Index: clang-tools-extra/clangd/DumpAST.cpp
===================================================================
--- clang-tools-extra/clangd/DumpAST.cpp
+++ clang-tools-extra/clangd/DumpAST.cpp
@@ -143,6 +143,7 @@
       TEMPLATE_ARGUMENT_KIND(Declaration);
       TEMPLATE_ARGUMENT_KIND(Template);
       TEMPLATE_ARGUMENT_KIND(TemplateExpansion);
+      TEMPLATE_ARGUMENT_KIND(UncommonValue);
 #undef TEMPLATE_ARGUMENT_KIND
     }
     llvm_unreachable("Unhandled ArgKind enum");
Index: clang-tools-extra/clangd/FindTarget.cpp
===================================================================
--- clang-tools-extra/clangd/FindTarget.cpp
+++ clang-tools-extra/clangd/FindTarget.cpp
@@ -1013,6 +1013,7 @@
     case TemplateArgument::Pack:
     case TemplateArgument::Type:
     case TemplateArgument::Expression:
+    case TemplateArgument::UncommonValue:
       break; // Handled by VisitType and VisitExpression.
     };
     return RecursiveASTVisitor::TraverseTemplateArgumentLoc(A);
Index: clang/include/clang/AST/ASTContext.h
===================================================================
--- clang/include/clang/AST/ASTContext.h
+++ clang/include/clang/AST/ASTContext.h
@@ -3042,8 +3042,8 @@
   /// for destruction.
   template <typename T> void addDestruction(T *Ptr) const {
     if (!std::is_trivially_destructible<T>::value) {
-      auto DestroyPtr = [](void *V) { static_cast<T *>(V)->~T(); };
-      AddDeallocation(DestroyPtr, Ptr);
+      auto DestroyPtr = [](void *V) { ((T *)V)->~T(); };
+      AddDeallocation(DestroyPtr, (void *)Ptr);
     }
   }
 
Index: clang/include/clang/AST/ODRHash.h
===================================================================
--- clang/include/clang/AST/ODRHash.h
+++ clang/include/clang/AST/ODRHash.h
@@ -25,6 +25,7 @@
 
 namespace clang {
 
+class APValue;
 class Decl;
 class IdentifierInfo;
 class NestedNameSpecifier;
@@ -105,6 +106,7 @@
 
 private:
   void AddDeclarationNameImpl(DeclarationName Name);
+  void AddUncommonValue(const APValue&);
 };
 
 }  // end namespace clang
Index: clang/include/clang/AST/PropertiesBase.td
===================================================================
--- clang/include/clang/AST/PropertiesBase.td
+++ clang/include/clang/AST/PropertiesBase.td
@@ -791,6 +791,20 @@
     return TemplateArgument(ctx, value, type, isDefaulted);
   }]>;
 }
+let Class = PropertyTypeCase<TemplateArgument, "UncommonValue"> in {
+  def : Property<"value", APValue> {
+    let Read = [{ node.getAsUncommonValue() }];
+  }
+  def : Property<"type", QualType> {
+    let Read = [{ node.getUncommonValueType() }];
+  }
+  def : Property<"isDefaulted", Bool> {
+    let Read = [{ node.getIsDefaulted() }];
+  }
+  def : Creator<[{
+    return TemplateArgument(ctx, type, value, isDefaulted);
+  }]>;
+}
 let Class = PropertyTypeCase<TemplateArgument, "Template"> in {
   def : Property<"name", TemplateName> {
     let Read = [{ node.getAsTemplateOrTemplatePattern() }];
Index: clang/include/clang/AST/RecursiveASTVisitor.h
===================================================================
--- clang/include/clang/AST/RecursiveASTVisitor.h
+++ clang/include/clang/AST/RecursiveASTVisitor.h
@@ -858,6 +858,7 @@
   case TemplateArgument::Declaration:
   case TemplateArgument::Integral:
   case TemplateArgument::NullPtr:
+  case TemplateArgument::UncommonValue:
     return true;
 
   case TemplateArgument::Type:
@@ -890,6 +891,7 @@
   case TemplateArgument::Declaration:
   case TemplateArgument::Integral:
   case TemplateArgument::NullPtr:
+  case TemplateArgument::UncommonValue:
     return true;
 
   case TemplateArgument::Type: {
Index: clang/include/clang/AST/TemplateArgumentVisitor.h
===================================================================
--- clang/include/clang/AST/TemplateArgumentVisitor.h
+++ clang/include/clang/AST/TemplateArgumentVisitor.h
@@ -37,6 +37,7 @@
       DISPATCH(Declaration);
       DISPATCH(NullPtr);
       DISPATCH(Integral);
+      DISPATCH(UncommonValue);
       DISPATCH(Template);
       DISPATCH(TemplateExpansion);
       DISPATCH(Expression);
@@ -59,6 +60,7 @@
   VISIT_METHOD(Declaration);
   VISIT_METHOD(NullPtr);
   VISIT_METHOD(Integral);
+  VISIT_METHOD(UncommonValue);
   VISIT_METHOD(Template);
   VISIT_METHOD(TemplateExpansion);
   VISIT_METHOD(Expression);
Index: clang/include/clang/AST/TemplateBase.h
===================================================================
--- clang/include/clang/AST/TemplateBase.h
+++ clang/include/clang/AST/TemplateBase.h
@@ -50,6 +50,7 @@
 
 namespace clang {
 
+class APValue;
 class ASTContext;
 class Expr;
 struct PrintingPolicy;
@@ -80,6 +81,12 @@
     /// that was provided for an integral non-type template parameter.
     Integral,
 
+    /// The template argument is a non-type template argument that can't be
+    /// represented by the special-case Declaration, NullPtr, or Integral
+    /// forms. These values are only ever produced by constant evaluation,
+    /// so cannot be dependent.
+    UncommonValue,
+
     /// The template argument is a template name that was provided for a
     /// template template parameter.
     Template,
@@ -125,6 +132,12 @@
     };
     void *Type;
   };
+  struct V {
+    unsigned Kind : 31;
+    unsigned IsDefaulted : 1;
+    const APValue *Value;
+    void *Type;
+  };
   struct A {
     unsigned Kind : 31;
     unsigned IsDefaulted : 1;
@@ -145,11 +158,19 @@
   union {
     struct DA DeclArg;
     struct I Integer;
+    struct V Value;
     struct A Args;
     struct TA TemplateArg;
     struct TV TypeOrValue;
   };
 
+  void initFromType(QualType, bool isNullPtr, bool IsDefaulted);
+  void initFromDeclaration(ValueDecl *, QualType, bool IsDefaulted);
+  void initFromIntegral(const ASTContext &, const llvm::APSInt &, QualType,
+                        bool IsDefaulted);
+  void initFromUncommon(const ASTContext &, QualType, const APValue &,
+                        bool IsDefaulted);
+
 public:
   /// Construct an empty, invalid template argument.
   constexpr TemplateArgument() : TypeOrValue({Null, 0, /* IsDefaulted */ 0}) {}
@@ -157,25 +178,22 @@
   /// Construct a template type argument.
   TemplateArgument(QualType T, bool isNullPtr = false,
                    bool IsDefaulted = false) {
-    TypeOrValue.Kind = isNullPtr ? NullPtr : Type;
-    TypeOrValue.IsDefaulted = IsDefaulted;
-    TypeOrValue.V = reinterpret_cast<uintptr_t>(T.getAsOpaquePtr());
+    initFromType(T, isNullPtr, IsDefaulted);
   }
 
-  /// Construct a template argument that refers to a
-  /// declaration, which is either an external declaration or a
-  /// template declaration.
+  /// Construct a template argument that refers to a (non-dependent)
+  /// declaration.
   TemplateArgument(ValueDecl *D, QualType QT, bool IsDefaulted = false) {
-    assert(D && "Expected decl");
-    DeclArg.Kind = Declaration;
-    DeclArg.IsDefaulted = IsDefaulted;
-    DeclArg.QT = QT.getAsOpaquePtr();
-    DeclArg.D = D;
+    initFromDeclaration(D, QT, IsDefaulted);
   }
 
   /// Construct an integral constant template argument. The memory to
   /// store the value is allocated with Ctx.
-  TemplateArgument(ASTContext &Ctx, const llvm::APSInt &Value, QualType Type,
+  TemplateArgument(const ASTContext &Ctx, const llvm::APSInt &Value,
+                   QualType Type, bool IsDefaulted = false);
+
+  /// Construct a template argument from an arbitrary constant value.
+  TemplateArgument(const ASTContext &Ctx, QualType Type, const APValue &Value,
                    bool IsDefaulted = false);
 
   /// Construct an integral constant template argument with the same
@@ -360,6 +378,14 @@
   /// default template parameter.
   bool getIsDefaulted() const { return (bool)TypeOrValue.IsDefaulted; }
 
+  /// Get the value of an UncommonValue.
+  const APValue &getAsUncommonValue() const { return *Value.Value; }
+
+  /// Get the type of an UncommonValue.
+  QualType getUncommonValueType() const {
+    return QualType::getFromOpaquePtr(Value.Type);
+  }
+
   /// If this is a non-type template argument, get its type. Otherwise,
   /// returns a null QualType.
   QualType getNonTypeTemplateArgumentType() const;
@@ -505,6 +531,7 @@
     assert(Argument.getKind() == TemplateArgument::NullPtr ||
            Argument.getKind() == TemplateArgument::Integral ||
            Argument.getKind() == TemplateArgument::Declaration ||
+           Argument.getKind() == TemplateArgument::UncommonValue ||
            Argument.getKind() == TemplateArgument::Expression);
   }
 
@@ -564,6 +591,11 @@
     return LocInfo.getAsExpr();
   }
 
+  Expr *getSourceUncommonValueExpression() const {
+    assert(Argument.getKind() == TemplateArgument::UncommonValue);
+    return LocInfo.getAsExpr();
+  }
+
   NestedNameSpecifierLoc getTemplateQualifierLoc() const {
     if (Argument.getKind() != TemplateArgument::Template &&
         Argument.getKind() != TemplateArgument::TemplateExpansion)
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -4985,8 +4985,6 @@
   "non-type template argument refers to subobject '%0'">;
 def err_non_type_template_arg_addr_label_diff : Error<
   "template argument / label address difference / what did you expect?">;
-def err_non_type_template_arg_unsupported : Error<
-  "sorry, non-type template argument of type %0 is not yet supported">;
 def err_template_arg_not_convertible : Error<
   "non-type template argument of type %0 cannot be converted to a value "
   "of type %1">;
@@ -5038,9 +5036,6 @@
   "non-type template argument does not refer to an object or function">;
 def err_template_arg_not_pointer_to_member_form : Error<
   "non-type template argument is not a pointer to member constant">;
-def err_template_arg_member_ptr_base_derived_not_supported : Error<
-  "sorry, non-type template argument of pointer-to-member type %1 that refers "
-  "to member %q0 of a different class is not supported yet">;
 def err_template_arg_invalid : Error<
   "non-type template argument '%0' is invalid">;
 def ext_template_arg_extra_parens : ExtWarn<
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -8416,8 +8416,8 @@
                                           QualType ParamType,
                                           SourceLocation Loc);
   ExprResult
-  BuildExpressionFromIntegralTemplateArgument(const TemplateArgument &Arg,
-                                              SourceLocation Loc);
+  BuildExpressionFromNonTypeTemplateArgument(const TemplateArgument &Arg,
+                                             SourceLocation Loc);
 
