From 273d0e6149a8326284ec1e385e83c8728c41ff68 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser@berlin.de>
Date: Fri, 27 Sep 2024 22:11:14 +0200
Subject: [PATCH] [Clang] Add __builtin_common_reference

---
 clang/docs/LanguageExtensions.rst             |  17 +
 clang/include/clang/Basic/BuiltinTemplates.td |  30 +-
 clang/include/clang/Sema/Sema.h               |  19 +
 clang/lib/Sema/SemaExprCXX.cpp                |  92 +----
 clang/lib/Sema/SemaTemplate.cpp               | 353 +++++++++++++++++-
 clang/lib/Sema/SemaType.cpp                   |  75 ++++
 .../SemaCXX/type-trait-common-reference.cpp   | 136 +++++++
 .../include/__type_traits/common_reference.h  |  37 +-
 libcxx/include/module.modulemap               |   1 +
 9 files changed, 642 insertions(+), 118 deletions(-)
 create mode 100644 clang/test/SemaCXX/type-trait-common-reference.cpp

diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index 3b8a9cac6587a..d5282758948cd 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -1677,6 +1677,23 @@ Builtin type aliases
 
 Clang provides a few builtin aliases to improve the throughput of certain metaprogramming facilities.
 
+__builtin_common_reference
+--------------------------
+
+.. code-block:: c++
+
+  template <template <class, class, template <class> class, template <class> class> class BasicCommonReferenceT,
+            template <class... Args> CommonTypeT,
+            template <class> HasTypeMember,
+            class HasNoTypeMember,
+            class... Ts>
+  using __builtin_common_reference = ...;
+
+This alias is used for implementing ``std::common_reference``. If ``std::common_reference`` should contain a ``type``
+member, it is an alias to ``HasTypeMember<TheCommonReference>``. Otherwse it is an alias to ``HasNoTypeMember``. The
+``CommonTypeT`` is usually ``std::common_type_t``. ``BasicCommonReferenceT`` is usually an alias template to
+``basic_common_reference<T, U, TX, UX>::type``.
+
 __builtin_common_type
 ---------------------
 
diff --git a/clang/include/clang/Basic/BuiltinTemplates.td b/clang/include/clang/Basic/BuiltinTemplates.td
index d46ce063d2f7e..5c79e89800829 100644
--- a/clang/include/clang/Basic/BuiltinTemplates.td
+++ b/clang/include/clang/Basic/BuiltinTemplates.td
@@ -10,11 +10,11 @@ class TemplateArg<string name> {
   string Name = name;
 }
 
-class Template<list<TemplateArg> args, string name> : TemplateArg<name> {
+class Template<list<TemplateArg> args, string name = ""> : TemplateArg<name> {
   list<TemplateArg> Args = args;
 }
 
-class Class<string name, bit is_variadic = 0> : TemplateArg<name> {
+class Class<string name = "", bit is_variadic = 0> : TemplateArg<name> {
   bit IsVariadic = is_variadic;
 }
 
@@ -50,3 +50,29 @@ def __builtin_common_type : BuiltinTemplate<
    Template<[Class<"TypeMember">], "HasTypeMember">,
    Class<"HasNoTypeMember">,
    Class<"Ts", /*is_variadic=*/1>]>;
+
+// template <template <class,"
+//                     class,"
+//                     template <class> class,"
+//                     template <class> class> class BasicCommonReferenceT,"
+//           template <class... Args> class CommonTypeT,"
+//           template <class> class HasTypeMember,"
+//           class HasNoTypeMember,"
+//           class... Ts>"
+def __builtin_common_reference : BuiltinTemplate<
+            [Template<[Class<>,
+                       Class<>,
+                       Template<[Class<>]>,
+                       Template<[Class<>]>], "BasicCommonReferenceT">,
+             Template<[Class<"Args", /*is_variadic=*/1>], "CommonTypeT">,
+             Template<[Class<>], "HasTypeMember">,
+             Class<"HasNoTypeMember">,
+             Class<"Ts", /*is_variadic=*/1>]>;
+
+foreach Ref = ["", "lvalue", "rvalue"] in {
+  foreach Const = ["", "const"] in {
+    foreach Volatile = ["", "volatile"] in {
+      def __clang_internal_xref_#Ref#Const#Volatile : BuiltinTemplate<[Class<>]>;
+    }
+  }
+}
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 066bce61c74c1..762cb851ee04f 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -15076,15 +15076,34 @@ class Sema final : public SemaBase {
   QualType BuiltinDecay(QualType BaseType, SourceLocation Loc);
   QualType BuiltinAddReference(QualType BaseType, UTTKind UKind,
                                SourceLocation Loc);
+
+  QualType BuiltinAddRValueReference(QualType BaseType, SourceLocation Loc) {
+    return BuiltinAddReference(BaseType, UnaryTransformType::AddRvalueReference,
+                               Loc);
+  }
+
+  QualType BuiltinAddLValueReference(QualType BaseType, SourceLocation Loc) {
+    return BuiltinAddReference(BaseType, UnaryTransformType::AddLvalueReference,
+                               Loc);
+  }
+
   QualType BuiltinRemoveExtent(QualType BaseType, UTTKind UKind,
                                SourceLocation Loc);
   QualType BuiltinRemoveReference(QualType BaseType, UTTKind UKind,
                                   SourceLocation Loc);
+
+  QualType BuiltinRemoveCVRef(QualType BaseType, SourceLocation Loc) {
+    return BuiltinRemoveReference(BaseType, UTTKind::RemoveCVRef, Loc);
+  }
+
   QualType BuiltinChangeCVRQualifiers(QualType BaseType, UTTKind UKind,
                                       SourceLocation Loc);
   QualType BuiltinChangeSignedness(QualType BaseType, UTTKind UKind,
                                    SourceLocation Loc);
 
