn_cast<TypeAliasTemplateDecl>(Template)) {
+
     // Find the canonical type for this type alias template specialization.
     TypeAliasDecl *Pattern = AliasTemplate->getTemplatedDecl();
     if (Pattern->isInvalidDecl())
@@ -3835,9 +3843,17 @@
 
   // Check that the template argument list is well-formed for this
   // template.
+  bool InstantiationDependent;
   SmallVector<TemplateArgument, 4> Converted;
   if (CheckTemplateArgumentList(VarTemplate, TemplateNameLoc, TemplateArgs,
-                                false, Converted))
+                                false, Converted,
+                                /*UpdateArgsWithConversion=*/true,
+                                &InstantiationDependent))
+    return true;
+
+  if (!InstantiationDependent
+      && EnsureTemplateArgumentListConstraints(VarTemplate, Converted,
+             SourceRange(TemplateNameLoc, RAngleLoc)))
     return true;
 
   // Find the variable template (partial) specialization declaration that
@@ -4004,11 +4020,18 @@
   assert(Template && "A variable template id without template?");
 
   // Check that the template argument list is well-formed for this template.
+  bool InstantiationDependentMatch;
   SmallVector<TemplateArgument, 4> Converted;
   if (CheckTemplateArgumentList(
           Template, TemplateNameLoc,
           const_cast<TemplateArgumentListInfo &>(TemplateArgs), false,
-          Converted))
+          Converted, /*UpdateArgsWithConversion=*/true,
+          &InstantiationDependentMatch))
+    return true;
+
+  if (!InstantiationDependentMatch
+      && EnsureTemplateArgumentListConstraints(Template, Converted,
+             SourceRange(TemplateNameLoc, TemplateArgs.getRAngleLoc())))
     return true;
 
   // Find the variable template specialization declaration that
@@ -4199,7 +4222,7 @@
                                 /*UpdateArgsWithConversion=*/false))
     return ExprError();
 
-  bool IsSatisfied = true;
+  ConstraintSatisfaction Satisfaction;
   bool IsInstantiationDependent = false;
   for (TemplateArgument &Arg : Converted) {
     if (Arg.isInstantiationDependent()) {
@@ -4207,23 +4230,21 @@
       break;
     }
   }
-  if (!IsInstantiationDependent) {
-    InstantiatingTemplate Inst(*this, ConceptNameLoc,
-        InstantiatingTemplate::ConstraintsCheck{}, NamedConcept, Converted,
-        SourceRange(SS.isSet() ? SS.getBeginLoc() : ConceptNameLoc,
-                    TemplateArgs->getRAngleLoc()));
-    MultiLevelTemplateArgumentList MLTAL;
-    MLTAL.addOuterTemplateArguments(Converted);
-    if (CalculateConstraintSatisfaction(NamedConcept, MLTAL,
-                                        NamedConcept->getConstraintExpr(),
-                                        IsSatisfied))
+  if (!IsInstantiationDependent)
+    if (CheckConstraintSatisfaction(NamedConcept,
+                                    {NamedConcept->getConstraintExpr()},
+                                    Converted,
+                                    SourceRange(SS.isSet() ? SS.getBeginLoc() :
+                                                             ConceptNameLoc,
+                                                TemplateArgs->getRAngleLoc()),
+                                    Satisfaction))
       return ExprError();
-  }
+
   return ConceptSpecializationExpr::Create(Context,
       SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc{},
       TemplateKWLoc, ConceptNameLoc, FoundDecl, NamedConcept,
       ASTTemplateArgumentListInfo::Create(Context, *TemplateArgs), Converted,
-      IsSatisfied);
+      Satisfaction);
 }
 
 ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
@@ -4266,7 +4287,7 @@
                               TemplateKWLoc, TemplateArgs);
   }
 