   /// Enumeration describing how template parameter lists are compared
   /// for equality.
Index: clang/lib/AST/ASTContext.cpp
===================================================================
--- clang/lib/AST/ASTContext.cpp
+++ clang/lib/AST/ASTContext.cpp
@@ -6892,6 +6892,11 @@
     case TemplateArgument::Integral:
       return TemplateArgument(Arg, getCanonicalType(Arg.getIntegralType()));
 
+    case TemplateArgument::UncommonValue:
+      return TemplateArgument(*this,
+                              getCanonicalType(Arg.getUncommonValueType()),
+                              Arg.getAsUncommonValue());
+
     case TemplateArgument::Type:
       return TemplateArgument(getCanonicalType(Arg.getAsType()),
                               /*isNullPtr*/ false, Arg.getIsDefaulted());
Index: clang/lib/AST/ASTImporter.cpp
===================================================================
--- clang/lib/AST/ASTImporter.cpp
+++ clang/lib/AST/ASTImporter.cpp
@@ -865,6 +865,17 @@
                             From.getIsDefaulted());
   }
 
+  case TemplateArgument::UncommonValue: {
+    ExpectedType ToTypeOrErr = import(From.getUncommonValueType());
+    if (!ToTypeOrErr)
+      return ToTypeOrErr.takeError();
+    Expected<APValue> ToValueOrErr = import(From.getAsUncommonValue());
+    if (!ToValueOrErr)
+      return ToValueOrErr.takeError();
+    return TemplateArgument(Importer.getToContext(), *ToTypeOrErr,
+                            *ToValueOrErr);
+  }
+
   case TemplateArgument::Template: {
     Expected<TemplateName> ToTemplateOrErr = import(From.getAsTemplate());
     if (!ToTemplateOrErr)
@@ -3382,6 +3393,8 @@
     case TemplateArgument::NullPtr:
       // FIXME: The type is not allowed to be in the function?
       return CheckType(Arg.getNullPtrType());
+    case TemplateArgument::UncommonValue:
+      return CheckType(Arg.getUncommonValueType());
     case TemplateArgument::Pack:
       for (const auto &PackArg : Arg.getPackAsArray())
         if (checkTemplateArgument(PackArg))
Index: clang/lib/AST/ASTStructuralEquivalence.cpp
===================================================================
--- clang/lib/AST/ASTStructuralEquivalence.cpp
+++ clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -604,6 +604,10 @@
     return IsStructurallyEquivalent(Context, Arg1.getAsExpr(),
                                     Arg2.getAsExpr());
 
+  case TemplateArgument::UncommonValue:
+    // FIXME: Do we need to customize the comparison?
+    return Arg1.structurallyEquals(Arg2);
+
   case TemplateArgument::Pack:
     return IsStructurallyEquivalent(Context, Arg1.pack_elements(),
                                     Arg2.pack_elements());
Index: clang/lib/AST/Decl.cpp
===================================================================
--- clang/lib/AST/Decl.cpp
+++ clang/lib/AST/Decl.cpp
@@ -343,6 +343,10 @@
       LV.merge(getTypeLinkageAndVisibility(Arg.getNullPtrType()));
       continue;
 
+    case TemplateArgument::UncommonValue:
+      LV.merge(getLVForValue(Arg.getAsUncommonValue(), computation));
+      continue;
+
     case TemplateArgument::Template:
     case TemplateArgument::TemplateExpansion:
       if (TemplateDecl *Template =
Index: clang/lib/AST/ItaniumMangle.cpp
===================================================================
--- clang/lib/AST/ItaniumMangle.cpp
+++ clang/lib/AST/ItaniumMangle.cpp
@@ -4497,9 +4497,27 @@
     E = cast<CXXStdInitializerListExpr>(E)->getSubExpr();
     goto recurse;
 
-  case Expr::SubstNonTypeTemplateParmExprClass:
+  case Expr::SubstNonTypeTemplateParmExprClass: {
+    // Mangle a substituted parameter the same way we mangle the template
+    // argument.
+    // As proposed in https://github.com/itanium-cxx-abi/cxx-abi/issues/111.
+    auto *SNTTPE = cast<SubstNonTypeTemplateParmExpr>(E);
+    if (auto *CE = dyn_cast<ConstantExpr>(SNTTPE->getReplacement())) {
+      // Pull out the constant value and mangle it as a template argument.
+      QualType ParamType = SNTTPE->getParameterType(Context.getASTContext());
+      if (CE->hasAPValueResult())
+        mangleValueInTemplateArg(ParamType, CE->getResultAsAPValue(), false,
+                                 /*NeedExactType=*/true);
+      else
+        mangleValueInTemplateArg(ParamType, CE->getAPValueResult(), false,
+                                 /*NeedExactType=*/true);
+      break;
+    }
+    // The remaining cases all happen to be substituted with expressions that
+    // mangle the same as a corresponding template argument anyway.
     E = cast<SubstNonTypeTemplateParmExpr>(E)->getReplacement();
     goto recurse;
+  }
 
   case Expr::UserDefinedLiteralClass:
     // We follow g++'s approach of mangling a UDL as a call to the literal
@@ -5532,6 +5550,10 @@
     mangleNullPointer(A.getNullPtrType());
     break;
   }
+  case TemplateArgument::UncommonValue:
+    mangleValueInTemplateArg(A.getUncommonValueType(), A.getAsUncommonValue(),
+                             /*TopLevel=*/true, NeedExactType);
+    break;
   case TemplateArgument::Pack: {
     //  <template-arg> ::= J <template-arg>* E
     Out << 'J';
@@ -5941,7 +5963,20 @@
       Out << "plcvPcad";
       Kind = Offset;
     } else {
-      if (!V.getLValuePath().empty() || V.isLValueOnePastTheEnd()) {
+      // Clang 11 and before mangled an array subject to array-to-pointer decay
+      // as if it were the declaration itself.
+      bool IsArrayToPointerDecayMangledAsDecl = false;
+      if (TopLevel && Ctx.getLangOpts().getClangABICompat() <=
+                          LangOptions::ClangABI::Ver11) {
+        QualType BType = B.getType();
+        IsArrayToPointerDecayMangledAsDecl =
+            BType->isArrayType() && V.getLValuePath().size() == 1 &&
+            V.getLValuePath()[0].getAsArrayIndex() == 0 &&
+            Ctx.hasSimilarType(T, Ctx.getDecayedType(BType));
+      }
+
+      if ((!V.getLValuePath().empty() || V.isLValueOnePastTheEnd()) &&
+          !IsArrayToPointerDecayMangledAsDecl) {
         NotPrimaryExpr();
         // A final conversion to the template parameter's type is usually
         // folded into the 'so' mangling, but we can't do that for 'void*'
Index: clang/lib/AST/MicrosoftMangle.cpp
===================================================================
--- clang/lib/AST/MicrosoftMangle.cpp
+++ clang/lib/AST/MicrosoftMangle.cpp
@@ -1546,6 +1546,22 @@
   }
 }
 
+/// If value V (with type T) represents a decayed pointer to the first element
+/// of an array, return that array.
+static ValueDecl *getAsArrayToPointerDecayedDecl(QualType T, const APValue &V) {
+  // Must be a pointer...
+  if (!T->isPointerType() || !V.isLValue() || !V.hasLValuePath() ||
+      !V.getLValueBase())
+    return nullptr;
+  // ... to element 0 of an array.
+  QualType BaseT = V.getLValueBase().getType();
+  if (!BaseT->isArrayType() || V.getLValuePath().size() != 1 ||
+      V.getLValuePath()[0].getAsArrayIndex() != 0)
+    return nullptr;
+  return const_cast<ValueDecl *>(
+      V.getLValueBase().dyn_cast<const ValueDecl *>());
+}
+
 void MicrosoftCXXNameMangler::mangleTemplateArg(const TemplateDecl *TD,
                                                 const TemplateArgument &TA,
                                                 const NamedDecl *Parm) {
@@ -1655,6 +1671,25 @@
                          cast<NonTypeTemplateParmDecl>(Parm), T);
     break;
   }
+  case TemplateArgument::UncommonValue:
+    if (ValueDecl *D = getAsArrayToPointerDecayedDecl(
+            TA.getUncommonValueType(), TA.getAsUncommonValue())) {
+      // Mangle the result of array-to-pointer decay as if it were a reference
+      // to the original declaration, to match MSVC's behavior. This can result
+      // in mangling collisions in some cases!
+      return mangleTemplateArg(
+          TD, TemplateArgument(D, TA.getUncommonValueType()), Parm);
+    }
+    Out << "$";
+    if (cast<NonTypeTemplateParmDecl>(Parm)
+            ->getType()
+            ->getContainedDeducedType()) {
+      Out << "M";
+      mangleType(TA.getNonTypeTemplateArgumentType(), SourceRange(), QMM_Drop);
+    }
+    mangleTemplateArgValue(TA.getUncommonValueType(), TA.getAsUncommonValue(),
+                           /*WithScalarType=*/false);
+    break;
   case TemplateArgument::Expression:
     mangleExpression(TA.getAsExpr(), cast<NonTypeTemplateParmDecl>(Parm));
     break;
Index: clang/lib/AST/ODRHash.cpp
===================================================================
--- clang/lib/AST/ODRHash.cpp
+++ clang/lib/AST/ODRHash.cpp
@@ -14,6 +14,11 @@
 
 #include "clang/AST/ODRHash.h"
 
+#include "clang/AST/APValue.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/CharUnits.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclVisitor.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/StmtVisitor.h"
@@ -180,6 +185,10 @@
       TA.getAsIntegral().Profile(ID);
       break;
     }
+    case TemplateArgument::UncommonValue:
+      AddQualType(TA.getUncommonValueType());
+      AddUncommonValue(TA.getAsUncommonValue());
+      break;
     case TemplateArgument::Template:
     case TemplateArgument::TemplateExpansion:
       AddTemplateName(TA.getAsTemplateOrTemplatePattern());
@@ -1288,3 +1297,60 @@
 void ODRHash::AddBoolean(bool Value) {
   Bools.push_back(Value);
 }
+
+void ODRHash::AddUncommonValue(const APValue &Value) {
+  ID.AddInteger(Value.getKind());
+
+  // 'APValue::Profile' uses pointer values to make hash for LValue and
+  // MemberPointer, but they differ from one compiler invocation to another.
+  // So, handle them explicitly here.
+
+  switch (Value.getKind()) {
+  case APValue::LValue: {
+    const APValue::LValueBase &Base = Value.getLValueBase();
+    if (!Base) {
+      ID.AddInteger(Value.getLValueOffset().getQuantity());
+      break;
+    }
+
+    assert(Base.is<const ValueDecl *>());
+    AddDecl(Base.get<const ValueDecl *>());
+    ID.AddInteger(Value.getLValueOffset().getQuantity());
+
+    bool OnePastTheEnd = Value.isLValueOnePastTheEnd();
+    if (Value.hasLValuePath()) {
+      QualType TypeSoFar = Base.getType();
+      for (APValue::LValuePathEntry E : Value.getLValuePath()) {
+        if (auto *AT = TypeSoFar->getAsArrayTypeUnsafe()) {
+          if (auto *CAT = dyn_cast<ConstantArrayType>(AT))
+            OnePastTheEnd |= CAT->getSize() == E.getAsArrayIndex();
+          TypeSoFar = AT->getElementType();
+        } else {
+          const Decl *D = E.getAsBaseOrMember().getPointer();
+          if (auto *FD = dyn_cast<FieldDecl>(D)) {
+            if (FD->getParent()->isUnion())
+              ID.AddInteger(FD->getFieldIndex());
+            TypeSoFar = FD->getType();
+          } else {
+            TypeSoFar =
+                D->getASTContext().getRecordType(cast<CXXRecordDecl>(D));
+          }
+        }
+      }
+    }
+    ID.AddInteger((Value.isNullPointer() ? 1 : 0) | (OnePastTheEnd ? 2 : 0) |
+                  (Value.hasLValuePath() ? 4 : 0));
+    break;
+  }
+  case APValue::MemberPointer: {
+    const ValueDecl *D = Value.getMemberPointerDecl();
+    assert(D);
+    AddDecl(D);
+    ID.AddInteger(
+        D->getASTContext().getMemberPointerPathAdjustment(Value).getQuantity());
+    break;
+  }
+  default:
+    Value.Profile(ID);
+  }
+}
\ No newline at end of file
Index: clang/lib/AST/StmtProfile.cpp
===================================================================
--- clang/lib/AST/StmtProfile.cpp
+++ clang/lib/AST/StmtProfile.cpp
@@ -2365,6 +2365,12 @@
     Arg.getAsIntegral().Profile(ID);
     break;
 