+  bool BuiltinIsConvertible(QualType From, QualType To, SourceLocation Loc,
+                            bool CheckNothrow = false);
+
   /// Ensure that the type T is a literal type.
   ///
   /// This routine checks whether the type @p T is a literal type. If @p T is an
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 46895db4a0756..5ca5a8a57fa0f 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -5739,76 +5739,6 @@ static bool EvaluateUnaryTypeTrait(Sema &Self, TypeTrait UTT,
 static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, const TypeSourceInfo *Lhs,
                                     const TypeSourceInfo *Rhs, SourceLocation KeyLoc);
 
-static ExprResult CheckConvertibilityForTypeTraits(
-    Sema &Self, const TypeSourceInfo *Lhs, const TypeSourceInfo *Rhs,
-    SourceLocation KeyLoc, llvm::BumpPtrAllocator &OpaqueExprAllocator) {
-
-  QualType LhsT = Lhs->getType();
-  QualType RhsT = Rhs->getType();
-
-  // C++0x [meta.rel]p4:
-  //   Given the following function prototype:
-  //
-  //     template <class T>
-  //       typename add_rvalue_reference<T>::type create();
-  //
-  //   the predicate condition for a template specialization
-  //   is_convertible<From, To> shall be satisfied if and only if
-  //   the return expression in the following code would be
-  //   well-formed, including any implicit conversions to the return
-  //   type of the function:
-  //
-  //     To test() {
-  //       return create<From>();
-  //     }
-  //
-  //   Access checking is performed as if in a context unrelated to To and
-  //   From. Only the validity of the immediate context of the expression
-  //   of the return-statement (including conversions to the return type)
-  //   is considered.
-  //
-  // We model the initialization as a copy-initialization of a temporary
-  // of the appropriate type, which for this expression is identical to the
-  // return statement (since NRVO doesn't apply).
-
-  // Functions aren't allowed to return function or array types.
-  if (RhsT->isFunctionType() || RhsT->isArrayType())
-    return ExprError();
-
-  // A function definition requires a complete, non-abstract return type.
-  if (!Self.isCompleteType(Rhs->getTypeLoc().getBeginLoc(), RhsT) ||
-      Self.isAbstractType(Rhs->getTypeLoc().getBeginLoc(), RhsT))
-    return ExprError();
-
-  // Compute the result of add_rvalue_reference.
-  if (LhsT->isObjectType() || LhsT->isFunctionType())
-    LhsT = Self.Context.getRValueReferenceType(LhsT);
-
-  // Build a fake source and destination for initialization.
-  InitializedEntity To(InitializedEntity::InitializeTemporary(RhsT));
-  Expr *From = new (OpaqueExprAllocator.Allocate<OpaqueValueExpr>())
-      OpaqueValueExpr(KeyLoc, LhsT.getNonLValueExprType(Self.Context),
-                      Expr::getValueKindForType(LhsT));
-  InitializationKind Kind =
-      InitializationKind::CreateCopy(KeyLoc, SourceLocation());
-
-  // Perform the initialization in an unevaluated context within a SFINAE
-  // trap at translation unit scope.
-  EnterExpressionEvaluationContext Unevaluated(
-      Self, Sema::ExpressionEvaluationContext::Unevaluated);
-  Sema::SFINAETrap SFINAE(Self, /*AccessCheckingSFINAE=*/true);
-  Sema::ContextRAII TUContext(Self, Self.Context.getTranslationUnitDecl());
-  InitializationSequence Init(Self, To, Kind, From);
-  if (Init.Failed())
-    return ExprError();
-
-  ExprResult Result = Init.Perform(Self, To, Kind, From);
-  if (Result.isInvalid() || SFINAE.hasErrorOccurred())
-    return ExprError();
-
-  return Result;
-}
-
 static APValue EvaluateSizeTTypeTrait(Sema &S, TypeTrait Kind,
                                       SourceLocation KWLoc,
                                       ArrayRef<TypeSourceInfo *> Args,
@@ -5958,9 +5888,8 @@ static bool EvaluateBooleanTypeTrait(Sema &S, TypeTrait Kind,
           S.Context.getPointerType(T.getNonReferenceType()));
       TypeSourceInfo *UPtr = S.Context.CreateTypeSourceInfo(
           S.Context.getPointerType(U.getNonReferenceType()));
-      return !CheckConvertibilityForTypeTraits(S, UPtr, TPtr, RParenLoc,
-                                               OpaqueExprAllocator)
-                  .isInvalid();
+      return S.BuiltinIsConvertible(UPtr->getType(), TPtr->getType(),
+                                    RParenLoc);
     }
 
     if (Kind == clang::TT_IsNothrowConstructible)
@@ -6200,20 +6129,9 @@ static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, const TypeSourceI
   }
   case BTT_IsConvertible:
   case BTT_IsConvertibleTo:
-  case BTT_IsNothrowConvertible: {
-    if (RhsT->isVoidType())
-      return LhsT->isVoidType();
-    llvm::BumpPtrAllocator OpaqueExprAllocator;
-    ExprResult Result = CheckConvertibilityForTypeTraits(Self, Lhs, Rhs, KeyLoc,
-                                                         OpaqueExprAllocator);
-    if (Result.isInvalid())
-      return false;
-
-    if (BTT != BTT_IsNothrowConvertible)
-      return true;
-
-    return Self.canThrow(Result.get()) == CT_Cannot;
-  }
+  case BTT_IsNothrowConvertible:
+    return Self.BuiltinIsConvertible(LhsT, RhsT, KeyLoc,
+                                     BTT == BTT_IsNothrowConvertible);
 
   case BTT_IsAssignable:
   case BTT_IsNothrowAssignable:
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index be81b6a46b2c0..47601c1840b92 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -3097,6 +3097,33 @@ void Sema::NoteAllFoundTemplates(TemplateName Name) {
   }
 }
 
