[Clang] Add __builtin_common_type

clang/include/clang/AST/ASTContext.h

@@ -399,6 +399,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// The identifier '__type_pack_element'.
   mutable IdentifierInfo *TypePackElementName = nullptr;
 
+  /// The identifier '__common_type'.
+  mutable IdentifierInfo *CommonTypeName = nullptr;
+
   QualType ObjCConstantStringType;
   mutable RecordDecl *CFConstantStringTagDecl = nullptr;
   mutable TypedefDecl *CFConstantStringTypeDecl = nullptr;
@@ -606,6 +609,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   mutable ExternCContextDecl *ExternCContext = nullptr;
   mutable BuiltinTemplateDecl *MakeIntegerSeqDecl = nullptr;
   mutable BuiltinTemplateDecl *TypePackElementDecl = nullptr;
+  mutable BuiltinTemplateDecl *CommonTypeDecl = nullptr;
 
   /// The associated SourceManager object.
   SourceManager &SourceMgr;
@@ -1107,6 +1111,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   ExternCContextDecl *getExternCContextDecl() const;
   BuiltinTemplateDecl *getMakeIntegerSeqDecl() const;
   BuiltinTemplateDecl *getTypePackElementDecl() const;
+  BuiltinTemplateDecl *getCommonTypeDecl() const;
 
   // Builtin Types.
   CanQualType VoidTy;
@@ -1984,6 +1989,12 @@ class ASTContext : public RefCountedBase<ASTContext> {
     return TypePackElementName;
   }
 
+  IdentifierInfo *getCommonTypeName() const {
+    if (!CommonTypeName)
+      CommonTypeName = &Idents.get("__common_type");
+    return CommonTypeName;
+  }
+
   /// Retrieve the Objective-C "instancetype" type, if already known;
   /// otherwise, returns a NULL type;
   QualType getObjCInstanceType() {

clang/include/clang/AST/DeclID.h

@@ -84,13 +84,16 @@ enum PredefinedDeclIDs {
 
   /// The internal '__type_pack_element' template.
   PREDEF_DECL_TYPE_PACK_ELEMENT_ID = 17,
+
+  /// The internal '__common_type' template.
+  PREDEF_DECL_COMMON_TYPE_ID = 18,
 };
 
 /// The number of declaration IDs that are predefined.
 ///
 /// For more information about predefined declarations, see the
 /// \c PredefinedDeclIDs type and the PREDEF_DECL_*_ID constants.
-const unsigned int NUM_PREDEF_DECL_IDS = 18;
+const unsigned int NUM_PREDEF_DECL_IDS = 19;
 
 /// GlobalDeclID means DeclID in the current ASTContext and LocalDeclID means
 /// DeclID specific to a certain ModuleFile. Specially, in ASTWriter, the

clang/include/clang/Basic/Builtins.h

@@ -309,7 +309,10 @@ enum BuiltinTemplateKind : int {
   BTK__make_integer_seq,
 
   /// This names the __type_pack_element BuiltinTemplateDecl.
-  BTK__type_pack_element
+  BTK__type_pack_element,
+
+  /// This names the __common_type BuiltinTemplateDecl.
+  BTK__common_type,
 };
 
 } // end namespace clang

clang/include/clang/Sema/Sema.h

@@ -2282,6 +2282,10 @@ class Sema final : public SemaBase {
   /// Check to see if a given expression could have '.c_str()' called on it.
   bool hasCStrMethod(const Expr *E);
 
+  // Check whether a type member 'Type::Name' exists, and if yes, return the
+  // type. If there is no type, the QualType is null
+  QualType getTypeMember(StringRef Name, QualType Type);
+
   /// Diagnose pointers that are always non-null.
   /// \param E the expression containing the pointer
   /// \param NullKind NPCK_NotNull if E is a cast to bool, otherwise, E is

clang/lib/AST/ASTContext.cpp

@@ -1170,6 +1170,13 @@ ASTContext::getTypePackElementDecl() const {
   return TypePackElementDecl;
 }
 