+  case TemplateArgument::UncommonValue:
+    VisitType(Arg.getUncommonValueType());
+    // FIXME: Do we need to recursively decompose this ourselves?
+    Arg.getAsUncommonValue().Profile(ID);
+    break;
+
   case TemplateArgument::Expression:
     Visit(Arg.getAsExpr());
     break;
Index: clang/lib/AST/TemplateBase.cpp
===================================================================
--- clang/lib/AST/TemplateBase.cpp
+++ clang/lib/AST/TemplateBase.cpp
@@ -160,8 +160,25 @@
 // TemplateArgument Implementation
 //===----------------------------------------------------------------------===//
 
-TemplateArgument::TemplateArgument(ASTContext &Ctx, const llvm::APSInt &Value,
-                                   QualType Type, bool IsDefaulted) {
+void TemplateArgument::initFromType(QualType T, bool isNullPtr,
+                                    bool IsDefaulted) {
+  TypeOrValue.Kind = isNullPtr ? NullPtr : Type;
+  TypeOrValue.IsDefaulted = IsDefaulted;
+  TypeOrValue.V = reinterpret_cast<uintptr_t>(T.getAsOpaquePtr());
+}
+
+void TemplateArgument::initFromDeclaration(ValueDecl *D, QualType QT,
+                                           bool IsDefaulted) {
+  assert(D && "Expected decl");
+  DeclArg.Kind = Declaration;
+  DeclArg.IsDefaulted = IsDefaulted;
+  DeclArg.QT = QT.getAsOpaquePtr();
+  DeclArg.D = D;
+}
+
+void TemplateArgument::initFromIntegral(const ASTContext &Ctx,
+                                        const llvm::APSInt &Value,
+                                        QualType Type, bool IsDefaulted) {
   Integer.Kind = Integral;
   Integer.IsDefaulted = IsDefaulted;
   // Copy the APSInt value into our decomposed form.
@@ -180,6 +197,56 @@
   Integer.Type = Type.getAsOpaquePtr();
 }
 
+void TemplateArgument::initFromUncommon(const ASTContext &Ctx, QualType Type,
+                                        const APValue &V, bool IsDefaulted) {
+  Value.Kind = UncommonValue;
+  Value.IsDefaulted = IsDefaulted;
+  Value.Value = new (Ctx) APValue(V);
+  Ctx.addDestruction(Value.Value);
+  Value.Type = Type.getAsOpaquePtr();
+}
+
+TemplateArgument::TemplateArgument(const ASTContext &Ctx,
+                                   const llvm::APSInt &Value, QualType Type,
+                                   bool IsDefaulted) {
+  initFromIntegral(Ctx, Value, Type, IsDefaulted);
+}
+
+static const ValueDecl *getAsSimpleValueDeclRef(const ASTContext &Ctx,
+                                                QualType T, const APValue &V) {
+  // Pointers to members are relatively easy.
+  if (V.isMemberPointer() && V.getMemberPointerPath().empty())
+    return V.getMemberPointerDecl();
+
+  // We model class non-type template parameters as their template parameter
+  // object declaration.
+  if (V.isStruct() || V.isUnion())
+    return Ctx.getTemplateParamObjectDecl(T, V);
+
+  // Pointers and references with an empty path use the special 'Declaration'
+  // representation.
+  if (V.isLValue() && V.hasLValuePath() && V.getLValuePath().empty() &&
+      !V.isLValueOnePastTheEnd())
+    return V.getLValueBase().dyn_cast<const ValueDecl *>();
+
+  // Everything else uses the 'uncommon' representation.
+  return nullptr;
+}
+
+TemplateArgument::TemplateArgument(const ASTContext &Ctx, QualType Type,
+                                   const APValue &V, bool IsDefaulted) {
+  if (Type->isIntegralOrEnumerationType() && V.isInt())
+    initFromIntegral(Ctx, V.getInt(), Type, IsDefaulted);
+  else if ((V.isLValue() && V.isNullPointer()) ||
+           (V.isMemberPointer() && !V.getMemberPointerDecl()))
+    initFromType(Type, /*isNullPtr=*/true, IsDefaulted);
+  else if (const ValueDecl *VD = getAsSimpleValueDeclRef(Ctx, Type, V))
+    // FIXME: The Declaration form should expose a const ValueDecl*.
+    initFromDeclaration(const_cast<ValueDecl *>(VD), Type, IsDefaulted);
+  else
+    initFromUncommon(Ctx, Type, V, IsDefaulted);
+}
+
 TemplateArgument
 TemplateArgument::CreatePackCopy(ASTContext &Context,
                                  ArrayRef<TemplateArgument> Args) {
@@ -220,6 +287,7 @@
 
   case NullPtr:
   case Integral:
+  case UncommonValue:
     return TemplateArgumentDependence::None;
 
   case Expression:
@@ -250,6 +318,7 @@
   case Null:
   case Declaration:
   case Integral:
+  case UncommonValue:
   case Pack:
   case Template:
   case NullPtr:
@@ -300,6 +369,9 @@
 
   case TemplateArgument::NullPtr:
     return getNullPtrType();
+
+  case TemplateArgument::UncommonValue:
+    return getUncommonValueType();
   }
 
   llvm_unreachable("Invalid TemplateArgument Kind!");
@@ -333,8 +405,13 @@
     break;
 
   case Integral:
-    getAsIntegral().Profile(ID);
     getIntegralType().Profile(ID);
+    getAsIntegral().Profile(ID);
+    break;
+
+  case UncommonValue:
+    getUncommonValueType().Profile(ID);
+    getAsUncommonValue().Profile(ID);
     break;
 
   case Expression:
@@ -371,6 +448,16 @@
     return getIntegralType() == Other.getIntegralType() &&
            getAsIntegral() == Other.getAsIntegral();
 
+  case UncommonValue: {
+    if (getUncommonValueType() != Other.getUncommonValueType())
+      return false;
+
+    llvm::FoldingSetNodeID A, B;
+    getAsUncommonValue().Profile(A);
+    Other.getAsUncommonValue().Profile(B);
+    return A == B;
+  }
+
   case Pack:
     if (Args.NumArgs != Other.Args.NumArgs) return false;
     for (unsigned I = 0, E = Args.NumArgs; I != E; ++I)
@@ -397,6 +484,7 @@
 
   case Declaration:
   case Integral:
+  case UncommonValue:
   case Pack:
   case Null:
   case Template:
@@ -439,6 +527,10 @@
     break;
   }
 
+  case UncommonValue:
+    getAsUncommonValue().printPretty(Out, Policy, getUncommonValueType());
+    break;
+
   case NullPtr:
     // FIXME: Include the type if it's not obvious from the context.
     Out << "nullptr";
@@ -522,6 +614,9 @@
   case TemplateArgument::Integral:
     return getSourceIntegralExpression()->getSourceRange();
 
+  case TemplateArgument::UncommonValue:
+    return getSourceUncommonValueExpression()->getSourceRange();
+
   case TemplateArgument::Pack:
   case TemplateArgument::Null:
     return SourceRange();
@@ -550,6 +645,18 @@
   case TemplateArgument::Integral:
     return DB << toString(Arg.getAsIntegral(), 10);
 
+  case TemplateArgument::UncommonValue: {
+    // FIXME: We're guessing at LangOptions!
+    SmallString<32> Str;
+    llvm::raw_svector_ostream OS(Str);
+    LangOptions LangOpts;
+    LangOpts.CPlusPlus = true;
+    PrintingPolicy Policy(LangOpts);
+    Arg.getAsUncommonValue().printPretty(OS, Policy,
+                                         Arg.getUncommonValueType());
+    return DB << OS.str();
+  }
+
   case TemplateArgument::Template:
     return DB << Arg.getAsTemplate();
 
Index: clang/lib/AST/TypeLoc.cpp
===================================================================
--- clang/lib/AST/TypeLoc.cpp
+++ clang/lib/AST/TypeLoc.cpp
@@ -585,6 +585,7 @@
     case TemplateArgument::Integral:
     case TemplateArgument::Declaration:
     case TemplateArgument::NullPtr:
+    case TemplateArgument::UncommonValue:
       ArgInfos[i] = TemplateArgumentLocInfo();
       break;
 
Index: clang/lib/CodeGen/CGDebugInfo.cpp
===================================================================
--- clang/lib/CodeGen/CGDebugInfo.cpp
+++ clang/lib/CodeGen/CGDebugInfo.cpp
@@ -2120,6 +2120,14 @@
       TemplateParams.push_back(DBuilder.createTemplateValueParameter(
           TheCU, Name, TTy, defaultParameter, V));
     } break;
+    case TemplateArgument::UncommonValue: {
+      QualType T = TA.getUncommonValueType();
+      llvm::DIType *TTy = getOrCreateType(T, Unit);
+      llvm::Constant *V = ConstantEmitter(CGM).emitAbstract(
+          SourceLocation(), TA.getAsUncommonValue(), T);
+      TemplateParams.push_back(DBuilder.createTemplateValueParameter(
+          TheCU, Name, TTy, defaultParameter, V));
+    } break;
     case TemplateArgument::Template: {
       std::string QualName;
       llvm::raw_string_ostream OS(QualName);
Index: clang/lib/Index/USRGeneration.cpp
===================================================================
--- clang/lib/Index/USRGeneration.cpp
+++ clang/lib/Index/USRGeneration.cpp
@@ -1029,6 +1029,10 @@
     VisitType(Arg.getIntegralType());
     Out << Arg.getAsIntegral();
     break;
+
+  case TemplateArgument::UncommonValue:
+    // FIXME: Visit value.
+    break;
   }
 }
 