+static QualType InstantiateTemplate(Sema &S, TemplateName Template,
+                                    ArrayRef<TemplateArgument> Args,
+                                    SourceLocation Loc) {
+  TemplateArgumentListInfo ArgList;
+  for (auto Arg : Args) {
+    if (Arg.getKind() == TemplateArgument::Type) {
+      ArgList.addArgument(TemplateArgumentLoc(
+          Arg, S.Context.getTrivialTypeSourceInfo(Arg.getAsType())));
+    } else {
+      ArgList.addArgument(
+          S.getTrivialTemplateArgumentLoc(Arg, QualType(), Loc));
+    }
+  }
+
+  EnterExpressionEvaluationContext UnevaluatedContext(
+      S, Sema::ExpressionEvaluationContext::Unevaluated);
+  Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+  Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
+
+  QualType Instantiation = S.CheckTemplateIdType(Template, Loc, ArgList);
+
+  if (SFINAE.hasErrorOccurred())
+    return QualType();
+
+  return Instantiation;
+}
+
 static QualType builtinCommonTypeImpl(Sema &S, TemplateName BaseTemplate,
                                       SourceLocation TemplateLoc,
                                       ArrayRef<TemplateArgument> Ts) {
@@ -3107,24 +3134,7 @@ static QualType builtinCommonTypeImpl(Sema &S, TemplateName BaseTemplate,
     if (T1.getAsType()->isBuiltinType() && T2.getAsType()->isBuiltinType())
       return builtinCommonTypeImpl(S, BaseTemplate, TemplateLoc, {T1, T2});
 
-    TemplateArgumentListInfo Args;
-    Args.addArgument(TemplateArgumentLoc(
-        T1, S.Context.getTrivialTypeSourceInfo(T1.getAsType())));
-    Args.addArgument(TemplateArgumentLoc(
-        T2, S.Context.getTrivialTypeSourceInfo(T2.getAsType())));
-
-    EnterExpressionEvaluationContext UnevaluatedContext(
-        S, Sema::ExpressionEvaluationContext::Unevaluated);
-    Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
-    Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
-
-    QualType BaseTemplateInst =
-        S.CheckTemplateIdType(BaseTemplate, TemplateLoc, Args);
-
-    if (SFINAE.hasErrorOccurred())
-      return QualType();
-
-    return BaseTemplateInst;
+    return InstantiateTemplate(S, BaseTemplate, {T1, T2}, TemplateLoc);
   };
 
   // Note A: For the common_type trait applied to a template parameter pack T of
@@ -3231,6 +3241,230 @@ static QualType builtinCommonTypeImpl(Sema &S, TemplateName BaseTemplate,
   }
 }
 