-  if (R.getAsSingle<ConceptDecl>() && !AnyDependentArguments()) {
+  if (R.getAsSingle<ConceptDecl>()) {
     return CheckConceptTemplateId(SS, TemplateKWLoc,
                                   R.getLookupNameInfo().getBeginLoc(),
                                   R.getFoundDecl(),
@@ -5170,7 +5191,8 @@
     TemplateDecl *Template, SourceLocation TemplateLoc,
     TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
     SmallVectorImpl<TemplateArgument> &Converted,
-    bool UpdateArgsWithConversions) {
+    bool UpdateArgsWithConversions, bool *InstantiationDependent) {
+
   // Make a copy of the template arguments for processing.  Only make the
   // changes at the end when successful in matching the arguments to the
   // template.
@@ -5274,6 +5296,10 @@
           ++ArgIdx;
         }
 
+        // We cannot determine yet whether the arguments are well-formed.
+        if (InstantiationDependent)
+          *InstantiationDependent = true;
+
         return false;
       }
 
@@ -5286,6 +5312,8 @@
         Converted.push_back(
             TemplateArgument::CreatePackCopy(Context, ArgumentPack));
 
+      if (InstantiationDependent)
+        *InstantiationDependent = false;
       return false;
     }
 
@@ -5424,6 +5452,9 @@
   if (UpdateArgsWithConversions)
     TemplateArgs = std::move(NewArgs);
 
+  if (InstantiationDependent)
+    *InstantiationDependent = false;
+
   return false;
 }
 
@@ -7753,9 +7784,17 @@
 
   // Check that the template argument list is well-formed for this
   // template.
+  bool InstantiationDependent;
   SmallVector<TemplateArgument, 4> Converted;
   if (CheckTemplateArgumentList(ClassTemplate, TemplateNameLoc,
-                                TemplateArgs, false, Converted))
+                                TemplateArgs, false, Converted,
+                                /*UpdateArgsWithConversion=*/true,
+                                &InstantiationDependent))
+    return true;
+
+  if (!InstantiationDependent
+      && EnsureTemplateArgumentListConstraints(ClassTemplate, Converted,
+             SourceRange(TemplateNameLoc, RAngleLoc)))
     return true;
 
   // Find the class template (partial) specialization declaration that
@@ -8999,10 +9038,19 @@
 
   // Check that the template argument list is well-formed for this
   // template.
+  bool InstantiationDependent;
   SmallVector<TemplateArgument, 4> Converted;
   if (CheckTemplateArgumentList(ClassTemplate, TemplateNameLoc,
-                                TemplateArgs, false, Converted))
+                                TemplateArgs, false, Converted,
+                                /*UpdateArgsWithConversion=*/true,
+                                &InstantiationDependent))
+    return true;
+
+  if (!InstantiationDependent
+      && EnsureTemplateArgumentListConstraints(ClassTemplate, Converted,
+             SourceRange(TemplateNameLoc, RAngleLoc))) {
     return true;
+  }
 
   // Find the class template specialization declaration that
   // corresponds to these arguments.
Index: lib/Sema/SemaTemplateDeduction.cpp
===================================================================
--- lib/Sema/SemaTemplateDeduction.cpp
+++ lib/Sema/SemaTemplateDeduction.cpp
@@ -2831,6 +2831,22 @@
   return Sema::TDK_Success;
 }
 
+template<typename TemplateDeclT>
+static Sema::TemplateDeductionResult
+CheckDeducedArgumentConstraints(Sema& S, TemplateDeclT *Template,
+                                ArrayRef<TemplateArgument> DeducedArgs,
+                                TemplateDeductionInfo& Info) {
+  llvm::SmallVector<const Expr *, 3> AssociatedConstraints;
+  Template->getAssociatedConstraints(AssociatedConstraints);
+  if (S.CheckConstraintSatisfaction(Template, AssociatedConstraints,
+                                    DeducedArgs, Info.getLocation(),
+                                    Info.AssociatedConstraintsSatisfaction)
+      || !Info.AssociatedConstraintsSatisfaction.IsSatisfied) {
+    Info.reset(TemplateArgumentList::CreateCopy(S.Context, DeducedArgs));
+    return Sema::TDK_ConstraintsNotSatisfied;
+  }
+  return Sema::TDK_Success;
+}
 
 /// Perform template argument deduction to determine whether
 /// the given template arguments match the given class template
@@ -2871,6 +2887,10 @@
   if (Trap.hasErrorOccurred())
     return Sema::TDK_SubstitutionFailure;
 