Index: clang/lib/Sema/SemaLookup.cpp
===================================================================
--- clang/lib/Sema/SemaLookup.cpp
+++ clang/lib/Sema/SemaLookup.cpp
@@ -2960,6 +2960,7 @@
     case TemplateArgument::Integral:
     case TemplateArgument::Expression:
     case TemplateArgument::NullPtr:
+    case TemplateArgument::UncommonValue:
       // [Note: non-type template arguments do not contribute to the set of
       //  associated namespaces. ]
       break;
Index: clang/lib/Sema/SemaOverload.cpp
===================================================================
--- clang/lib/Sema/SemaOverload.cpp
+++ clang/lib/Sema/SemaOverload.cpp
@@ -5935,7 +5935,9 @@
 
     if (Notes.empty()) {
       // It's a constant expression.
-      Expr *E = ConstantExpr::Create(S.Context, Result.get(), Value);
+      Expr *E = Result.get();
+      if (!isa<ConstantExpr>(E))
+        E = ConstantExpr::Create(S.Context, Result.get(), Value);
       if (ReturnPreNarrowingValue)
         Value = std::move(PreNarrowingValue);
       return E;
Index: clang/lib/Sema/SemaTemplate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplate.cpp
+++ clang/lib/Sema/SemaTemplate.cpp
@@ -4290,6 +4290,7 @@
   case TemplateArgument::NullPtr:
   case TemplateArgument::Integral:
   case TemplateArgument::Declaration:
+  case TemplateArgument::UncommonValue:
   case TemplateArgument::Pack:
   case TemplateArgument::TemplateExpansion:
     return false;
@@ -5615,6 +5616,7 @@
 
     case TemplateArgument::Declaration:
     case TemplateArgument::Integral:
+    case TemplateArgument::UncommonValue:
     case TemplateArgument::NullPtr:
       // We've already checked this template argument, so just copy
       // it to the list of converted arguments.
@@ -5769,11 +5771,10 @@
     return true;
 
   case TemplateArgument::Declaration:
-    llvm_unreachable("Declaration argument with template template parameter");
   case TemplateArgument::Integral:
-    llvm_unreachable("Integral argument with template template parameter");
+  case TemplateArgument::UncommonValue:
   case TemplateArgument::NullPtr:
-    llvm_unreachable("Null pointer argument with template template parameter");
+    llvm_unreachable("non-type argument with template template parameter");
 
   case TemplateArgument::Pack:
     llvm_unreachable("Caller must expand template argument packs");
@@ -7243,44 +7244,9 @@
       return ArgResult;
     }
 
-    // Convert the APValue to a TemplateArgument.
-    switch (Value.getKind()) {
-    case APValue::None:
-      assert(ParamType->isNullPtrType());
-      SugaredConverted = TemplateArgument(ParamType, /*isNullPtr=*/true);
-      CanonicalConverted = TemplateArgument(CanonParamType, /*isNullPtr=*/true);
-      break;
-    case APValue::Indeterminate:
-      llvm_unreachable("result of constant evaluation should be initialized");
-      break;
-    case APValue::Int:
-      assert(ParamType->isIntegralOrEnumerationType());
-      SugaredConverted = TemplateArgument(Context, Value.getInt(), ParamType);
-      CanonicalConverted =
-          TemplateArgument(Context, Value.getInt(), CanonParamType);
-      break;
-    case APValue::MemberPointer: {
-      assert(ParamType->isMemberPointerType());
-
-      // FIXME: We need TemplateArgument representation and mangling for these.
-      if (!Value.getMemberPointerPath().empty()) {
-        Diag(Arg->getBeginLoc(),
-             diag::err_template_arg_member_ptr_base_derived_not_supported)
-            << Value.getMemberPointerDecl() << ParamType
-            << Arg->getSourceRange();
-        return ExprError();
-      }
-
-      auto *VD = const_cast<ValueDecl*>(Value.getMemberPointerDecl());
-      SugaredConverted = VD ? TemplateArgument(VD, ParamType)
-                            : TemplateArgument(ParamType, /*isNullPtr=*/true);
-      CanonicalConverted =
-          VD ? TemplateArgument(cast<ValueDecl>(VD->getCanonicalDecl()),
-                                CanonParamType)
-             : TemplateArgument(CanonParamType, /*isNullPtr=*/true);
-      break;
-    }
-    case APValue::LValue: {
+    // Prior to C++20, enforce restrictions on possible template argument
+    // values.
+    if (!getLangOpts().CPlusPlus20 && Value.isLValue()) {
       //   For a non-type template-parameter of pointer or reference type,
       //   the value of the constant expression shall not refer to
       assert(ParamType->isPointerType() || ParamType->isReferenceType() ||
@@ -7298,8 +7264,7 @@
             << Arg->getSourceRange();
         return ExprError();
       }
-      // -- a subobject
-      // FIXME: Until C++20
+      // -- a subobject [until C++20]
       if (Value.hasLValuePath() && Value.getLValuePath().size() == 1 &&
           VD && VD->getType()->isArrayType() &&
           Value.getLValuePath()[0].getAsArrayIndex() == 0 &&
@@ -7317,37 +7282,13 @@
              "null reference should not be a constant expression");
       assert((!VD || !ParamType->isNullPtrType()) &&
              "non-null value of type nullptr_t?");
-
-      SugaredConverted = VD ? TemplateArgument(VD, ParamType)
-                            : TemplateArgument(ParamType, /*isNullPtr=*/true);
-      CanonicalConverted =
-          VD ? TemplateArgument(cast<ValueDecl>(VD->getCanonicalDecl()),
-                                CanonParamType)
-             : TemplateArgument(CanonParamType, /*isNullPtr=*/true);
-      break;
-    }
-    case APValue::Struct:
-    case APValue::Union: {
-      // Get or create the corresponding template parameter object.
-      TemplateParamObjectDecl *D =
-          Context.getTemplateParamObjectDecl(ParamType, Value);
-      SugaredConverted = TemplateArgument(D, ParamType);
-      CanonicalConverted =
-          TemplateArgument(D->getCanonicalDecl(), CanonParamType);
-      break;
     }
-    case APValue::AddrLabelDiff:
+
+    if (Value.isAddrLabelDiff())
       return Diag(StartLoc, diag::err_non_type_template_arg_addr_label_diff);
-    case APValue::FixedPoint:
-    case APValue::Float:
-    case APValue::ComplexInt:
-    case APValue::ComplexFloat:
-    case APValue::Vector:
-    case APValue::Array:
-      return Diag(StartLoc, diag::err_non_type_template_arg_unsupported)
-             << ParamType;
-    }
 
+    SugaredConverted = TemplateArgument(Context, ParamType, Value);
+    CanonicalConverted = TemplateArgument(Context, CanonParamType, Value);
     return ArgResult.get();
   }
 
@@ -7895,12 +7836,9 @@
 /// This routine takes care of the mapping from an integral template
 /// argument (which may have any integral type) to the appropriate
 /// literal value.
-ExprResult
-Sema::BuildExpressionFromIntegralTemplateArgument(const TemplateArgument &Arg,
-                                                  SourceLocation Loc) {
-  assert(Arg.getKind() == TemplateArgument::Integral &&
-         "Operation is only valid for integral template arguments");
-  QualType OrigT = Arg.getIntegralType();
+static Expr *BuildExpressionFromIntegralTemplateArgumentValue(
+    Sema &S, QualType OrigT, const llvm::APSInt &Int, SourceLocation Loc) {
+  assert(OrigT->isIntegralOrEnumerationType());
 
   // If this is an enum type that we're instantiating, we need to use an integer
   // type the same size as the enumerator.  We don't want to build an
@@ -7916,7 +7854,7 @@
     CharacterLiteral::CharacterKind Kind;
     if (T->isWideCharType())
       Kind = CharacterLiteral::Wide;
-    else if (T->isChar8Type() && getLangOpts().Char8)
+    else if (T->isChar8Type() && S.getLangOpts().Char8)
       Kind = CharacterLiteral::UTF8;
     else if (T->isChar16Type())
       Kind = CharacterLiteral::UTF16;
@@ -7925,29 +7863,133 @@
     else
       Kind = CharacterLiteral::Ascii;
 
-    E = new (Context) CharacterLiteral(Arg.getAsIntegral().getZExtValue(),
-                                       Kind, T, Loc);
+    E = new (S.Context) CharacterLiteral(Int.getZExtValue(), Kind, T, Loc);
   } else if (T->isBooleanType()) {
-    E = CXXBoolLiteralExpr::Create(Context, Arg.getAsIntegral().getBoolValue(),
-                                   T, Loc);
-  } else if (T->isNullPtrType()) {
-    E = new (Context) CXXNullPtrLiteralExpr(Context.NullPtrTy, Loc);
+    E = CXXBoolLiteralExpr::Create(S.Context, Int.getBoolValue(), T, Loc);
   } else {
-    E = IntegerLiteral::Create(Context, Arg.getAsIntegral(), T, Loc);
+    E = IntegerLiteral::Create(S.Context, Int, T, Loc);
   }
 
   if (OrigT->isEnumeralType()) {
     // FIXME: This is a hack. We need a better way to handle substituted
     // non-type template parameters.
-    E = CStyleCastExpr::Create(Context, OrigT, VK_PRValue, CK_IntegralCast, E,
-                               nullptr, CurFPFeatureOverrides(),
-                               Context.getTrivialTypeSourceInfo(OrigT, Loc),
+    E = CStyleCastExpr::Create(S.Context, OrigT, VK_PRValue, CK_IntegralCast, E,
+                               nullptr, S.CurFPFeatureOverrides(),
+                               S.Context.getTrivialTypeSourceInfo(OrigT, Loc),
                                Loc, Loc);
   }
 