+static QualType CopyCV(QualType From, QualType To) {
+  if (From.isConstQualified())
+    To.addConst();
+  if (From.isVolatileQualified())
+    To.addVolatile();
+  return To;
+}
+
+// COND-RES(X, Y) be decltype(false ? declval<X(&)()>()() : declval<Y(&)()>()())
+static QualType CondRes(Sema &S, QualType X, QualType Y, SourceLocation Loc) {
+  EnterExpressionEvaluationContext UnevaluatedContext(
+      S, Sema::ExpressionEvaluationContext::Unevaluated);
+  Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+  Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
+
+  // false
+  OpaqueValueExpr CondExpr(SourceLocation(), S.Context.BoolTy, VK_PRValue);
+  ExprResult Cond = &CondExpr;
+
+  // declval<X(&)()>()()
+  OpaqueValueExpr LHSExpr(Loc, X.getNonLValueExprType(S.Context),
+                          Expr::getValueKindForType(X));
+  ExprResult LHS = &LHSExpr;
+
+  // declval<Y(&)()>()()
+  OpaqueValueExpr RHSExpr(Loc, Y.getNonLValueExprType(S.Context),
+                          Expr::getValueKindForType(Y));
+  ExprResult RHS = &RHSExpr;
+
+  ExprValueKind VK = VK_PRValue;
+  ExprObjectKind OK = OK_Ordinary;
+
+  // decltype(false ? declval<X(&)()>()() : declval<Y(&)()>()())
+  QualType Result = S.CheckConditionalOperands(Cond, LHS, RHS, VK, OK, Loc);
+
+  if (SFINAE.hasErrorOccurred())
+    return QualType();
+  if (VK == VK_LValue)
+    return S.BuiltinAddLValueReference(Result, Loc);
+  if (VK == VK_XValue)
+    return S.BuiltinAddRValueReference(Result, Loc);
+  return Result;
+}
+
+static QualType CommonRef(Sema &S, QualType A, QualType B, SourceLocation Loc) {
+  // Given types A and B, let X be remove_reference_t<A>, let Y be
+  // remove_reference_t<B>, and let COMMON-​REF(A, B) be:
+  assert(A->isReferenceType() && B->isReferenceType() &&
+         "A and B have to be ref qualified for a COMMON-REF");
+  auto X = A.getNonReferenceType();
+  auto Y = B.getNonReferenceType();
+
+  // If A and B are both lvalue reference types, COMMON-REF(A, B) is
+  // COND-RES(COPYCV(X, Y) &, COPYCV(​Y, X) &) if that type exists and is a
+  // reference type.
+  if (A->isLValueReferenceType() && B->isLValueReferenceType()) {
+    auto CR = CondRes(S, S.BuiltinAddLValueReference(CopyCV(X, Y), Loc),
+                      S.BuiltinAddLValueReference(CopyCV(Y, X), Loc), Loc);
+    if (CR.isNull() || !CR->isReferenceType())
+      return QualType();
+    return CR;
+  }
+
+  // Otherwise, let C be remove_reference_t<COMMON-REF(X&, Y&)>&&. If A and B
+  // are both rvalue reference types, C is well-formed, and
+  // is_convertible_v<A, C> && is_convertible_v<B, C> is true, then
+  // COMMON-REF(A, B) is C.
+  if (A->isRValueReferenceType() && B->isRValueReferenceType()) {
+    auto C = CommonRef(S, S.BuiltinAddLValueReference(X, Loc),
+                       S.BuiltinAddLValueReference(Y, Loc), Loc);
+    if (C.isNull())
+      return QualType();
+
+    C = C.getNonReferenceType();
+
+    if (S.BuiltinIsConvertible(A, C, Loc) && S.BuiltinIsConvertible(B, C, Loc))
+      return S.BuiltinAddRValueReference(C, Loc);
+    return QualType();
+  }
+
+  // Otherwise, if A is an lvalue reference and B is an rvalue reference, then
+  // COMMON-REF(A, B) is COMMON-REF(B, A).
+  if (A->isLValueReferenceType() && B->isRValueReferenceType())
+    std::swap(A, B);
+
+  // Otherwise, let D be COMMON-REF(const X&, Y&). If A is an rvalue reference
+  // and B is an lvalue reference and D is well-formed and
+  // is_convertible_v<A, D> is true, then COMMON-REF(A, B) is D.
+  if (A->isRValueReferenceType() && B->isLValueReferenceType()) {
+    auto X2 = X;
+    X2.addConst();
+    auto D = CommonRef(S, S.BuiltinAddLValueReference(X2, Loc),
+                       S.BuiltinAddLValueReference(Y, Loc), Loc);
+    if (!D.isNull() && S.BuiltinIsConvertible(A, D, Loc))
+      return D;
+    return QualType();
+  }
+
+  // Otherwise, COMMON-REF(A, B) is ill-formed.
+  // This is implemented by returning from the individual branches above.
+
+  llvm_unreachable("The above cases should be exhaustive");
+}
+
+static QualType builtinCommonReferenceImpl(Sema &S,
+                                           TemplateName CommonReference,
+                                           TemplateName CommonType,
+                                           SourceLocation TemplateLoc,
+                                           ArrayRef<TemplateArgument> Ts) {
+  switch (Ts.size()) {
+  // If sizeof...(T) is zero, there shall be no member type.
+  case 0:
+    return QualType();
+
+  // Otherwise, if sizeof...(T) is one, let T0 denote the sole type in the
+  // pack T. The member typedef type shall denote the same type as T0.
+  case 1:
+    return Ts[0].getAsType();
+
+  // Otherwise, if sizeof...(T) is two, let T1 and T2 denote the two types in
+  // the pack T. Then
+  case 2: {
+    auto T1 = Ts[0].getAsType();
+    auto T2 = Ts[1].getAsType();
+
+    // Let R be COMMON-REF(T1, T2). If T1 and T2 are reference types, R is
+    // well-formed, and is_convertible_v<add_pointer_t<T1>, add_pointer_t<R>> &&
+    // is_convertible_v<add_pointer_t<T2>, add_pointer_t<R>> is true, then the
+    // member typedef type denotes R.
+    if (T1->isReferenceType() && T2->isReferenceType()) {
+      QualType R = CommonRef(S, T1, T2, TemplateLoc);
+      if (!R.isNull()) {
+        if (S.BuiltinIsConvertible(S.BuiltinAddPointer(T1, TemplateLoc),
+                                   S.BuiltinAddPointer(R, TemplateLoc),
+                                   TemplateLoc) &&
+            S.BuiltinIsConvertible(S.BuiltinAddPointer(T2, TemplateLoc),
+                                   S.BuiltinAddPointer(R, TemplateLoc),
+                                   TemplateLoc)) {
+          return R;
+        }
+      }
+    }
+
+    // Otherwise, if basic_common_reference<remove_cvref_t<T1>,
+    // remove_cvref_t<T2>, ​XREF(​T1), XREF(T2)>​::​type is well-formed,
+    // then the member typedef type denotes that type.
+    {
+      auto getXRef = [&](QualType T) {
+        static BuiltinTemplateDecl *Quals[12] = {
+            S.Context.get__clang_internal_xref_Decl(),
+            S.Context.get__clang_internal_xref_constDecl(),
+            S.Context.get__clang_internal_xref_volatileDecl(),
+            S.Context.get__clang_internal_xref_constvolatileDecl(),
+            S.Context.get__clang_internal_xref_lvalueDecl(),
+            S.Context.get__clang_internal_xref_lvalueconstDecl(),
+            S.Context.get__clang_internal_xref_lvaluevolatileDecl(),
+            S.Context.get__clang_internal_xref_lvalueconstvolatileDecl(),
+            S.Context.get__clang_internal_xref_rvalueDecl(),
+            S.Context.get__clang_internal_xref_rvalueconstDecl(),
+            S.Context.get__clang_internal_xref_rvaluevolatileDecl(),