+  if (TemplateDeductionResult Result
+        = CheckDeducedArgumentConstraints(*this, Partial, DeducedArgs, Info))
+    return Result;
+
   return ::FinishTemplateArgumentDeduction(
       *this, Partial, /*PartialOrdering=*/false, TemplateArgs, Deduced, Info);
 }
@@ -2912,6 +2932,10 @@
   if (Trap.hasErrorOccurred())
     return Sema::TDK_SubstitutionFailure;
 
+  if (TemplateDeductionResult Result
+        = CheckDeducedArgumentConstraints(*this, Partial, DeducedArgs, Info))
+    return Result;
+
   return ::FinishTemplateArgumentDeduction(
       *this, Partial, /*PartialOrdering=*/false, TemplateArgs, Deduced, Info);
 }
@@ -3021,6 +3045,12 @@
     = TemplateArgumentList::CreateCopy(Context, Builder);
   Info.setExplicitArgs(ExplicitArgumentList);
 
+  if (TemplateDeductionResult Result
+        = CheckDeducedArgumentConstraints(*this, FunctionTemplate,
+                                          ExplicitArgumentList->asArray(),
+                                          Info))
+    return Result;
+
   // Template argument deduction and the final substitution should be
   // done in the context of the templated declaration.  Explicit
   // argument substitution, on the other hand, needs to happen in the
@@ -3333,6 +3363,11 @@
           PartialOverloading))
     return Result;
 
+  if (TemplateDeductionResult Result
+        = CheckDeducedArgumentConstraints(*this, FunctionTemplate, DeducedArgs,
+                                          Info))
+    return Result;
+
   // C++ [temp.deduct.call]p10: [DR1391]
   //   If deduction succeeds for all parameters that contain
   //   template-parameters that participate in template argument deduction,
@@ -5028,6 +5063,7 @@
 static bool isAtLeastAsSpecializedAs(Sema &S, QualType T1, QualType T2,
                                      TemplateLikeDecl *P2,
                                      TemplateDeductionInfo &Info) {
+  // TODO: Concepts: Regard constraints
   // C++ [temp.class.order]p1:
   //   For two class template partial specializations, the first is at least as
   //   specialized as the second if, given the following rewrite to two
Index: lib/Sema/SemaTemplateInstantiate.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiate.cpp
+++ lib/Sema/SemaTemplateInstantiate.cpp
@@ -361,7 +361,7 @@
 
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
-    ConstraintsCheck, TemplateDecl *Template,
+    ConstraintsCheck, NamedDecl *Template,
     ArrayRef<TemplateArgument> TemplateArgs, SourceRange InstantiationRange)
     : InstantiatingTemplate(
           SemaRef, CodeSynthesisContext::ConstraintsCheck,
@@ -370,7 +370,7 @@
 
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
-    ConstraintSubstitution, TemplateDecl *Template,
+    ConstraintSubstitution, NamedDecl *Template,
     sema::TemplateDeductionInfo &DeductionInfo, SourceRange InstantiationRange)
     : InstantiatingTemplate(
           SemaRef, CodeSynthesisContext::ConstraintSubstitution,
@@ -680,24 +680,27 @@
     case CodeSynthesisContext::Memoization:
       break;
     