   return E;
 }
 
+static Expr *BuildExpressionFromNonTypeTemplateArgumentValue(
+    Sema &S, QualType T, const APValue &Val, SourceLocation Loc) {
+  auto MakeInitList = [&](ArrayRef<Expr *> Elts) -> Expr * {
+    auto *ILE = new (S.Context) InitListExpr(S.Context, Loc, Elts, Loc);
+    ILE->setType(T);
+    return ILE;
+  };
+
+  switch (Val.getKind()) {
+  case APValue::AddrLabelDiff:
+    // This cannot occur in a template argument at all.
+  case APValue::Array:
+  case APValue::Struct:
+  case APValue::Union:
+    // These can only occur within a template parameter object, which is
+    // represented as a TemplateArgument::Declaration.
+    llvm_unreachable("unexpected template argument value");
+
+  case APValue::Int:
+    return BuildExpressionFromIntegralTemplateArgumentValue(S, T, Val.getInt(),
+                                                            Loc);
+
+  case APValue::Float:
+    return FloatingLiteral::Create(S.Context, Val.getFloat(), /*IsExact=*/true,
+                                   T, Loc);
+
+  case APValue::FixedPoint:
+    return FixedPointLiteral::CreateFromRawInt(
+        S.Context, Val.getFixedPoint().getValue(), T, Loc,
+        Val.getFixedPoint().getScale());
+
+  case APValue::ComplexInt: {
+    QualType ElemT = T->castAs<ComplexType>()->getElementType();
+    return MakeInitList({BuildExpressionFromIntegralTemplateArgumentValue(
+                             S, ElemT, Val.getComplexIntReal(), Loc),
+                         BuildExpressionFromIntegralTemplateArgumentValue(
+                             S, ElemT, Val.getComplexIntImag(), Loc)});
+  }
+
+  case APValue::ComplexFloat: {
+    QualType ElemT = T->castAs<ComplexType>()->getElementType();
+    return MakeInitList(
+        {FloatingLiteral::Create(S.Context, Val.getComplexFloatReal(), true,
+                                 ElemT, Loc),
+         FloatingLiteral::Create(S.Context, Val.getComplexFloatImag(), true,
+                                 ElemT, Loc)});
+  }
+
+  case APValue::Vector: {
+    QualType ElemT = T->castAs<VectorType>()->getElementType();
+    llvm::SmallVector<Expr *, 8> Elts;
+    for (unsigned I = 0, N = Val.getVectorLength(); I != N; ++I)
+      Elts.push_back(BuildExpressionFromNonTypeTemplateArgumentValue(
+          S, ElemT, Val.getVectorElt(I), Loc));
+    return MakeInitList(Elts);
+  }
+
+  case APValue::None:
+  case APValue::Indeterminate:
+    // FIXME: Are these values possible?
+  case APValue::LValue:
+  case APValue::MemberPointer:
+    // There isn't necessarily a valid equivalent source-level syntax for
+    // these; in particular, a naive lowering might violate access control.
+    // So for now we lower to a ConstantExpr holding the value, wrapped around
+    // an OpaqueValueExpr.
+    // FIXME: We should have a better representation for this.
+    ExprValueKind VK = VK_PRValue;
+    if (T->isReferenceType()) {
+      T = T->getPointeeType();
+      VK = VK_LValue;
+    }
+    auto *OVE = new (S.Context) OpaqueValueExpr(Loc, T, VK);
+    return ConstantExpr::Create(S.Context, OVE, Val);
+  }
+  llvm_unreachable("Unhandled APValue::ValueKind enum");
+}
+
+ExprResult
+Sema::BuildExpressionFromNonTypeTemplateArgument(const TemplateArgument &Arg,
+                                                 SourceLocation Loc) {
+  switch (Arg.getKind()) {
+  case TemplateArgument::Null:
+  case TemplateArgument::Type:
+  case TemplateArgument::Template:
+  case TemplateArgument::TemplateExpansion:
+  case TemplateArgument::Pack:
+    llvm_unreachable("not a non-type template argument");
+
+  case TemplateArgument::Expression:
+    return Arg.getAsExpr();
+
+  case TemplateArgument::NullPtr:
+  case TemplateArgument::Declaration:
+    return BuildExpressionFromDeclTemplateArgument(
+        Arg, Arg.getNonTypeTemplateArgumentType(), Loc);
+
+  case TemplateArgument::Integral:
+    return BuildExpressionFromIntegralTemplateArgumentValue(
+        *this, Arg.getIntegralType(), Arg.getAsIntegral(), Loc);
+
+  case TemplateArgument::UncommonValue:
+    return BuildExpressionFromNonTypeTemplateArgumentValue(
+        *this, Arg.getUncommonValueType(), Arg.getAsUncommonValue(), Loc);
+  }
+  llvm_unreachable("Unhandled TemplateArgument::ArgKind enum");
+}
+
 /// Match two template parameters within template parameter lists.
 static bool MatchTemplateParameterKind(
     Sema &S, NamedDecl *New, const NamedDecl *NewInstFrom, NamedDecl *Old,
Index: clang/lib/Sema/SemaTemplateDeduction.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateDeduction.cpp
+++ clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -266,6 +266,16 @@
     // All other combinations are incompatible.
     return DeducedTemplateArgument();
 
+  case TemplateArgument::UncommonValue:
+    // If we deduced a value and a dependent expression, keep the value.
+    if (Y.getKind() == TemplateArgument::Expression ||
+        (Y.getKind() == TemplateArgument::UncommonValue &&
+         X.structurallyEquals(Y)))
+      return X;
+
+    // All other combinations are incompatible.
+    return DeducedTemplateArgument();
+
   case TemplateArgument::Template:
     if (Y.getKind() == TemplateArgument::Template &&
         Context.hasSameTemplateName(X.getAsTemplate(), Y.getAsTemplate()))
@@ -2283,27 +2293,45 @@
     Info.SecondArg = A;
     return Sema::TDK_NonDeducedMismatch;
 
+  case TemplateArgument::UncommonValue:
+    if (A.getKind() == TemplateArgument::UncommonValue &&
+        A.structurallyEquals(P))
+      return Sema::TDK_Success;
+
+    Info.FirstArg = P;
+    Info.SecondArg = A;
+    return Sema::TDK_NonDeducedMismatch;
+
   case TemplateArgument::Expression:
     if (const NonTypeTemplateParmDecl *NTTP =
             getDeducedParameterFromExpr(Info, P.getAsExpr())) {
-      if (A.getKind() == TemplateArgument::Integral)
+      switch (A.getKind()) {
+      case TemplateArgument::Integral:
+      case TemplateArgument::Expression:
+      case TemplateArgument::UncommonValue:
         return DeduceNonTypeTemplateArgument(
-            S, TemplateParams, NTTP, A.getAsIntegral(), A.getIntegralType(),
-            /*ArrayBound=*/false, Info, Deduced);
-      if (A.getKind() == TemplateArgument::NullPtr)
+            S, TemplateParams, NTTP, DeducedTemplateArgument(A),
+            A.getNonTypeTemplateArgumentType(), Info, Deduced);
+
+      case TemplateArgument::NullPtr:
         return DeduceNullPtrTemplateArgument(S, TemplateParams, NTTP,
                                              A.getNullPtrType(), Info, Deduced);
-      if (A.getKind() == TemplateArgument::Expression)
-        return DeduceNonTypeTemplateArgument(S, TemplateParams, NTTP,
-                                             A.getAsExpr(), Info, Deduced);
-      if (A.getKind() == TemplateArgument::Declaration)
+
+      case TemplateArgument::Declaration:
         return DeduceNonTypeTemplateArgument(
             S, TemplateParams, NTTP, A.getAsDecl(), A.getParamTypeForDecl(),
             Info, Deduced);
 
-      Info.FirstArg = P;
-      Info.SecondArg = A;
-      return Sema::TDK_NonDeducedMismatch;
+      case TemplateArgument::Null:
+      case TemplateArgument::Type:
+      case TemplateArgument::Template:
+      case TemplateArgument::TemplateExpansion:
+      case TemplateArgument::Pack:
+        Info.FirstArg = P;
+        Info.SecondArg = A;
+        return Sema::TDK_NonDeducedMismatch;
+      }
+      llvm_unreachable("Unknown template argument kind");
     }
 
     // Can't deduce anything, but that's okay.
@@ -2488,6 +2516,9 @@
     case TemplateArgument::Integral:
       return hasSameExtendedValue(X.getAsIntegral(), Y.getAsIntegral());
 
+    case TemplateArgument::UncommonValue:
+      return X.structurallyEquals(Y);
+
     case TemplateArgument::Expression: {
       llvm::FoldingSetNodeID XID, YID;
       X.getAsExpr()->Profile(XID, Context, true);
@@ -2568,9 +2599,9 @@
                                E);
   }
 
-  case TemplateArgument::Integral: {
-    Expr *E =
-        BuildExpressionFromIntegralTemplateArgument(Arg, Loc).getAs<Expr>();
+  case TemplateArgument::Integral:
+  case TemplateArgument::UncommonValue: {
+    Expr *E = BuildExpressionFromNonTypeTemplateArgument(Arg, Loc).get();
     return TemplateArgumentLoc(TemplateArgument(E), E);
   }
 
@@ -6215,11 +6246,8 @@
   case TemplateArgument::Null:
   case TemplateArgument::Integral:
   case TemplateArgument::Declaration:
-    break;
-
   case TemplateArgument::NullPtr:
-    MarkUsedTemplateParameters(Ctx, TemplateArg.getNullPtrType(), OnlyDeduced,
-                               Depth, Used);
+  case TemplateArgument::UncommonValue:
     break;
 
   case TemplateArgument::Type:
Index: clang/lib/Sema/SemaTemplateInstantiate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1808,9 +1808,8 @@
     }
   } else if (arg.getKind() == TemplateArgument::Declaration ||
              arg.getKind() == TemplateArgument::NullPtr) {
-    ValueDecl *VD;
     if (arg.getKind() == TemplateArgument::Declaration) {
-      VD = arg.getAsDecl();
+      ValueDecl *VD = arg.getAsDecl();
 
       // Find the instantiation of the template argument.  This is
       // required for nested templates.
@@ -1818,21 +1817,20 @@
              getSema().FindInstantiatedDecl(loc, VD, TemplateArgs));
       if (!VD)
         return ExprError();
-    } else {
-      // Propagate NULL template argument.
-      VD = nullptr;
     }
 
-    QualType paramType = VD ? arg.getParamTypeForDecl() : arg.getNullPtrType();
+    QualType paramType = arg.getNonTypeTemplateArgumentType();
     assert(!paramType.isNull() && "type substitution failed for param type");
     assert(!paramType->isDependentType() && "param type still dependent");
     result = SemaRef.BuildExpressionFromDeclTemplateArgument(arg, paramType, loc);
     refParam = paramType->isReferenceType();
   } else {
-    result = SemaRef.BuildExpressionFromIntegralTemplateArgument(arg, loc);
+    QualType paramType = arg.getNonTypeTemplateArgumentType();
+    result = SemaRef.BuildExpressionFromNonTypeTemplateArgument(arg, loc);
+    refParam = paramType->isReferenceType();
     assert(result.isInvalid() ||
            SemaRef.Context.hasSameType(result.get()->getType(),
-                                       arg.getIntegralType()));
+                                       paramType.getNonReferenceType()));
   }
 
   if (result.isInvalid())
Index: clang/lib/Sema/SemaTemplateVariadic.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateVariadic.cpp
+++ clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -1111,6 +1111,7 @@
   case TemplateArgument::NullPtr:
   case TemplateArgument::Template:
   case TemplateArgument::Integral:
+  case TemplateArgument::UncommonValue:
   case TemplateArgument::Pack:
   case TemplateArgument::Null:
     return TemplateArgumentLoc();
@@ -1161,6 +1162,7 @@
   case TemplateArgument::NullPtr:
   case TemplateArgument::TemplateExpansion:
   case TemplateArgument::Integral:
+  case TemplateArgument::UncommonValue:
   case TemplateArgument::Pack:
   case TemplateArgument::Null:
     return std::nullopt;
Index: clang/lib/Sema/TreeTransform.h
===================================================================
--- clang/lib/Sema/TreeTransform.h
+++ clang/lib/Sema/TreeTransform.h
@@ -3814,6 +3814,7 @@
     case TemplateArgument::Null:
     case TemplateArgument::Integral:
     case TemplateArgument::Declaration:
+    case TemplateArgument::UncommonValue:
     case TemplateArgument::Pack:
     case TemplateArgument::TemplateExpansion:
     case TemplateArgument::NullPtr:
@@ -4493,7 +4494,8 @@
 