+BuiltinTemplateDecl *ASTContext::getCommonTypeDecl() const {
+  if (!CommonTypeDecl)
+    CommonTypeDecl =
+        buildBuiltinTemplateDecl(BTK__common_type, getCommonTypeName());
+  return CommonTypeDecl;
+}
+
 RecordDecl *ASTContext::buildImplicitRecord(StringRef Name,
                                             RecordDecl::TagKind TK) const {
   SourceLocation Loc;

clang/lib/AST/ASTImporter.cpp

@@ -5408,6 +5408,9 @@ ExpectedDecl ASTNodeImporter::VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D) {
   case BuiltinTemplateKind::BTK__type_pack_element:
     ToD = Importer.getToContext().getTypePackElementDecl();
     break;
+  case BuiltinTemplateKind::BTK__common_type:
+    ToD = Importer.getToContext().getCommonTypeDecl();
+    break;
   }
   assert(ToD && "BuiltinTemplateDecl of unsupported kind!");
   Importer.MapImported(D, ToD);

clang/lib/AST/DeclTemplate.cpp

@@ -1599,13 +1599,66 @@ createTypePackElementParameterList(const ASTContext &C, DeclContext *DC) {
                                        nullptr);
 }
 
+static TemplateParameterList *createCommonTypeList(const ASTContext &C,
+                                                   DeclContext *DC) {
+  // class... Args
+  auto *Args = TemplateTypeParmDecl::Create(
+      C, DC, {}, {}, /*Depth=*/1, /*Position=*/0, /*Id=*/nullptr,
+      /*Typename=*/false, /*ParameterPack=*/true);
+  Args->setImplicit();
+
+  // <class... Args>
+  auto *BaseTemplateList =
+      TemplateParameterList::Create(C, {}, {}, Args, {}, nullptr);
+
+  // template <class... Args> class BaseTemplate
+  auto *BaseTemplate = TemplateTemplateParmDecl::Create(
+      C, DC, {}, /*Depth=*/0, /*Position=*/0, /*ParameterPack=*/false, {},
+      /*Typename=*/false, BaseTemplateList);
+
+  // class TypeMember
+  auto *TypeMember = TemplateTypeParmDecl::Create(
+      C, DC, {}, {}, /*Depth=*/1, /*Position=*/0, /*Id=*/nullptr,
+      /*Typename=*/false, /*ParameterPack=*/false);
+
+  // <class TypeMember>
+  auto *HasTypeMemberList =
+      TemplateParameterList::Create(C, {}, {}, TypeMember, {}, nullptr);
+
+  // template <class TypeMember> class HasTypeMember
+  auto *HasTypeMember =
+      TemplateTemplateParmDecl::Create(C, DC, {}, /*Depth=*/0, /*Position=*/1,
+                                       /*ParameterPack=*/false, {},
+                                       /*Typename=*/false, HasTypeMemberList);
+
+  // class HasNoTypeMember
+  auto *HasNoTypeMember = TemplateTypeParmDecl::Create(
+      C, DC, {}, {}, /*Depth=*/0, /*Position=*/2, /*Id=*/nullptr,
+      /*Typename=*/false, /*ParameterPack=*/false);
+
+  // class... Ts
+  auto *Ts = TemplateTypeParmDecl::Create(
+      C, DC, {}, {}, /*Depth=*/0, /*Position=*/3,
+      /*Id=*/nullptr, /*Typename=*/false, /*ParameterPack=*/true);
+  Ts->setImplicit();
+
+  // template <template <class... Args> class BaseTemplate,
+  //   template <class TypeMember> class HasTypeMember, class HasNoTypeMember,
+  //   class... Ts>
+  return TemplateParameterList::Create(
+      C, {}, {}, {BaseTemplate, HasTypeMember, HasNoTypeMember, Ts}, {},
+      nullptr);
+}
+
 static TemplateParameterList *createBuiltinTemplateParameterList(
     const ASTContext &C, DeclContext *DC, BuiltinTemplateKind BTK) {
   switch (BTK) {
   case BTK__make_integer_seq:
     return createMakeIntegerSeqParameterList(C, DC);
   case BTK__type_pack_element:
     return createTypePackElementParameterList(C, DC);
+  case BTK__common_type:
+    return createCommonTypeList(C, DC);
   }
 
   llvm_unreachable("unhandled BuiltinTemplateKind!");

clang/lib/Lex/PPMacroExpansion.cpp

@@ -1822,6 +1822,7 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
               // Report builtin templates as being builtins.
               .Case("__make_integer_seq", getLangOpts().CPlusPlus)
               .Case("__type_pack_element", getLangOpts().CPlusPlus)
+              .Case("__common_type", getLangOpts().CPlusPlus)
               // Likewise for some builtin preprocessor macros.
               // FIXME: This is inconsistent; we usually suggest detecting
               // builtin macros via #ifdef. Don't add more cases here.

clang/lib/Sema/SemaChecking.cpp

@@ -6844,6 +6844,14 @@ CXXRecordMembersNamed(StringRef Name, Sema &S, QualType Ty) {
   return Results;
 }
 