-    case CodeSynthesisContext::ConstraintsCheck:
-      if (auto *CD = dyn_cast<ConceptDecl>(Active->Entity)) {
-        SmallVector<char, 128> TemplateArgsStr;
-        llvm::raw_svector_ostream OS(TemplateArgsStr);
-        CD->printName(OS);
-        printTemplateArgumentList(OS, Active->template_arguments(),
-                                  getPrintingPolicy());
-        Diags.Report(Active->PointOfInstantiation,
-                     diag::note_concept_specialization_here)
-          << OS.str()
-          << Active->InstantiationRange;
-        break;
+    case CodeSynthesisContext::ConstraintsCheck: {
+      unsigned DiagID = 0;
+      if (isa<ConceptDecl>(Active->Entity))
+        DiagID = diag::note_concept_specialization_here;
+      else if (isa<TemplateDecl>(Active->Entity))
+        DiagID = diag::note_checking_constraints_for_template_id_here;
+      else if (isa<VarTemplatePartialSpecializationDecl>(Active->Entity))
+        DiagID = diag::note_checking_constraints_for_var_spec_id_here;
+      else {
+        assert(isa<ClassTemplatePartialSpecializationDecl>(Active->Entity));
+        DiagID = diag::note_checking_constraints_for_class_spec_id_here;
       }
-      // TODO: Concepts - implement this for constrained templates and partial
-      // specializations.
-      llvm_unreachable("only concept constraints are supported right now");
+      SmallVector<char, 128> TemplateArgsStr;
+      llvm::raw_svector_ostream OS(TemplateArgsStr);
+      cast<NamedDecl>(Active->Entity)->printName(OS);
+      printTemplateArgumentList(OS, Active->template_arguments(),
+                                getPrintingPolicy());
+      Diags.Report(Active->PointOfInstantiation, DiagID) << OS.str()
+        << Active->InstantiationRange;
       break;
-      
+    }
     case CodeSynthesisContext::ConstraintSubstitution:
       Diags.Report(Active->PointOfInstantiation,
                    diag::note_constraint_substitution_here)
Index: lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3202,12 +3202,21 @@
 
   // Check that the template argument list is well-formed for this
   // class template.
+  bool InstantiationDependent;
   SmallVector<TemplateArgument, 4> Converted;
   if (SemaRef.CheckTemplateArgumentList(InstClassTemplate,
                                         D->getLocation(),
                                         InstTemplateArgs,
                                         false,
-                                        Converted))
+                                        Converted,
+                                        /*UpdateArgsWithConversion=*/true,
+                                        &InstantiationDependent))
+    return nullptr;
+
+  if (!InstantiationDependent
+      && SemaRef.EnsureTemplateArgumentListConstraints(InstClassTemplate,
+                                                       Converted,
+                                                       D->getLocation()))
     return nullptr;
 
   // Figure out where to insert this class template explicit specialization
@@ -3326,9 +3335,18 @@
     return nullptr;
 
   // Check that the template argument list is well-formed for this template.
+  bool InstantiationDependent;
   SmallVector<TemplateArgument, 4> Converted;
   if (SemaRef.CheckTemplateArgumentList(InstVarTemplate, D->getLocation(),
-                                        VarTemplateArgsInfo, false, Converted))
+                                        VarTemplateArgsInfo, false, Converted,
+                                        /*UpdateArgsWithConversion=*/true,
+                                        &InstantiationDependent))
+    return nullptr;
+
+  if (!InstantiationDependent
+      && SemaRef.EnsureTemplateArgumentListConstraints(InstVarTemplate,
+          Converted, SourceRange(D->getLocation(),
+                                 VarTemplateArgsInfo.getRAngleLoc())))
     return nullptr;
 
   // Check whether we've already seen a declaration of this specialization.
Index: lib/Serialization/ASTReaderStmt.cpp
===================================================================
--- lib/Serialization/ASTReaderStmt.cpp
+++ lib/Serialization/ASTReaderStmt.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Serialization/ASTReader.h"
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/AttrIterator.h"
 #include "clang/AST/Decl.h"
@@ -742,14 +743,33 @@
   E->TemplateKWLoc = Record.readSourceLocation();
   E->ConceptNameLoc = Record.readSourceLocation();
   E->FoundDecl = ReadDeclAs<NamedDecl>();
-  E->NamedConcept.setPointer(ReadDeclAs<ConceptDecl>());
+  E->NamedConcept = ReadDeclAs<ConceptDecl>();
   const ASTTemplateArgumentListInfo *ArgsAsWritten =
       Record.readASTTemplateArgumentListInfo();
   llvm::SmallVector<TemplateArgument, 4> Args;
   for (unsigned I = 0; I < NumTemplateArgs; ++I)
     Args.push_back(Record.readTemplateArgument());
   E->setTemplateArguments(ArgsAsWritten, Args);
-  E->NamedConcept.setInt(Record.readInt() == 1);
+  ConstraintSatisfaction Satisfaction;
+  Satisfaction.IsSatisfied = Record.readInt();
+  if (!Satisfaction.IsSatisfied) {
+    unsigned NumDetailRecords = Record.readInt();
+    for (unsigned i = 0; i != NumDetailRecords; ++i) {
+      Expr *ConstraintExpr = Record.readExpr();
+      bool IsDiagnostic = Record.readInt();
+      if (IsDiagnostic) {
+        SourceLocation DiagLocation = Record.readSourceLocation();
+        std::string DiagMessage = Record.readString();
+        Satisfaction.Details.emplace_back(
+            ConstraintExpr, new (Record.getContext())
+                                ConstraintSatisfaction::SubstitutionDiagnostic{
+                                    DiagLocation, DiagMessage});
+      } else
+        Satisfaction.Details.emplace_back(ConstraintExpr, Record.readExpr());
+    }
+  }
+  E->Satisfaction = ASTConstraintSatisfaction::Create(Record.getContext(),
+                                                      Satisfaction);
 }
 