   case TemplateArgument::Integral:
   case TemplateArgument::NullPtr:
-  case TemplateArgument::Declaration: {
+  case TemplateArgument::Declaration:
+  case TemplateArgument::UncommonValue: {
     // Transform a resolved template argument straight to a resolved template
     // argument. We get here when substituting into an already-substituted
     // template type argument during concept satisfaction checking.
@@ -4520,9 +4522,15 @@
     else if (Arg.getKind() == TemplateArgument::NullPtr)
       Output = TemplateArgumentLoc(TemplateArgument(NewT, /*IsNullPtr=*/true),
                                    TemplateArgumentLocInfo());
-    else
+    else if (Arg.getKind() == TemplateArgument::Declaration)
       Output = TemplateArgumentLoc(TemplateArgument(NewD, NewT),
                                    TemplateArgumentLocInfo());
+    else if (Arg.getKind() == TemplateArgument::UncommonValue)
+      Output = TemplateArgumentLoc(
+          TemplateArgument(getSema().Context, NewT, Arg.getAsUncommonValue()),
+          TemplateArgumentLocInfo());
+    else
+      llvm_unreachable("unexpected template argument kind");
 
     return false;
   }
Index: clang/lib/Serialization/ASTReader.cpp
===================================================================
--- clang/lib/Serialization/ASTReader.cpp
+++ clang/lib/Serialization/ASTReader.cpp
@@ -7269,6 +7269,7 @@
   case TemplateArgument::Integral:
   case TemplateArgument::Declaration:
   case TemplateArgument::NullPtr:
+  case TemplateArgument::UncommonValue:
   case TemplateArgument::Pack:
     // FIXME: Is this right?
     return TemplateArgumentLocInfo();
Index: clang/lib/Serialization/ASTWriter.cpp
===================================================================
--- clang/lib/Serialization/ASTWriter.cpp
+++ clang/lib/Serialization/ASTWriter.cpp
@@ -5514,6 +5514,7 @@
   case TemplateArgument::Integral:
   case TemplateArgument::Declaration:
   case TemplateArgument::NullPtr:
+  case TemplateArgument::UncommonValue:
   case TemplateArgument::Pack:
     // FIXME: Is this right?
     break;
Index: clang/test/CXX/temp/temp.arg/temp.arg.nontype/p1.cpp
===================================================================
--- clang/test/CXX/temp/temp.arg/temp.arg.nontype/p1.cpp
+++ clang/test/CXX/temp/temp.arg/temp.arg.nontype/p1.cpp
@@ -200,7 +200,9 @@
   // cxx17-note@-3 {{reinterpret_cast}}
   X0<__builtin_constant_p(0) ? (int*)1 : (int*)1> x0d;
   // precxx17-error@-1 {{non-type template argument '(int *)1' is invalid}}
-  // cxx17-error@-2 {{non-type template argument refers to subobject '(int *)1'}}
+#if __cplusplus == 201703L
+  // cxx17-error@-3 {{non-type template argument refers to subobject '(int *)1'}}
+#endif
 }
 #endif // CPP11ONLY
 
Index: clang/test/CodeGenCXX/mangle-ms-templates.cpp
===================================================================
--- clang/test/CodeGenCXX/mangle-ms-templates.cpp
+++ clang/test/CodeGenCXX/mangle-ms-templates.cpp
@@ -2,6 +2,21 @@
 // RUN: %clang_cc1 -std=c++11 -fms-compatibility-version=19 -emit-llvm %s -o - -fms-extensions -fdelayed-template-parsing -triple=x86_64-pc-win32 | FileCheck -check-prefix X64 %s
 // RUN: %clang_cc1 -std=c++17 -fms-compatibility-version=19 -emit-llvm %s -o - -fms-extensions -fdelayed-template-parsing -triple=i386-pc-win32 | FileCheck %s
 // RUN: %clang_cc1 -std=c++17 -fms-compatibility-version=19 -emit-llvm %s -o - -fms-extensions -fdelayed-template-parsing -triple=x86_64-pc-win32 | FileCheck -check-prefix X64 %s
+// RUN: %clang_cc1 -std=c++20 -fms-compatibility-version=19 -emit-llvm %s -o - -fms-extensions -fdelayed-template-parsing -triple=x86_64-pc-win32 | FileCheck -check-prefix CXX20-X64 %s
+
+// Check that array-to-pointer decay is mangled as the underlying declaration.
+extern const char arr[4] = "foo";
+template<const char*> struct Decay1 {};
+// CHECK: "?decay1@@3U?$Decay1@$1?arr@@3QBDB@@A"
+Decay1<arr> decay1;
+#if __cplusplus >= 201702L
+// Note that this mangling approach can lead to collisions.
+template<const void*> struct Decay2 {};
+// CXX20-X64: "?decay2a@@3U?$Decay2@$1?arr@@3QBDB@@A"
+Decay2<(const void*)arr> decay2a;
+// CXX20-X64: "?decay2b@@3U?$Decay2@$1?arr@@3QBDB@@A"
+Decay2<(const void*)&arr> decay2b;
+#endif
 
 template<typename T>
 class Class {
@@ -327,3 +342,12 @@
 // X64: define {{.*}} @"?fun_uint128@@YAXU?$UInt128@$0DPPPPPPPPPPPPPPPAAAAAAAAAAAAAAAB@@@@Z"(
 void fun_uint128(UInt128<(unsigned __int128)9223372036854775807 * (unsigned __int128)9223372036854775807>) {}
 #endif
+
+#if __cplusplus >= 202002L
+template<float> struct Float {};
+// CXX20-X64: define {{.*}} @"?f@@YAXU?$Float@$ADPIAAAAA@@@@Z"(
+void f(Float<1.0f>) {}
+template<auto> struct Auto {};
+// CXX20-X64: define {{.*}} @"?f@@YAXU?$Auto@$MMADPIAAAAA@@@@Z"(
+void f(Auto<1.0f>) {}
+#endif
Index: clang/test/CodeGenCXX/mangle-template.cpp
===================================================================
--- clang/test/CodeGenCXX/mangle-template.cpp
+++ clang/test/CodeGenCXX/mangle-template.cpp
@@ -226,6 +226,16 @@
 namespace cxx20 {
   template<auto> struct A {};
   template<typename T, T V> struct B {};
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AILf3f800000EEE(
+  void f(A<1.0f>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AILd3ff0000000000000EEE(
+  void f(A<1.0>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AILe3fff8000000000000000EEE(
+  void f(A<1.0l>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXtlCiLi0ELi1EEEEE(
+  void f(A<1i>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXtlCdLd0000000000000000ELd3ff0000000000000EEEEE(
+  void f(A<1.0i>) {}
 
   int x;
   // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXadL_ZNS_1xEEEEE(
@@ -245,7 +255,24 @@
   // CXX20: define {{.*}} @_ZN5cxx201fENS_1BIPKvXadL_ZNS_1xEEEEE(
   void f(B<const void*, (const void*)&x>) {}
 
-  struct Q { int x; };
+  struct Q { int x; } q;
+
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXadsoiL_ZNS_1qEEEEEE(
+  void f(A<&q.x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1BIPiXadsoiL_ZNS_1qEEEEEE(
+  void f(B<int*, &q.x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXadsoKiL_ZNS_1qEEEEEE(
+  void f(A<(const int*)&q.x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1BIPKiXadsoS1_L_ZNS_1qEEEEEE
+  void f(B<const int*, (const int*)&q.x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXcvPvadsoiL_ZNS_1qEEEEEE(
+  void f(A<(void*)&q.x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1BIPvXadsoiL_ZNS_1qEEEEEE(
+  void f(B<void*, (void*)&q.x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXcvPKvadsoiL_ZNS_1qEEEEEE(
+  void f(A<(const void*)&q.x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1BIPKvXadsoiL_ZNS_1qEEEEEE(
+  void f(B<const void*, (const void*)&q.x>) {}
 
   // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXadL_ZNS_1Q1xEEEEE(
   void f(A<&Q::x>) {}
@@ -255,6 +282,17 @@
   void f(A<(const int Q::*)&Q::x>) {}
   // CXX20: define {{.*}} @_ZN5cxx201fENS_1BIMNS_1QEKiXadL_ZNS1_1xEEEEE(
   void f(B<const int Q::*, (const int Q::*)&Q::x>) {}
+
+  struct R : Q {};
+
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXmcMNS_1REiadL_ZNS_1Q1xEEEEEE(
+  void f(A<(int R::*)&Q::x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1BIMNS_1REiXmcS2_adL_ZNS_1Q1xEEEEEE(
+  void f(B<int R::*, (int R::*)&Q::x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1AIXmcMNS_1REKiadL_ZNS_1Q1xEEEEEE(
+  void f(A<(const int R::*)&Q::x>) {}
+  // CXX20: define {{.*}} @_ZN5cxx201fENS_1BIMNS_1REKiXmcS3_adL_ZNS_1Q1xEEEEEE(
+  void f(B<const int R::*, (const int R::*)&Q::x>) {}
 }
 #endif
 