+QualType Sema::getTypeMember(StringRef Name, QualType Type) {
+  auto Results = CXXRecordMembersNamed<TypeDecl>(Name, *this, Type);
+  assert(Results.size() <= 1);
+  if (Results.empty())
+    return {};
+  return Context.getTypeDeclType(*Results.begin());
+}
+
 /// Check if we could call '.c_str()' on an object.
 ///
 /// FIXME: This returns the wrong results in some cases (if cv-qualifiers don't

clang/lib/Sema/SemaLookup.cpp

@@ -932,6 +932,10 @@ bool Sema::LookupBuiltin(LookupResult &R) {
           R.addDecl(getASTContext().getTypePackElementDecl());
           return true;
         }
+        if (II == getASTContext().getCommonTypeName()) {
+          R.addDecl(getASTContext().getCommonTypeDecl());
+          return true;
+        }
       }
 
       // Check if this is an OpenCL Builtin, and if so, insert its overloads.

clang/lib/Sema/SemaTemplate.cpp

@@ -3058,6 +3058,141 @@ void Sema::NoteAllFoundTemplates(TemplateName Name) {
   }
 }
 
+static std::optional<QualType> commonTypeImpl(Sema &S,
+                                              TemplateName BaseTemplate,
+                                              SourceLocation TemplateLoc,
+                                              ArrayRef<TemplateArgument> Ts) {
+  auto lookUpCommonType = [&](TemplateArgument T1,
+                              TemplateArgument T2) -> std::optional<QualType> {
+    // Don't bother looking for other specializations if both types are
+    // builtins - users aren't allowed to specialize for them
+    if (T1.getAsType()->isBuiltinType() && T2.getAsType()->isBuiltinType())
+      return commonTypeImpl(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())));
+    QualType BaseTemplateInst =
+        S.CheckTemplateIdType(BaseTemplate, TemplateLoc, Args);
+    if (S.RequireCompleteType(TemplateLoc, BaseTemplateInst,
+                              diag::err_incomplete_type))
+      return std::nullopt;
+    if (QualType Type = S.getTypeMember("type", BaseTemplateInst);
+        !Type.isNull()) {
+      return Type;
+    }
+    return std::nullopt;
+  };
+
+  // Note A: For the common_type trait applied to a template parameter pack T of
+  // types, the member type shall be either defined or not present as follows:
+  switch (Ts.size()) {
+
+  // If sizeof...(T) is zero, there shall be no member type.
+  case 0:
+    return std::nullopt;
+
+  // If sizeof...(T) is one, let T0 denote the sole type constituting the
+  // pack T. The member typedef-name type shall denote the same type, if any, as
+  // common_type_t<T0, T0>; otherwise there shall be no member type.
+  case 1:
+    return lookUpCommonType(Ts[0], Ts[0]);
+
+  // If sizeof...(T) is two, let the first and second types constituting T be
+  // denoted by T1 and T2, respectively, and let D1 and D2 denote the same types
+  // as decay_t<T1> and decay_t<T2>, respectively.
+  case 2: {
+    QualType T1 = Ts[0].getAsType();
+    QualType T2 = Ts[1].getAsType();
+    QualType D1 = S.BuiltinDecay(T1, {});
+    QualType D2 = S.BuiltinDecay(T2, {});
+
+    // If is_same_v<T1, D1> is false or is_same_v<T2, D2> is false, let C denote
+    // the same type, if any, as common_type_t<D1, D2>.
+    if (!S.Context.hasSameType(T1, D1) || !S.Context.hasSameType(T2, D2)) {
+      return lookUpCommonType(D1, D2);
+    }
+
+    // Otherwise, if decay_t<decltype(false ? declval<D1>() : declval<D2>())>
+    // denotes a valid type, let C denote that type.
+    {
+      auto CheckConditionalOperands =
+          [&](bool ConstRefQual) -> std::optional<QualType> {
+        EnterExpressionEvaluationContext UnevaluatedContext(
+            S, Sema::ExpressionEvaluationContext::Unevaluated);
+        Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+        Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
+
+        // false
+        OpaqueValueExpr CondExpr({}, S.Context.BoolTy,
+                                 ExprValueKind::VK_PRValue);
+        ExprResult Cond = &CondExpr;
+
+        auto EVK =
+            ConstRefQual ? ExprValueKind::VK_LValue : ExprValueKind::VK_PRValue;
+        if (ConstRefQual) {
+          D1.addConst();
+          D2.addConst();
+        }
+
+        // declval<D1>()
+        OpaqueValueExpr LHSExpr(TemplateLoc, D1, EVK);
+        ExprResult LHS = &LHSExpr;
+
+        // declval<D2>()
+        OpaqueValueExpr RHSExpr(TemplateLoc, D2, EVK);
+        ExprResult RHS = &RHSExpr;
+
+        ExprValueKind VK = VK_PRValue;
+        ExprObjectKind OK = OK_Ordinary;
+
+        // decltype(false ? declval<D1>() : declval<D2>())
+        QualType Result =
+            S.CheckConditionalOperands(Cond, LHS, RHS, VK, OK, TemplateLoc);
+
+        if (Result.isNull() || SFINAE.hasErrorOccurred())
+          return std::nullopt;
+
+        // decay_t<decltype(false ? declval<D1>() : declval<D2>())>
+        return S.BuiltinDecay(Result, TemplateLoc);
+      };
+
+      if (auto Res = CheckConditionalOperands(false))
+        return Res;
+
+      // Let:
+      // CREF(A) be add_lvalue_reference_t<const remove_reference_t<A>>,
+      // COND-RES(X, Y) be
+      //   decltype(false ? declval<X(&)()>()() : declval<Y(&)()>()()).
+
+      // C++20 only
+      // Otherwise, if COND-RES(CREF(D1), CREF(D2)) denotes a type, let C denote
+      // the type decay_t<COND-RES(CREF(D1), CREF(D2))>.
+      if (!S.Context.getLangOpts().CPlusPlus20)
+        return std::nullopt;
+      return CheckConditionalOperands(true);
+    }
+  }
+
+  // If sizeof...(T) is greater than two, let T1, T2, and R, respectively,
+  // denote the first, second, and (pack of) remaining types constituting T. Let
+  // C denote the same type, if any, as common_type_t<T1, T2>. If there is such
+  // a type C, the member typedef-name type shall denote the same type, if any,
+  // as common_type_t<C, R...>. Otherwise, there shall be no member type.
+  default: {
+    std::optional<QualType> Result = Ts[Ts.size() - 1].getAsType();
+    for (size_t i = Ts.size() - 1; i != 0; --i) {
+      Result = lookUpCommonType(Ts[i - 1].getAsType(), *Result);
+      if (!Result)
+        return std::nullopt;
+    }
+    return Result;
+  }
+  }
+}
+
 static QualType
 checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
                            ArrayRef<TemplateArgument> Converted,
@@ -3114,7 +3249,7 @@ checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
                                        TemplateLoc, SyntheticTemplateArgs);
   }
 