 void ASTStmtReader::VisitArraySubscriptExpr(ArraySubscriptExpr *E) {
Index: lib/Serialization/ASTWriterStmt.cpp
===================================================================
--- lib/Serialization/ASTWriterStmt.cpp
+++ lib/Serialization/ASTWriterStmt.cpp
@@ -401,7 +401,25 @@
   Record.AddASTTemplateArgumentListInfo(E->getTemplateArgsAsWritten());
   for (const TemplateArgument &Arg : TemplateArgs)
     Record.AddTemplateArgument(Arg);
-  Record.push_back(E->isSatisfied());
+  const ASTConstraintSatisfaction &Satisfaction = E->getSatisfaction();
+  Record.push_back(Satisfaction.IsSatisfied);
+  if (!Satisfaction.IsSatisfied) {
+    Record.push_back(Satisfaction.NumRecords);
+    for (const auto &DetailRecord : Satisfaction) {
+      Record.AddStmt(const_cast<Expr *>(DetailRecord.first));
+      auto *E = DetailRecord.second.dyn_cast<Expr *>();
+      Record.push_back(E == nullptr);
+      if (E)
+        Record.AddStmt(E);
+      else {
+        auto *Diag = DetailRecord.second.get<std::pair<SourceLocation,
+                                                       StringRef> *>();
+        Record.AddSourceLocation(Diag->first);
+        Record.AddString(Diag->second);
+      }
+    }
+  }
+
   Code = serialization::EXPR_CONCEPT_SPECIALIZATION;
 }
 
Index: test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp
===================================================================
--- test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp
+++ test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp
@@ -72,6 +72,15 @@
 static_assert(IsTypePredicate<T2>);
 static_assert(!IsTypePredicate<T1>);
 
+template<typename T, typename U, typename... Ts>
+concept OneOf = (Same<T, Ts> || ...);
+
+template<typename... X>
+constexpr bool S = OneOf<X..., int, int>;
+
+static_assert(S<int, long, int>);
+static_assert(!S<long, int, char, char>);
+
 namespace piecewise_substitution {
   template <typename T>
   concept True = true;
@@ -151,3 +160,25 @@
 struct X { static constexpr bool a = SameSize<T>; };
 