Index: clang/test/CodeGenCXX/template-arguments.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/template-arguments.cpp
@@ -0,0 +1,79 @@
+// RUN: %clang_cc1 -std=c++20 %s -emit-llvm -o - -triple x86_64-linux -DCONSTEXPR= | FileCheck %s
+// RUN: %clang_cc1 -std=c++20 %s -emit-llvm -o - -triple x86_64-linux -DCONSTEXPR=constexpr | FileCheck %s --check-prefix=CONST
+
+template<typename T> CONSTEXPR T id(T v) { return v; }
+template<auto V> auto value = id(V);
+
+// CHECK: call {{.*}} @_Z2idIiET_S0_(i32 noundef 1)
+// CONST: @_Z5valueILi1EE = weak_odr {{.*}} i32 1,
+template int value<1>;
+
+// CHECK: call {{.*}} @_Z2idIyET_S0_(i64 noundef -1)
+// CONST: @_Z5valueILy18446744073709551615EE = weak_odr {{.*}} i64 -1,
+template unsigned long long value<-1ULL>;
+
+// CHECK: call {{.*}} @_Z2idIfET_S0_(float noundef 1.000000e+00)
+// CONST: @_Z5valueILf3f800000EE = weak_odr {{.*}} float 1.000000e+00,
+template float value<1.0f>;
+// CHECK: call {{.*}} @_Z2idIdET_S0_(double noundef 1.000000e+00)
+// CONST: @_Z5valueILd3ff0000000000000EE = weak_odr {{.*}} double 1.000000e+00,
+template double value<1.0>;
+
+int n;
+// CHECK: call {{.*}} @_Z2idIPiET_S1_(ptr noundef @n)
+// CONST: @_Z5valueIXadL_Z1nEEE = weak_odr {{.*}} ptr @n,
+template int *value<&n>;
+
+struct A { int a[3]; } a;
+// CHECK: call {{.*}} @_Z2idIPiET_S1_(ptr noundef @a)
+// CONST: @_Z5valueIXadsoiL_Z1aEEEE = weak_odr {{.*}} ptr @a,
+template int *value<&a.a[0]>;
+// CHECK: call {{.*}} @_Z2idIPiET_S1_(ptr noundef getelementptr (i8, ptr @a, i64 4))
+// CONST: @_Z5valueIXadsoiL_Z1aE4EEE = weak_odr {{.*}} ptr getelementptr (i8, ptr @a, i64 4),
+template int *value<&a.a[1]>;
+// CHECK: call {{.*}} @_Z2idIPiET_S1_(ptr noundef getelementptr (i8, ptr @a, i64 8))
+// CONST: @_Z5valueIXadsoiL_Z1aE8EEE = weak_odr {{.*}} ptr getelementptr (i8, ptr @a, i64 8),
+template int *value<&a.a[2]>;
+// CHECK: call {{.*}} @_Z2idIPiET_S1_(ptr noundef getelementptr (i8, ptr @a, i64 12))
+// CONST: @_Z5valueIXadsoiL_Z1aE12pEEE = weak_odr {{.*}} ptr getelementptr (i8, ptr @a, i64 12),
+template int *value<&a.a[3]>;
+
+struct B { int x, y; };
+// CHECK: call {{.*}} @_Z2idIM1BiET_S2_(i64 0)
+// CONST: @_Z5valueIXadL_ZN1B1xEEEE = weak_odr {{.*}} i64 0,
+template int B::*value<&B::x>;
+// CHECK: call {{.*}} @_Z2idIM1BiET_S2_(i64 4)
+// CONST: @_Z5valueIXadL_ZN1B1yEEEE = weak_odr {{.*}} i64 4,
+template int B::*value<&B::y>;
+
+struct C : A, B { int z; };
+// CHECK: call {{.*}} @_Z2idIM1CiET_S2_(i64 12)
+// CONST: @_Z5valueIXmcM1CiadL_ZN1B1xEE12EEE = weak_odr {{.*}} i64 12,
+template int C::*value<(int C::*)&B::x>;
+// CHECK: call {{.*}} @_Z2idIM1BiET_S2_(i64 8)
+// CONST: @_Z5valueIXmcM1BiadL_ZN1C1zEEn12EEE = weak_odr {{.*}} i64 8,
+template int B::*value<(int B::*)&C::z>;
+
+// CHECK: store i32 1,
+// CHECK: store i32 2,
+// CHECK: load i64,
+// CHECK: call {{.*}} @_Z2idICiET_S1_(i64 noundef %
+// CONST: @_Z5valueIXtlCiLi1ELi2EEEE = weak_odr {{.*}} { i32, i32 } { i32 1, i32 2 },
+template _Complex int value<1 + 2j>;
+
+// CHECK: store float 1.000000e+00,
+// CHECK: store float 2.000000e+00,
+// CHECK: load <2 x float>,
+// CHECK: call {{.*}} @_Z2idICfET_S1_(<2 x float> noundef %
+// CONST: @_Z5valueIXtlCfLf3f800000ELf40000000EEEE = weak_odr {{.*}} { float, float } { float 1.000000e+00, float 2.000000e+00 },
+template _Complex float value<1.0f + 2.0fj>;
+
+using V3i __attribute__((ext_vector_type(3))) = int;
+// CHECK: call {{.*}} @_Z2idIDv3_iET_S1_(<3 x i32> noundef <i32 1, i32 2, i32 3>)
+// CONST: @_Z5valueIXtlDv3_iLi1ELi2ELi3EEEE = weak_odr {{.*}} <3 x i32> <i32 1, i32 2, i32 3>
+template V3i value<V3i{1, 2, 3}>;
+
+using V3f [[gnu::vector_size(12)]] = float;
+// CHECK: call {{.*}} @_Z2idIDv3_fET_S1_(<3 x float> noundef <float 1.000000e+00, float 2.000000e+00, float 3.000000e+00>)
+// CONST: @_Z5valueIXtlDv3_fLf3f800000ELf40000000ELf40400000EEEE = weak_odr {{.*}} <3 x float> <float 1.000000e+00, float 2.000000e+00, float 3.000000e+00>
+template V3f value<V3f{1, 2, 3}>;
Index: clang/test/Modules/odr_hash.cpp
===================================================================
--- clang/test/Modules/odr_hash.cpp
+++ clang/test/Modules/odr_hash.cpp
@@ -13,8 +13,8 @@
 // RUN: cat %s                >> %t/Inputs/second.h
 
 // Test that each header can compile
-// RUN: %clang_cc1 -fsyntax-only -x c++ -std=c++1z %t/Inputs/first.h
-// RUN: %clang_cc1 -fsyntax-only -x c++ -std=c++1z %t/Inputs/second.h
+// RUN: %clang_cc1 -fsyntax-only -x c++ -std=c++20 %t/Inputs/first.h
+// RUN: %clang_cc1 -fsyntax-only -x c++ -std=c++20 %t/Inputs/second.h
 
 // Build module map file
 // RUN: echo "module FirstModule {"     >> %t/Inputs/module.map
@@ -25,7 +25,7 @@
 // RUN: echo "}"                        >> %t/Inputs/module.map
 
 // Run test
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -x c++ -std=c++1z \
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -x c++ -std=c++20 \
 // RUN:   -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache \
 // RUN:   -I%t/Inputs -verify %s
 
@@ -2097,6 +2097,193 @@
 S21 s21;
 #endif
 
+#if defined(FIRST)
+struct S22 {
+  template <double> void f(){};
+  template <> void f<1.5>(){};
+};
+#elif defined(SECOND)
+struct S22 {
+  template <double> void f(){};
+  template <> void f<1.7>(){};
+};
+#else
+S22 s22;
+// expected-error@second.h:* {{'TemplateArgument::S22' has different definitions in different modules; first difference is definition in module 'SecondModule' found method 'f' with 1.700000e+00 for 1st template argument}}
+// expected-note@first.h:* {{but in 'FirstModule' found method 'f' with 1.500000e+00 for 1st template argument}}
+#endif
+
+#if defined(FIRST)
+struct S23 {
+  template <double> void f(){};
+  template <> void f<2.7>(){};
+};
+#elif defined(SECOND)
+struct S23 {
+  template <double> void f(){};
+  template <> void f<2.7>(){};
+};
+#else
+S23 s23;
+#endif
+
+#if defined(FIRST) || defined(SECOND)
+struct Composite {
+  int n1[4];
+  int n2[4];
+};
+extern Composite composite;
+#endif
+
+#if defined(FIRST)
+struct S24 {
+  template <int&> void f(){};
+  template <> void f<composite.n1[1]>(){};
+};
+#elif defined(SECOND)
+struct S24 {
+  template <int&> void f(){};
+  template <> void f<composite.n1[2]>(){};
+};
+#else
+S24 s24;
+// expected-error@second.h:* {{'TemplateArgument::S24' has different definitions in different modules; first difference is definition in module 'SecondModule' found method 'f' with composite.n1[2] for 1st template argument}}
+// expected-note@first.h:* {{but in 'FirstModule' found method 'f' with composite.n1[1] for 1st template argument}}
+#endif
+
+#if defined(FIRST) || defined(SECOND)
+struct S25 {
+  template <int&> void f();
+  template <> void f<composite.n1[2]>();
+};
+#else
+S25 s25;
+#endif
+
+#if defined(FIRST)
+struct S26 {
+  template <int*> void f(){};
+  template <> void f<&composite.n1[4]>(){}; // Past-the-end pointer.
+};
+#elif defined(SECOND)
+struct S26 {
+  template <int*> void f(){};
+  template <> void f<&composite.n2[0]>(){};
+};
+#else
+S26 s26;
+// expected-error@second.h:* {{'TemplateArgument::S26' has different definitions in different modules; first difference is definition in module 'SecondModule' found method 'f' with &composite.n2[0] for 1st template argument}}
+// expected-note@first.h:* {{but in 'FirstModule' found method 'f' with &composite.n1[4] for 1st template argument}}
+#endif
+
+#if defined(FIRST) || defined(SECOND)
+union Union {
+  int i1;
+  int i2;
+};
+extern Union u;
+#endif
+
+#if defined(FIRST)
+struct S27 {
+  template <int&> void f(){};
+  template <> void f<u.i1>(){};
+};
+#elif defined(SECOND)
+struct S27 {
+  template <int&> void f(){};
+  template <> void f<u.i2>(){};
+};
+#else
+S27 s27;
+// expected-error@second.h:* {{'TemplateArgument::S27' has different definitions in different modules; first difference is definition in module 'SecondModule' found method 'f' with u.i2 for 1st template argument}}
+// expected-note@first.h:* {{but in 'FirstModule' found method 'f' with u.i1 for 1st template argument}}
+#endif
+
+#if defined(FIRST) || defined(SECOND)
+struct S28 {
+  template <int&> void f(){};
+  template <> void f<u.i1>(){};
+};
+#else
+S28 s28;
+#endif
+
+#if defined(FIRST) || defined(SECOND)
+struct A {
+  int a;
+};
+struct B : A {};
+struct C : A {};
+struct D : B, C {};
+#endif
+
+#if defined(FIRST)
+struct S29 {
+  template <int D::*> void f(){};
+  template <> void f<(int D::*)(int B::*)&A::a>(){};
+};
+#elif defined(SECOND)
+struct S29 {
+  template <int D::*> void f(){};
+  template <> void f<(int D::*)(int C::*)&A::a>(){};
+};
+#else
+S29 s29;
+// expected-error@second.h:* {{'TemplateArgument::S29' has different definitions in different modules; first difference is definition in module 'SecondModule' found method 'f' with &A::a for 1st template argument}}
+// expected-note@first.h:* {{but in 'FirstModule' found method 'f' with &A::a for 1st template argument}}
+#endif
+
+#if defined(FIRST) || defined(SECOND)
+struct S30 {
+  template <int D::*> void f(){};
+  template <> void f<(int D::*)(int B::*)&A::a>(){};
+};
+#else
+S30 s30;
+#endif
+
+#if defined(FIRST)
+struct S31 {
+  template <auto*> void f(){};
+  template <> void f<&composite.n1[2]>(){};
+};
+#elif defined(SECOND)
+struct S31 {
+  template <auto*> void f(){};
+  template <> void f<(void*)&composite.n1[2]>(){};
+};
+#else
+S31 s31;
+// expected-error@second.h:* {{'TemplateArgument::S31' has different definitions in different modules; first difference is definition in module 'SecondModule' found method 'f' with &composite.n1[2] for 1st template argument}}
+// expected-note@first.h:* {{but in 'FirstModule' found method 'f' with &composite.n1[2] for 1st template argument}}
+#endif
+
+#if defined(FIRST)
+struct S32 {
+  template <int*> void f(){};
+  template <> void f<__builtin_constant_p(0) ? (int*)1 : (int*)1>(){};
+};
+#elif defined(SECOND)
+struct S32 {
+  template <int*> void f(){};
+  template <> void f<__builtin_constant_p(0) ? (int*)2 : (int*)2>(){};
+};
+#else
+S32 s32;
+// expected-error@second.h:* {{'TemplateArgument::S32' has different definitions in different modules; first difference is definition in module 'SecondModule' found method 'f' with (int *)2 for 1st template argument}}
+// expected-note@first.h:* {{but in 'FirstModule' found method 'f' with (int *)1 for 1st template argument}}
+#endif
+
+#if defined(FIRST) || defined(SECOND)
+struct S33 {
+  template <int*> void f(){};
+  template <> void f<__builtin_constant_p(0) ? (int*)1 : (int*)1>(){};
+};
+#else
+S33 s33;
+#endif
+
 #define DECLS                   \
   OneClass<int> a;              \
   OneInt<1> b;                  \
Index: clang/test/SemaCXX/warn-bool-conversion.cpp
===================================================================
--- clang/test/SemaCXX/warn-bool-conversion.cpp
+++ clang/test/SemaCXX/warn-bool-conversion.cpp
@@ -186,6 +186,7 @@
   }
 }
 