-  case BTK__type_pack_element:
+  case BTK__type_pack_element: {
     // Specializations of
     //    __type_pack_element<Index, T_1, ..., T_N>
     // are treated like T_Index.
@@ -3140,6 +3275,29 @@ checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
     int64_t N = Index.getExtValue();
     return Ts.getPackAsArray()[N].getAsType();
   }
+
+  case BTK__common_type: {
+    assert(Converted.size() == 4);
+    if (Converted[0].isDependent() || Converted[1].isDependent() ||
+        Converted[2].isDependent() || Converted[3].isDependent())
+      return Context.getCanonicalTemplateSpecializationType(TemplateName(BTD),
+                                                            Converted);
+
+    TemplateName BaseTemplate = Converted[0].getAsTemplate();
+    TemplateName HasTypeMember = Converted[1].getAsTemplate();
+    QualType HasNoTypeMember = Converted[2].getAsType();
+    ArrayRef<TemplateArgument> Ts = Converted[3].getPackAsArray();
+    if (auto CT = commonTypeImpl(SemaRef, BaseTemplate, TemplateLoc, Ts)) {
+      TemplateArgumentListInfo TAs;
+      TAs.addArgument(TemplateArgumentLoc(
+          TemplateArgument(*CT), SemaRef.Context.getTrivialTypeSourceInfo(
+                                     *CT, TemplateArgs[1].getLocation())));
+
+      return SemaRef.CheckTemplateIdType(HasTypeMember, TemplateLoc, TAs);
+    }
+    return HasNoTypeMember;
+  }
+  }
   llvm_unreachable("unexpected BuiltinTemplateDecl!");
 }
 

clang/lib/Serialization/ASTReader.cpp

@@ -7789,6 +7789,9 @@ static Decl *getPredefinedDecl(ASTContext &Context, PredefinedDeclIDs ID) {
 
   case PREDEF_DECL_TYPE_PACK_ELEMENT_ID:
     return Context.getTypePackElementDecl();
+
+  case PREDEF_DECL_COMMON_TYPE_ID:
+    return Context.getCommonTypeDecl();
   }
   llvm_unreachable("PredefinedDeclIDs unknown enum value");
 }