 static_assert(X<unsigned>::a);
+
+// static_assert concept diagnostics
+template<typename T>
+concept Large = sizeof(T) > 100;
+// expected-note@-1 2{{because 'sizeof(small) > 100' (1 > 100) evaluated to false}}
+
+struct small { };
+static_assert(Large<small>);
+// expected-error@-1 {{static_assert failed}}
+// expected-note@-2 {{because 'small' does not satisfy 'Large'}}
+static_assert(Large<small>, "small isn't large");
+// expected-error@-1 {{static_assert failed "small isn't large"}}
+// expected-note@-2 {{because 'small' does not satisfy 'Large'}}
+
+// Make sure access-checking can fail a concept specialization
+
+class T4 { static constexpr bool f = true; };
+template<typename T> concept AccessPrivate = T{}.f;
+// expected-note@-1{{because substituted constraint expression is ill-formed: 'f' is a private member of 'T4'}}
+static_assert(AccessPrivate<T4>);
+// expected-error@-1{{static_assert failed}}
+// expected-note@-2{{because 'T4' does not satisfy 'AccessPrivate'}}
Index: test/CXX/concepts-ts/temp/temp.constr/temp.constr.constr/function-templates.cpp
===================================================================
--- /dev/null
+++ test/CXX/concepts-ts/temp/temp.constr/temp.constr.constr/function-templates.cpp
@@ -0,0 +1,43 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
+
+template<typename T>
+constexpr bool is_ptr_v = false;
+
+template<typename T>
+constexpr bool is_ptr_v<T*> = true;
+
+template<typename T, typename U>
+constexpr bool is_same_v = false;
+
+template<typename T>
+constexpr bool is_same_v<T, T> = true;
+
+template<typename T> requires is_ptr_v<T> // expected-note   {{because 'is_ptr_v<int>' evaluated to false}}
+                         // expected-note@-1{{because 'is_ptr_v<char>' evaluated to false}}
+auto dereference(T t) { // expected-note   {{candidate template ignored: constraints not satisfied [with T = int]}}
+                        // expected-note@-1{{candidate template ignored: constraints not satisfied [with T = char]}}
+  return *t;
+}
+
+static_assert(is_same_v<decltype(dereference<int*>(nullptr)), int>);
+static_assert(is_same_v<decltype(dereference(2)), int>); // expected-error {{no matching function for call to 'dereference'}}
+static_assert(is_same_v<decltype(dereference<char>('a')), char>); // expected-error {{no matching function for call to 'dereference'}}
+
+
+template<typename T> requires T{} + T{} // expected-note {{because substituted constraint expression is ill-formed: invalid operands to binary expression ('A' and 'A')}}
+auto foo(T t) { // expected-note {{candidate template ignored: constraints not satisfied [with T = A]}}
+  return t + t;
+}
+
+
+template<typename T> requires !((T{} - T{}) && (T{} + T{})) || false
+// expected-note@-1{{because substituted constraint expression is ill-formed: invalid operands to binary expression ('A' and 'A')}}
+// expected-note@-2{{and 'false' evaluated to false}}
+auto bar(T t) { // expected-note {{candidate template ignored: constraints not satisfied [with T = A]}}
+  return t + t;
+}
+
+struct A { };
+
+static_assert(foo(A{})); // expected-error {{no matching function for call to 'foo'}}
+static_assert(bar(A{})); // expected-error {{no matching function for call to 'bar'}}
\ No newline at end of file
Index: test/CXX/concepts-ts/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
===================================================================
--- /dev/null
+++ test/CXX/concepts-ts/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
@@ -0,0 +1,92 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
+
+template<typename T> requires sizeof(T) >= 2 // expected-note{{because 'sizeof(char) >= 2' (1 >= 2) evaluated to false}}
+struct A {
+  static constexpr int value = sizeof(T);
+};
+
+static_assert(A<int>::value == 4);
+static_assert(A<char>::value == 1); // expected-error{{constraints not satisfied for class template 'A' [with T = char]}}
+
+template<typename T, typename U>
+  requires sizeof(T) != sizeof(U) // expected-note{{because 'sizeof(int) != sizeof(char [4])' (4 != 4) evaluated to false}}
+           && sizeof(T) >= 4 // expected-note{{because 'sizeof(char) >= 4' (1 >= 4) evaluated to false}}
+constexpr int SizeDiff = sizeof(T) > sizeof(U) ? sizeof(T) - sizeof(U) : sizeof(U) - sizeof(T);
+
+static_assert(SizeDiff<int, char> == 3);
+static_assert(SizeDiff<int, char[4]> == 0); // expected-error{{constraints not satisfied for variable template 'SizeDiff' [with T = int, U = char [4]]}}