+#if __cplusplus < 201703L
 namespace Template {
   // FIXME: These cases should not warn.
   template<int *p> void f() { if (p) {} } // expected-warning 2{{will always evaluate to 'true'}} expected-cxx11-warning {{implicit conversion of nullptr}}
@@ -205,3 +206,4 @@
 #endif
   template void h<d>();
 }
+#endif // __cplusplus < 201703L
Index: clang/test/SemaTemplate/temp_arg_nontype_cxx1z.cpp
===================================================================
--- clang/test/SemaTemplate/temp_arg_nontype_cxx1z.cpp
+++ clang/test/SemaTemplate/temp_arg_nontype_cxx1z.cpp
@@ -2,7 +2,7 @@
 
 template<typename T, T val> struct A {};
 
-template<typename T, typename U> constexpr bool is_same = false; // expected-note +{{here}}
+template<typename T, typename U> constexpr bool is_same = false;
 template<typename T> constexpr bool is_same<T, T> = true;
 
 namespace String {
@@ -84,34 +84,32 @@
   constexpr int B::*b = &B::b;
   constexpr int C::*cb = b;
   constexpr int D::*db = b;
-  constexpr int E::*ecb = cb; // expected-note +{{here}}
-  constexpr int E::*edb = db; // expected-note +{{here}}
+  constexpr int E::*ecb = cb;
+  constexpr int E::*edb = db;
 
   constexpr int E::*e = &E::e;
   constexpr int D::*de = (int D::*)e;
   constexpr int C::*ce = (int C::*)e;
-  constexpr int B::*bde = (int B::*)de; // expected-note +{{here}}
-  constexpr int B::*bce = (int B::*)ce; // expected-note +{{here}}
+  constexpr int B::*bde = (int B::*)de;
+  constexpr int B::*bce = (int B::*)ce;
 
-  // FIXME: This should all be accepted, but we don't yet have a representation
-  // nor mangling for this form of template argument.
   using Ab = A<int B::*, b>;
   using Ab = A<int B::*, &B::b>;
-  using Abce = A<int B::*, bce>; // expected-error {{not supported}}
-  using Abde = A<int B::*, bde>; // expected-error {{not supported}}
-  static_assert(!is_same<Ab, Abce>, ""); // expected-error {{undeclared}} expected-error {{must be a type}}
-  static_assert(!is_same<Ab, Abde>, ""); // expected-error {{undeclared}} expected-error {{must be a type}}
-  static_assert(!is_same<Abce, Abde>, ""); // expected-error 2{{undeclared}} expected-error {{must be a type}}
-  static_assert(is_same<Abce, A<int B::*, (int B::*)(int C::*)&E::e>>, ""); // expected-error {{undeclared}} expected-error {{not supported}}
+  using Abce = A<int B::*, bce>;
+  using Abde = A<int B::*, bde>;
+  static_assert(!is_same<Ab, Abce>, "");
+  static_assert(!is_same<Ab, Abde>, "");
+  static_assert(!is_same<Abce, Abde>, "");
+  static_assert(is_same<Abce, A<int B::*, (int B::*)(int C::*)&E::e>>, "");
 
   using Ae = A<int E::*, e>;
   using Ae = A<int E::*, &E::e>;
-  using Aecb = A<int E::*, ecb>; // expected-error {{not supported}}
-  using Aedb = A<int E::*, edb>; // expected-error {{not supported}}
-  static_assert(!is_same<Ae, Aecb>, ""); // expected-error {{undeclared}} expected-error {{must be a type}}
-  static_assert(!is_same<Ae, Aedb>, ""); // expected-error {{undeclared}} expected-error {{must be a type}}
-  static_assert(!is_same<Aecb, Aedb>, ""); // expected-error 2{{undeclared}} expected-error {{must be a type}}
-  static_assert(is_same<Aecb, A<int E::*, (int E::*)(int C::*)&B::b>>, ""); // expected-error {{undeclared}} expected-error {{not supported}}
+  using Aecb = A<int E::*, ecb>;
+  using Aedb = A<int E::*, edb>;
+  static_assert(!is_same<Ae, Aecb>, "");
+  static_assert(!is_same<Ae, Aedb>, "");
+  static_assert(!is_same<Aecb, Aedb>, "");
+  static_assert(is_same<Aecb, A<int E::*, (int E::*)(int C::*)&B::b>>, "");
 
   using An = A<int E::*, nullptr>;
   using A0 = A<int E::*, (int E::*)0>;
@@ -205,9 +203,9 @@
 
     struct Y : X {};
     void type_affects_identity(B<&X::n>) {}
-    void type_affects_identity(B<(int Y::*)&X::n>) {} // FIXME: expected-error {{sorry}}
+    void type_affects_identity(B<(int Y::*)&X::n>) {}
     void type_affects_identity(B<(const int X::*)&X::n>) {}
-    void type_affects_identity(B<(const int Y::*)&X::n>) {} // FIXME: expected-error {{sorry}}
+    void type_affects_identity(B<(const int Y::*)&X::n>) {}
 
     // A case where we need to do auto-deduction, and check whether the
     // resulting dependent types match during partial ordering. These
Index: clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
===================================================================
--- clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
+++ clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
@@ -8,8 +8,8 @@
 
 // floating-point arguments
 template<float> struct Float {};
-using F1 = Float<1.0f>; // FIXME expected-error {{sorry}}
-using F1 = Float<2.0f / 2>; // FIXME expected-error {{sorry}}
+using F1 = Float<1.0f>;
+using F1 = Float<2.0f / 2>;
 
 struct S { int n[3]; } s; // expected-note 1+{{here}}
 union U { int a, b; } u;
@@ -17,24 +17,24 @@
 
 // pointers to subobjects
 template<int *> struct IntPtr {};
-using IPn = IntPtr<&n + 1>; // FIXME expected-error {{refers to subobject}}
-using IPn = IntPtr<&n + 1>; // FIXME expected-error {{refers to subobject}}
+using IPn = IntPtr<&n + 1>;
+using IPn = IntPtr<&n + 1>;
 
-using IP2 = IntPtr<&s.n[2]>; // FIXME expected-error {{refers to subobject}}
-using IP2 = IntPtr<s.n + 2>; // FIXME expected-error {{refers to subobject}}
+using IP2 = IntPtr<&s.n[2]>;
+using IP2 = IntPtr<s.n + 2>;
 
-using IP3 = IntPtr<&s.n[3]>; // FIXME expected-error {{refers to subobject}}
-using IP3 = IntPtr<s.n + 3>; // FIXME expected-error {{refers to subobject}}
+using IP3 = IntPtr<&s.n[3]>;
+using IP3 = IntPtr<s.n + 3>;
 
 template<int &> struct IntRef {};
-using IPn = IntRef<*(&n + 1)>; // expected-error {{not a constant expression}} expected-note {{dereferenced pointer past the end of 'n'}}
-using IPn = IntRef<*(&n + 1)>; // expected-error {{not a constant expression}} expected-note {{dereferenced pointer past the end of 'n'}}
+using IRn = IntRef<*(&n + 1)>; // expected-error {{not a constant expression}} expected-note {{dereferenced pointer past the end of 'n'}}
+using IRn = IntRef<*(&n + 1)>; // expected-error {{not a constant expression}} expected-note {{dereferenced pointer past the end of 'n'}}
 
-using IP2 = IntRef<s.n[2]>; // FIXME expected-error {{refers to subobject}}
-using IP2 = IntRef<*(s.n + 2)>; // FIXME expected-error {{refers to subobject}}
+using IR2 = IntRef<s.n[2]>;
+using IR2 = IntRef<*(s.n + 2)>;
 
-using IP3 = IntRef<s.n[3]>; // expected-error {{not a constant expression}} expected-note {{dereferenced pointer past the end of subobject of 's'}}
-using IP3 = IntRef<*(s.n + 3)>; // expected-error {{not a constant expression}} expected-note {{dereferenced pointer past the end of subobject of 's'}}
+using IR3 = IntRef<s.n[3]>; // expected-error {{not a constant expression}} expected-note {{dereferenced pointer past the end of subobject of 's'}}
+using IR3 = IntRef<*(s.n + 3)>; // expected-error {{not a constant expression}} expected-note {{dereferenced pointer past the end of subobject of 's'}}
 
 // classes
 template<S> struct Struct {};
@@ -48,12 +48,12 @@
 
 // miscellaneous scalar types
 template<_Complex int> struct ComplexInt {};
-using CI = ComplexInt<1 + 3i>; // FIXME: expected-error {{sorry}}
-using CI = ComplexInt<1 + 3i>; // FIXME: expected-error {{sorry}}
+using CI = ComplexInt<1 + 3i>;
+using CI = ComplexInt<3i + 1>;
 
 template<_Complex float> struct ComplexFloat {};
-using CF = ComplexFloat<1.0f + 3.0fi>; // FIXME: expected-error {{sorry}}
-using CF = ComplexFloat<1.0f + 3.0fi>; // FIXME: expected-error {{sorry}}
+using CF = ComplexFloat<1.0f + 3.0fi>;
+using CF = ComplexFloat<3.0fi + 1.0f>;
 
 namespace ClassNTTP {
   struct A { // expected-note 2{{candidate}}
Index: clang/tools/libclang/CIndex.cpp
===================================================================
--- clang/tools/libclang/CIndex.cpp
+++ clang/tools/libclang/CIndex.cpp
@@ -1570,6 +1570,11 @@
       return Visit(MakeCXCursor(E, StmtParent, TU, RegionOfInterest));
     return false;
 
+  case TemplateArgument::UncommonValue:
+    if (Expr *E = TAL.getSourceUncommonValueExpression())
+      return Visit(MakeCXCursor(E, StmtParent, TU, RegionOfInterest));
+    return false;
+
   case TemplateArgument::NullPtr:
     if (Expr *E = TAL.getSourceNullPtrExpression())
       return Visit(MakeCXCursor(E, StmtParent, TU, RegionOfInterest));
Index: clang/tools/libclang/CXCursor.cpp
===================================================================
--- clang/tools/libclang/CXCursor.cpp
+++ clang/tools/libclang/CXCursor.cpp
@@ -1463,6 +1463,9 @@
     return CXTemplateArgumentKind_NullPtr;
   case TemplateArgument::Integral:
     return CXTemplateArgumentKind_Integral;
+  case TemplateArgument::UncommonValue:
+    // FIXME: Expose these values.
+    return CXTemplateArgumentKind_Invalid;
   case TemplateArgument::Template:
     return CXTemplateArgumentKind_Template;
   case TemplateArgument::TemplateExpansion:
Index: lldb/include/lldb/lldb-enumerations.h
===================================================================
--- lldb/include/lldb/lldb-enumerations.h
+++ lldb/include/lldb/lldb-enumerations.h
@@ -834,6 +834,7 @@
   eTemplateArgumentKindExpression,
   eTemplateArgumentKindPack,
   eTemplateArgumentKindNullPtr,
+  eTemplateArgumentKindUncommonValue,
 };
 
 /// Type of match to be performed when looking for a formatter for a data type.
Index: lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
===================================================================
--- lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
+++ lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
@@ -7293,6 +7293,9 @@
 
   case clang::TemplateArgument::Pack:
     return eTemplateArgumentKindPack;
+
+  case clang::TemplateArgument::UncommonValue:
+    return eTemplateArgumentKindUncommonValue;
   }
   llvm_unreachable("Unhandled clang::TemplateArgument::ArgKind");
 }