+            S.Context.get__clang_internal_xref_rvalueconstvolatileDecl(),
+        };
+        size_t Index = 0;
+        if (T->isLValueReferenceType()) {
+          T = T.getNonReferenceType();
+          Index += 4;
+        } else if (T->isRValueReferenceType()) {
+          T = T.getNonReferenceType();
+          Index += 8;
+        }
+        if (T.isConstQualified())
+          Index += 1;
+
+        if (T.isVolatileQualified())
+          Index += 2;
+
+        return Quals[Index];
+      };
+
+      auto BCR = InstantiateTemplate(S, CommonReference,
+                                     {S.BuiltinRemoveCVRef(T1, TemplateLoc),
+                                      S.BuiltinRemoveCVRef(T2, TemplateLoc),
+                                      TemplateName{getXRef(T1)},
+                                      TemplateName{getXRef(T2)}},
+                                     TemplateLoc);
+      if (!BCR.isNull())
+        return BCR;
+    }
+
+    // Otherwise, if COND-RES(T1, T2) is well-formed, then the member typedef
+    // type denotes that type.
+    if (auto CR = CondRes(S, T1, T2, TemplateLoc); !CR.isNull())
+      return CR;
+
+    // Otherwise, if common_type_t<T1, T2> is well-formed, then the member
+    // typedef type denotes that type.
+    if (auto CT = InstantiateTemplate(S, CommonType, {T1, T2}, TemplateLoc);
+        !CT.isNull())
+      return CT;
+
+    // Otherwise, there shall be no member type.
+    return QualType();
+  }
+
+  // Otherwise, if sizeof...(T) is greater than two, let T1, T2, and Rest,
+  // respectively, denote the first, second, and (pack of) remaining types
+  // comprising T. Let C be the type common_reference_t<T1, T2>. Then:
+  default: {
+    auto T1 = Ts[0];
+    auto T2 = Ts[1];
+    auto Rest = Ts.drop_front(2);
+    auto C = builtinCommonReferenceImpl(S, CommonReference, CommonType,
+                                        TemplateLoc, {T1, T2});
+    if (C.isNull())
+      return QualType();
+    llvm::SmallVector<TemplateArgument, 4> Args;
+    Args.emplace_back(C);
+    Args.append(Rest.begin(), Rest.end());
+    return builtinCommonReferenceImpl(S, CommonReference, CommonType,
+                                      TemplateLoc, Args);
+  }
+  }
+}
+
 static QualType
 checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
                            ArrayRef<TemplateArgument> Converted,
@@ -3335,6 +3569,89 @@ checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
     }
     return HasNoTypeMember;
   }
+
+  case BTK__builtin_common_reference: {
+    assert(Converted.size() == 5);
+    if (llvm::any_of(Converted, [](auto &C) { return C.isDependent(); }))
+      return Context.getCanonicalTemplateSpecializationType(TemplateName(BTD),
+                                                            Converted);
+
+    TemplateName BasicCommonReference = Converted[0].getAsTemplate();
+    TemplateName CommonType = Converted[1].getAsTemplate();
+    TemplateName HasTypeMember = Converted[2].getAsTemplate();
+    QualType HasNoTypeMember = Converted[3].getAsType();
+    ArrayRef<TemplateArgument> Ts = Converted[4].getPackAsArray();
+    if (auto CR = builtinCommonReferenceImpl(SemaRef, BasicCommonReference,
+                                             CommonType, TemplateLoc, Ts);
+        !CR.isNull()) {
+      TemplateArgumentListInfo TAs;
+      TAs.addArgument(TemplateArgumentLoc(
+          TemplateArgument(CR), SemaRef.Context.getTrivialTypeSourceInfo(
+                                    CR, TemplateArgs[1].getLocation())));
+      return SemaRef.CheckTemplateIdType(HasTypeMember, TemplateLoc, TAs);
+    }
+    return HasNoTypeMember;
+  }
+
+  case BTK__clang_internal_xref_:
+  case BTK__clang_internal_xref_const:
+  case BTK__clang_internal_xref_volatile:
+  case BTK__clang_internal_xref_constvolatile:
+  case BTK__clang_internal_xref_lvalue:
+  case BTK__clang_internal_xref_lvalueconst:
+  case BTK__clang_internal_xref_lvaluevolatile:
+  case BTK__clang_internal_xref_lvalueconstvolatile:
+  case BTK__clang_internal_xref_rvalue:
+  case BTK__clang_internal_xref_rvalueconst:
+  case BTK__clang_internal_xref_rvaluevolatile:
+  case BTK__clang_internal_xref_rvalueconstvolatile: {
+    if (llvm::any_of(Converted, [](auto &C) { return C.isDependent(); }))
+      return Context.getCanonicalTemplateSpecializationType(TemplateName(BTD),
+                                                            Converted);
+
+    auto BTK = BTD->getBuiltinTemplateKind();
+    auto anyOf = [&](auto... Vals) { return ((BTK == Vals) || ...); };
+
+    bool AddCV = anyOf(BTK__clang_internal_xref_constvolatile,
+                       BTK__clang_internal_xref_lvalueconstvolatile,
+                       BTK__clang_internal_xref_rvalueconstvolatile);
+
+    bool AddConst = AddCV || anyOf(BTK__clang_internal_xref_const,
+                                   BTK__clang_internal_xref_lvalueconst,
+                                   BTK__clang_internal_xref_rvalueconst);
+
+    bool AddVolatile = AddCV || anyOf(BTK__clang_internal_xref_volatile,
+                                      BTK__clang_internal_xref_lvaluevolatile,
+                                      BTK__clang_internal_xref_rvaluevolatile);
+
+    bool AddLValue = anyOf(BTK__clang_internal_xref_lvalue,
+                           BTK__clang_internal_xref_lvalueconst,
+                           BTK__clang_internal_xref_lvaluevolatile,
+                           BTK__clang_internal_xref_lvalueconstvolatile);
+
+    bool AddRValue = anyOf(BTK__clang_internal_xref_rvalue,
+                           BTK__clang_internal_xref_rvalueconst,
+                           BTK__clang_internal_xref_rvaluevolatile,
+                           BTK__clang_internal_xref_rvalueconstvolatile);
+
+    assert(Converted.size() == 1);
+
+    QualType T = Converted[0].getAsType();
+
+    if (AddConst)
+      T.addConst();
+
+    if (AddVolatile)
+      T.addVolatile();
+
+    if (AddLValue)
+      T = SemaRef.BuiltinAddLValueReference(T, TemplateLoc);
+
+    if (AddRValue)
+      T = SemaRef.BuiltinAddRValueReference(T, TemplateLoc);
+
+    return T;
+  }
   }
   llvm_unreachable("unexpected BuiltinTemplateDecl!");
 }
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 776d6e55acc18..81e9c6c72761e 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -32,6 +32,8 @@
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/DeclSpec.h"
 #include "clang/Sema/DelayedDiagnostic.h"
+#include "clang/Sema/EnterExpressionEvaluationContext.h"
+#include "clang/Sema/Initialization.h"
 #include "clang/Sema/Lookup.h"
 #include "clang/Sema/ParsedAttr.h"
 #include "clang/Sema/ParsedTemplate.h"
@@ -9904,6 +9906,79 @@ QualType Sema::BuiltinChangeSignedness(QualType BaseType, UTTKind UKind,
   return Context.getQualifiedType(Underlying, BaseType.getQualifiers());
 }
 