+static_assert(SizeDiff<char, int> == 3); // expected-error{{constraints not satisfied for variable template 'SizeDiff' [with T = char, U = int]}}
+
+template<typename... Ts>
+  requires ((sizeof(Ts) == 4) || ...) // expected-note{{because 'sizeof(char) == 4' (1 == 4) evaluated to false}} expected-note{{'sizeof(long long) == 4' (8 == 4) evaluated to false}} expected-note{{'sizeof(int [20]) == 4' (80 == 4) evaluated to false}}
+constexpr auto SumSizes = (sizeof(Ts) + ...);
+
+static_assert(SumSizes<char, long long, int> == 13);
+static_assert(SumSizes<char, long long, int[20]> == 89); // expected-error{{constraints not satisfied for variable template 'SumSizes' [with Ts = <char, long long, int [20]>]}}
+
+template<typename T>
+concept IsBig = sizeof(T) > 100; // expected-note{{because 'sizeof(int) > 100' (4 > 100) evaluated to false}}
+
+template<typename T>
+  requires IsBig<T> // expected-note{{'int' does not satisfy 'IsBig'}}
+using BigPtr = T*;
+
+static_assert(sizeof(BigPtr<int>)); // expected-error{{constraints not satisfied for alias template 'BigPtr' [with T = int]}}}}
+
+template<typename T> requires T::value // expected-note{{because substituted constraint expression is ill-formed: type 'int' cannot be used prior to '::' because it has no members}}
+struct S { static constexpr bool value = true; };
+
+struct S2 { static constexpr bool value = true; };
+
+static_assert(S<int>::value); // expected-error{{constraints not satisfied for class template 'S' [with T = int]}}
+static_assert(S<S2>::value);
+
+template<typename T>
+struct AA
+{
+    template<typename U> requires sizeof(U) == sizeof(T) // expected-note{{because 'sizeof(int [2]) == sizeof(int)' (8 == 4) evaluated to false}}
+    struct B
+    {
+        static constexpr int a = 0;
+    };
+
+    template<typename U> requires sizeof(U) == sizeof(T) // expected-note{{because 'sizeof(int [2]) == sizeof(int)' (8 == 4) evaluated to false}}
+    static constexpr int b = 1;
+
+    template<typename U> requires sizeof(U) == sizeof(T) // expected-note{{because 'sizeof(int [2]) == sizeof(int)' (8 == 4) evaluated to false}}
+    static constexpr int getB() { // expected-note{{candidate template ignored: constraints not satisfied [with U = int [2]]}}
+        return 2;
+    }
+
+    static auto foo()
+    {
+        return B<T[2]>::a; // expected-error{{constraints not satisfied for class template 'B' [with U = int [2]]}}
+    }
+
+    static auto foo1()
+    {
+        return b<T[2]>; // expected-error{{constraints not satisfied for variable template 'b' [with U = int [2]]}}
+    }
+
+    static auto foo2()
+    {
+        return AA<T>::getB<T[2]>(); // expected-error{{no matching function for call to 'getB'}}
+    }
+};
+
+constexpr auto x = AA<int>::foo(); // expected-note{{in instantiation of member function 'AA<int>::foo' requested here}}
+constexpr auto x1 = AA<int>::foo1(); // expected-note{{in instantiation of member function 'AA<int>::foo1' requested here}}
+constexpr auto x2 = AA<int>::foo2(); // expected-note{{in instantiation of member function 'AA<int>::foo2' requested here}}
+
+template<typename T>
+struct B { using type = typename T::type; }; // expected-error{{type 'int' cannot be used prior to '::' because it has no members}}
+
+template<typename T> requires B<T>::type // expected-note{{in instantiation of template class 'B<int>' requested here}}
+                                         // expected-note@-1{{while substituting template arguments into constraint expression here}}
+struct C { };
+
+template<typename T> requires T{} // expected-error{{atomic constraint must be of type 'bool' (found 'int')}}
+struct D { };
+
+static_assert(C<int>{}); // expected-note{{while checking constraint satisfaction for template 'C<int>' required here}}
+static_assert(D<int>{}); // expected-note{{while checking constraint satisfaction for template 'D<int>' required here}}
\ No newline at end of file
Index: test/CXX/concepts-ts/temp/temp.constr/temp.constr.constr/partial-specializations.cpp
===================================================================
--- /dev/null
+++ test/CXX/concepts-ts/temp/temp.constr/temp.constr.constr/partial-specializations.cpp
@@ -0,0 +1,67 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
+
+namespace class_templates
+{