+bool Sema::BuiltinIsConvertible(QualType From, QualType To, SourceLocation Loc,
+                                bool CheckNothrow) {
+  if (To->isVoidType())
+    return From->isVoidType();
+
+  if ((!From->isIncompleteArrayType() && !From->isVoidType() &&
+       RequireCompleteType(
+           Loc, From, diag::err_incomplete_type_used_in_type_trait_expr)) ||
+      (!To->isIncompleteArrayType() && !To->isVoidType() &&
+       RequireCompleteType(Loc, To,
+                           diag::err_incomplete_type_used_in_type_trait_expr)))
+    return false;
+
+  // C++11 [meta.rel]p4:
+  //   Given the following function prototype:
+  //
+  //     template <class T>
+  //       typename add_rvalue_reference<T>::type create();
+  //
+  //   the predicate condition for a template specialization
+  //   is_convertible<From, To> shall be satisfied if and only if
+  //   the return expression in the following code would be
+  //   well-formed, including any implicit conversions to the return
+  //   type of the function:
+  //
+  //     To test() {
+  //       return create<From>();
+  //     }
+  //
+  //   Access checking is performed as if in a context unrelated to To and
+  //   From. Only the validity of the immediate context of the expression
+  //   of the return-statement (including conversions to the return type)
+  //   is considered.
+  //
+  // We model the initialization as a copy-initialization of a temporary
+  // of the appropriate type, which for this expression is identical to the
+  // return statement (since NRVO doesn't apply).
+
+  // Functions aren't allowed to return function or array types.
+  if (To->isFunctionType() || To->isArrayType())
+    return false;
+
+  // A function definition requires a non-abstract return type.
+  if (isAbstractType(Loc, To))
+    return false;
+
+  From = BuiltinAddRValueReference(From, Loc);
+
+  // Build a fake source and destination for initialization.
+  InitializedEntity ToEntity(InitializedEntity::InitializeTemporary(To));
+  OpaqueValueExpr FromExpr(Loc, From.getNonLValueExprType(Context),
+                           Expr::getValueKindForType(From));
+  InitializationKind Kind =
+      InitializationKind::CreateCopy(Loc, SourceLocation());
+
+  // Perform the initialization in an unevaluated context within a SFINAE
+  // trap at translation unit scope.
+  EnterExpressionEvaluationContext Unevaluated(
+      *this, Sema::ExpressionEvaluationContext::Unevaluated);
+  Sema::SFINAETrap SFINAE(*this, /*AccessCheckingSFINAE=*/true);
+  Sema::ContextRAII TUContext(*this, Context.getTranslationUnitDecl());
+  Expr *FromExprPtr = &FromExpr;
+  InitializationSequence Init(*this, ToEntity, Kind, FromExprPtr);
+  if (Init.Failed())
+    return false;
+
+  ExprResult Result = Init.Perform(*this, ToEntity, Kind, FromExprPtr);
+  if (Result.isInvalid() || SFINAE.hasErrorOccurred())
+    return false;
+
+  return !CheckNothrow || canThrow(Result.get()) == CT_Cannot;
+}
+
 QualType Sema::BuildUnaryTransformType(QualType BaseType, UTTKind UKind,
                                        SourceLocation Loc) {
   if (BaseType->isDependentType())
diff --git a/clang/test/SemaCXX/type-trait-common-reference.cpp b/clang/test/SemaCXX/type-trait-common-reference.cpp
new file mode 100644
index 0000000000000..cc4fefc49886d
--- /dev/null
+++ b/clang/test/SemaCXX/type-trait-common-reference.cpp
@@ -0,0 +1,136 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fsyntax-only -verify -std=c++17 -Wno-vla-cxx-extension %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fsyntax-only -verify -std=c++20 -Wno-vla-cxx-extension %s
+
+#if !__has_builtin(__builtin_common_reference)
+#  error
+#endif
+
+// expected-note@*:* {{template <template <class, class, template <class> class, template <class> class> class, template <class ...> class, template <class> class, class, class ...>}}
+
+void test() {
+  __builtin_common_reference<> a; // expected-error {{too few template arguments for template '__builtin_common_reference'}}
+  __builtin_common_reference<1> b; // expected-error {{template argument for template template parameter must be a class template or type alias template}}
+  __builtin_common_reference<int, 1> c; // expected-error {{template argument for template template parameter must be a class template or type alias template}}
+}
+
+struct empty_type {};
+
+template <class T>
+struct type_identity {
+  using type = T;
+};
+
+template <class...>
+struct common_type;
+
+template <class... Args>
+using common_type_t = typename common_type<Args...>::type;
+
+template <class, class, template <class> class, template <class> class>
+struct basic_common_reference {};
+
+template <class T, class U, template <class> class TX, template <class> class UX>
+using basic_common_reference_t = typename basic_common_reference<T, U, TX, UX>::type;
+
+void test_vla() {
+  int i = 4;
+  int VLA[i];
+  __builtin_common_reference<basic_common_reference_t, common_type_t, type_identity, empty_type, decltype(VLA)> d; // expected-error {{variably modified type 'decltype(VLA)' (aka 'int[i]') cannot be used as a template argument}}
+}
+
+template <class... Args>
+using common_reference_base = __builtin_common_reference<basic_common_reference_t, common_type_t, type_identity, empty_type, Args...>;
+
+template <class... Args>
+struct common_reference : common_reference_base<Args...> {};
+
+template <class... Args>
+using common_reference_t = typename __builtin_common_reference<basic_common_reference_t, common_type_t, type_identity, empty_type, Args...>::type;
+
+struct Incomplete;
+
+template<>
+struct common_type<Incomplete, Incomplete>;
+
+static_assert(__is_same(common_reference_base<>, empty_type));
+
+static_assert(__is_same(common_reference_base<Incomplete>, type_identity<Incomplete>));
+static_assert(__is_same(common_reference_base<char>, type_identity<char>));
+static_assert(__is_same(common_reference_base<int>, type_identity<int>));
+static_assert(__is_same(common_reference_base<const int>, type_identity<const int>));
+static_assert(__is_same(common_reference_base<volatile int>, type_identity<volatile int>));
+static_assert(__is_same(common_reference_base<const volatile int>, type_identity<const volatile int>));
+static_assert(__is_same(common_reference_base<int[]>, type_identity<int[]>));
+static_assert(__is_same(common_reference_base<const int[]>, type_identity<const int[]>));
+static_assert(__is_same(common_reference_base<void(&)()>, type_identity<void(&)()>));
+
+static_assert(__is_same(common_reference_base<int[], int[]>, type_identity<int*>));
+static_assert(__is_same(common_reference_base<int, int>, type_identity<int>));
+static_assert(__is_same(common_reference_base<int, long>, type_identity<long>));
+static_assert(__is_same(common_reference_base<long, int>, type_identity<long>));
+static_assert(__is_same(common_reference_base<long, long>, type_identity<long>));
+
+static_assert(__is_same(common_reference_base<const int, long>, type_identity<long>));
+static_assert(__is_same(common_reference_base<const volatile int, long>, type_identity<long>));
+static_assert(__is_same(common_reference_base<int, const long>, type_identity<long>));
+static_assert(__is_same(common_reference_base<int, const volatile long>, type_identity<long>));
+
+static_assert(__is_same(common_reference_base<int*, long*>, empty_type));
+static_assert(__is_same(common_reference_base<const unsigned int *const &, const unsigned int *const &>, type_identity<const unsigned int *const &>));
+
+static_assert(__is_same(common_reference_base<int, long, float>, type_identity<float>));
+static_assert(__is_same(common_reference_base<unsigned, char, long>, type_identity<long>));
+static_assert(__is_same(common_reference_base<long long, long long, long>, type_identity<long long>));
+
+static_assert(__is_same(common_reference_base<int [[clang::address_space(1)]]>, type_identity<int [[clang::address_space(1)]]>));
+static_assert(__is_same(common_reference_base<int [[clang::address_space(1)]], int>, type_identity<int>));
+static_assert(__is_same(common_reference_base<long [[clang::address_space(1)]], int>, type_identity<long>));
+static_assert(__is_same(common_reference_base<long [[clang::address_space(1)]], int [[clang::address_space(1)]]>, type_identity<long>));
+static_assert(__is_same(common_reference_base<long [[clang::address_space(1)]], long [[clang::address_space(1)]]>, type_identity<long>));
+static_assert(__is_same(common_reference_base<long [[clang::address_space(1)]], long [[clang::address_space(2)]]>, type_identity<long>));
+
+struct S {};
+struct T : S {};
+struct U {};
+
+static_assert(__is_same(common_reference_base<S&&, T&&>, type_identity<S&&>));
+
+static_assert(__is_same(common_reference_base<int S::*, int S::*>, type_identity<int S::*>));
+static_assert(__is_same(common_reference_base<int S::*, int T::*>, type_identity<int T::*>));
+static_assert(__is_same(common_reference_base<int S::*, long S::*>, empty_type));
+
+static_assert(__is_same(common_reference_base<int (S::*)(), int (S::*)()>, type_identity<int (S::*)()>));
+static_assert(__is_same(common_reference_base<int (S::*)(), int (T::*)()>, type_identity<int (T::*)()>));
+static_assert(__is_same(common_reference_base<int (S::*)(), long (S::*)()>, empty_type));
+
+static_assert(__is_same(common_reference_base<int&, int&>, type_identity<int&>));
+static_assert(__is_same(common_reference_base<int&, const int&>, type_identity<const int&>));
+static_assert(__is_same(common_reference_base<volatile int&, const int&>, type_identity<const volatile int&>));
+
+template <class T, class U>
+struct my_pair;
+
+template <class T1, class U1, class T2, class U2, template <class> class TX, template <class> class UX>
+struct basic_common_reference<my_pair<T1, U1>, my_pair<T2, U2>, TX, UX> {
+  using type = my_pair<common_reference_t<TX<T1>, UX<T2>>, common_reference_t<TX<U1>, UX<U2>>>;
+};
+
+static_assert(__is_same(common_reference_base<my_pair<const int&, int&>, my_pair<int&, volatile int&>>, type_identity<my_pair<const int&, volatile int&>>));
+static_assert(__is_same(common_reference_base<const my_pair<int, int>&, my_pair<int&, volatile int&>>, type_identity<my_pair<const int&, const volatile int&>>));
+static_assert(__is_same(common_reference_base<const int&, const volatile int&>, type_identity<const volatile int&>));
+static_assert(__is_same(common_reference_base<int&&, const volatile int&>, type_identity<int>));
+static_assert(__is_same(common_reference_base<my_pair<int, int>&&, my_pair<int&, volatile int&>>, type_identity<my_pair<const int&, int>>));
+static_assert(__is_same(common_reference_base<my_pair<int, int>&&, my_pair<int&, int>&&>, type_identity<my_pair<const int&, int&&>>));
+
+struct conversion_operator {
+  operator volatile int&&() volatile;
+};
+
+static_assert(__is_same(common_reference_base<volatile conversion_operator&&, volatile int&&>, type_identity<volatile int&&>));
+
+struct reference_wrapper {
+  reference_wrapper(int&);
+  operator int&() const;
+};
+
+static_assert(__is_same(common_reference_base<const reference_wrapper&, int&>, empty_type));
diff --git a/libcxx/include/__type_traits/common_reference.h b/libcxx/include/__type_traits/common_reference.h
index c27da5251b9b7..f27a85836c105 100644
--- a/libcxx/include/__type_traits/common_reference.h
+++ b/libcxx/include/__type_traits/common_reference.h
@@ -17,16 +17,37 @@
 #include <__type_traits/is_reference.h>
 #include <__type_traits/remove_cvref.h>
 #include <__type_traits/remove_reference.h>
+#include <__type_traits/type_identity.h>
 #include <__utility/declval.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-// common_reference
-#if _LIBCPP_STD_VER >= 20
+template <class...>
+struct common_reference;
+
+template <class... _Types>
+using common_reference_t = typename common_reference<_Types...>::type;
+
+template <class, class, template <class> class, template <class> class>
+struct basic_common_reference {};
+
+#  if __has_builtin(__builtin_common_reference)
+
+template <class _Tp, class _Up, template <class> class _Tx, template <class> class _Ux>
+using __basic_common_reference_t = basic_common_reference<_Tp, _Up, _Tx, _Ux>::type;
+
+template <class... _Args>
+struct common_reference
+    : __builtin_common_reference<__basic_common_reference_t, common_type_t, type_identity, __empty, _Args...> {};
+
+#  else
+
 // Let COND_RES(X, Y) be:
 template <class _Xp, class _Yp>
 using __cond_res _LIBCPP_NODEBUG = decltype(false ? std::declval<_Xp (&)()>()() : std::declval<_Yp (&)()>()());
@@ -108,12 +129,6 @@ struct __common_ref {};
 