+  template<typename T, typename U> requires sizeof(T) >= 4 // expected-note {{because 'sizeof(char) >= 4' (1 >= 4) evaluated to false}}
+  struct is_same { static constexpr bool value = false; };
+
+  template<typename T> requires sizeof(T*) >= 4 && sizeof(T) >= 4
+  struct is_same<T*, T*> { static constexpr bool value = true; };
+
+  static_assert(!is_same<char*, char*>::value);
+  static_assert(!is_same<short*, short*>::value);
+  static_assert(is_same<int*, int*>::value);
+  static_assert(is_same<char, char>::value); // expected-error {{constraints not satisfied for class template 'is_same' [with T = char, U = char]}}
+
+  template<typename T>
+  struct A { using type = typename T::type; }; // expected-error{{type 'int *' cannot be used prior to '::' because it has no members}}
+
+  template<typename T>
+  struct B {};
+
+  template<typename T> requires A<T>::type // expected-note{{in instantiation of template class 'class_templates::A<int *>' requested here}}
+                                           // expected-note@-1{{while substituting template arguments into constraint expression here}}
+  struct B<T*> {};
+
+  template<typename T> requires T{} // expected-error{{atomic constraint must be of type 'bool' (found 'int')}}
+  struct B<T**> {};
+
+  static_assert((B<int**>{}, true)); // expected-note{{while checking constraint satisfaction for class template partial specialization 'B<int *>' required here}}
+  // expected-note@-1{{while checking constraint satisfaction for class template partial specialization 'B<int>' required here}}
+  // expected-note@-2{{during template argument deduction for class template partial specialization 'B<T *>' [with T = int *]}}
+  // expected-note@-3{{during template argument deduction for class template partial specialization 'B<T **>' [with T = int]}}
+  // expected-note@-4 2{{in instantiation of template class 'class_templates::B<int **>' requested here}}
+}
+
+namespace variable_templates
+{
+  template<typename T, typename U> requires sizeof(T) >= 4
+  constexpr bool is_same_v = false;
+
+  template<typename T> requires sizeof(T*) >= 4 && sizeof(T) >= 4
+  constexpr bool is_same_v<T*, T*> = true;
+
+  static_assert(!is_same_v<char*, char*>);
+  static_assert(!is_same_v<short*, short*>);
+  static_assert(is_same_v<int*, int*>);
+
+  template<typename T>
+  struct A { using type = typename T::type; }; // expected-error{{type 'int *' cannot be used prior to '::' because it has no members}}
+
+  template<typename T>
+  constexpr bool v1 = false;
+
+  template<typename T> requires A<T>::type // expected-note{{in instantiation of template class 'variable_templates::A<int *>' requested here}}
+                                           // expected-note@-1{{while substituting template arguments into constraint expression here}}
+  constexpr bool v1<T*> = true;
+
+  template<typename T> requires T{} // expected-error{{atomic constraint must be of type 'bool' (found 'int')}}
+  constexpr bool v1<T**> = true;
+
+  static_assert(v1<int**>); // expected-note{{while checking constraint satisfaction for variable template partial specialization 'v1<int *>' required here}}
+  // expected-note@-1{{while checking constraint satisfaction for variable template partial specialization 'v1<int>' required here}}
+  // expected-note@-2{{during template argument deduction for variable template partial specialization 'v1<T *>' [with T = int *]}}
+  // expected-note@-3{{during template argument deduction for variable template partial specialization 'v1<T **>' [with T = int]}}
+  // expected-error@-4{{static_assert failed due to requirement 'v1<int **>'}}
+
+}
\ No newline at end of file
Index: test/CodeGenCXX/mangle-concept.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCXX/mangle-concept.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -verify -Wno-return-type -Wno-main -std=c++2a -fconcepts-ts -emit-llvm -triple %itanium_abi_triple -o - %s | FileCheck %s
+// expected-no-diagnostics
+
+namespace test1 {
+template <bool> struct S {};
+template <typename> concept C = true;
+template <typename T = int> S<C<T>> f0() { return S<C<T>>{}; }
+template S<C<int>> f0<>();
+// CHECK: void @_ZN5test12f0IiEENS_1SIXL_ZNS_1CIT_EEEEEEv()
+}
+
+template <bool> struct S {};
+template <typename> concept C = true;
+template <typename T = int> S<C<T>> f0() { return S<C<T>>{}; }
+template S<C<int>> f0<>();
+// CHECK: void @_Z2f0IiE1SIXL_Z1CIT_EEEEv()
\ No newline at end of file