 // Note C: For the common_reference trait applied to a parameter pack [...]
 
-template <class...>
-struct common_reference;
-
-template <class... _Types>
-using common_reference_t = typename common_reference<_Types...>::type;
-
 // bullet 1 - sizeof...(T) == 0
 template <>
 struct common_reference<> {};
@@ -145,8 +160,6 @@ struct __common_reference_sub_bullet1<_Tp, _Up> {
 
 // sub-bullet 2 - Otherwise, if basic_common_reference<remove_cvref_t<T1>, remove_cvref_t<T2>, XREF(T1), XREF(T2)>::type
 // is well-formed, then the member typedef `type` denotes that type.
-template <class, class, template <class> class, template <class> class>
-struct basic_common_reference {};
 
 template <class _Tp, class _Up>
 using __basic_common_reference_t _LIBCPP_NODEBUG =
@@ -185,8 +198,10 @@ struct common_reference<_Tp, _Up, _Vp, _Rest...> : common_reference<common_refer
 template <class...>
 struct common_reference {};
 
-#endif // _LIBCPP_STD_VER >= 20
+#  endif // __has_builtin(__builtin_common_reference)
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 20
+
 #endif // _LIBCPP___TYPE_TRAITS_COMMON_REFERENCE_H
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index 0ce42fc4d3633..0ec36e3e8c897 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -1503,6 +1503,7 @@ module std [system] {
       header "__iterator/iterator_traits.h"
       export std_core.type_traits.integral_constant
       export std_core.type_traits.is_convertible
+      export std_core.type_traits.nat
     }
     module iterator_with_data         { header "__iterator/iterator_with_data.h" }
     module iterator                   { header "__iterator/iterator.h" }