[Clang] Add __builtin_common_type (#99473)

This implements the logic of the `common_type` base template as a
builtin alias. If there should be no `type` member, an empty class is
returned. Otherwise a specialization of a `type_identity`-like class is
returned. The base template (i.e. `std::common_type`) as well as the
empty class and `type_identity`-like struct are given as arguments to
the builtin.

clang/docs/LanguageExtensions.rst

@@ -1516,6 +1516,46 @@ Attributes (N2335)                                                            C2
 ``#embed`` (N3017)                                                            C23           C89, C++
 ============================================ ================================ ============= =============
 
+Builtin type aliases
+====================
+
+Clang provides a few builtin aliases to improve the throughput of certain metaprogramming facilities.
+
+__builtin_common_type
+---------------------
+
+.. code-block:: c++
+
+  template <template <class... Args> class BaseTemplate,
+            template <class TypeMember> class HasTypeMember,
+            class HasNoTypeMember,
+            class... Ts>
+  using __builtin_common_type = ...;
+
+This alias is used for implementing ``std::common_type``. If ``std::common_type`` should contain a ``type`` member,
+it is an alias to ``HasTypeMember<TheCommonType>``. Otherwise it is an alias to ``HasNoTypeMember``. The
+``BaseTemplate`` is usually ``std::common_type``. ``Ts`` are the arguments to ``std::common_type``.
+
+__type_pack_element
+-------------------
+
+.. code-block:: c++
+
+  template <std::size_t Index, class... Ts>
+  using __type_pack_element = ...;
+
+This alias returns the type at ``Index`` in the parameter pack ``Ts``.
+
+__make_integer_seq
+------------------
+
+.. code-block:: c++
+
+  template <template <class IntSeqT, IntSeqT... Ints> class IntSeq, class T, T N>
+  using __make_integer_seq = ...;
+
+This alias returns ``IntSeq`` instantiated with ``IntSeqT = T``and ``Ints`` being the pack ``0, ..., N - 1``.
+
 Type Trait Primitives
 =====================
 

clang/docs/ReleaseNotes.rst

@@ -123,6 +123,9 @@ C++ Language Changes
 
 - Add ``__builtin_elementwise_popcount`` builtin for integer types only.
 
+- The builtin type alias ``__builtin_common_type`` has been added to improve the
+  performance of ``std::common_type``.
+
 C++2c Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/AST/ASTContext.h

@@ -403,6 +403,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// The identifier '__type_pack_element'.
   mutable IdentifierInfo *TypePackElementName = nullptr;
 
+  /// The identifier '__builtin_common_type'.
+  mutable IdentifierInfo *BuiltinCommonTypeName = nullptr;
+
   QualType ObjCConstantStringType;
   mutable RecordDecl *CFConstantStringTagDecl = nullptr;
   mutable TypedefDecl *CFConstantStringTypeDecl = nullptr;
@@ -610,6 +613,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   mutable ExternCContextDecl *ExternCContext = nullptr;
   mutable BuiltinTemplateDecl *MakeIntegerSeqDecl = nullptr;
   mutable BuiltinTemplateDecl *TypePackElementDecl = nullptr;
+  mutable BuiltinTemplateDecl *BuiltinCommonTypeDecl = nullptr;
 
   /// The associated SourceManager object.
   SourceManager &SourceMgr;
@@ -1134,6 +1138,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   ExternCContextDecl *getExternCContextDecl() const;
   BuiltinTemplateDecl *getMakeIntegerSeqDecl() const;
   BuiltinTemplateDecl *getTypePackElementDecl() const;
+  BuiltinTemplateDecl *getBuiltinCommonTypeDecl() const;
 
   // Builtin Types.
   CanQualType VoidTy;
@@ -2025,6 +2030,12 @@ class ASTContext : public RefCountedBase<ASTContext> {
     return TypePackElementName;
   }
 
+  IdentifierInfo *getBuiltinCommonTypeName() const {
+    if (!BuiltinCommonTypeName)
+      BuiltinCommonTypeName = &Idents.get("__builtin_common_type");
+    return BuiltinCommonTypeName;
+  }
+
   /// Retrieve the Objective-C "instancetype" type, if already known;
   /// otherwise, returns a NULL type;
   QualType getObjCInstanceType() {

clang/include/clang/AST/DeclID.h

@@ -83,6 +83,9 @@ enum PredefinedDeclIDs {
   /// The internal '__type_pack_element' template.
   PREDEF_DECL_TYPE_PACK_ELEMENT_ID,
 
+  /// The internal '__builtin_common_type' template.
+  PREDEF_DECL_COMMON_TYPE_ID,
+
   /// The number of declaration IDs that are predefined.
   NUM_PREDEF_DECL_IDS
 };

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 __builtin_common_type BuiltinTemplateDecl.
+  BTK__builtin_common_type,
 };
 
 } // end namespace clang

clang/lib/AST/ASTContext.cpp

@@ -1184,6 +1184,13 @@ ASTContext::getTypePackElementDecl() const {
   return TypePackElementDecl;
 }
 
+BuiltinTemplateDecl *ASTContext::getBuiltinCommonTypeDecl() const {
+  if (!BuiltinCommonTypeDecl)
+    BuiltinCommonTypeDecl = buildBuiltinTemplateDecl(
+        BTK__builtin_common_type, getBuiltinCommonTypeName());
+  return BuiltinCommonTypeDecl;
+}
+
 RecordDecl *ASTContext::buildImplicitRecord(StringRef Name,
                                             RecordDecl::TagKind TK) const {
   SourceLocation Loc;

clang/lib/AST/ASTImporter.cpp

@@ -5467,6 +5467,9 @@ ExpectedDecl ASTNodeImporter::VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D) {
   case BuiltinTemplateKind::BTK__type_pack_element:
     ToD = Importer.getToContext().getTypePackElementDecl();
     break;
+  case BuiltinTemplateKind::BTK__builtin_common_type:
+    ToD = Importer.getToContext().getBuiltinCommonTypeDecl();
+    break;
   }
   assert(ToD && "BuiltinTemplateDecl of unsupported kind!");
   Importer.MapImported(D, ToD);

clang/lib/AST/DeclTemplate.cpp

@@ -1608,13 +1608,69 @@ createTypePackElementParameterList(const ASTContext &C, DeclContext *DC) {
                                        nullptr);
 }
 
+static TemplateParameterList *createBuiltinCommonTypeList(const ASTContext &C,
+                                                          DeclContext *DC) {
+  // class... Args
+  auto *Args =
+      TemplateTypeParmDecl::Create(C, DC, SourceLocation(), SourceLocation(),
+                                   /*Depth=*/1, /*Position=*/0, /*Id=*/nullptr,
+                                   /*Typename=*/false, /*ParameterPack=*/true);
+
+  // <class... Args>
+  auto *BaseTemplateList = TemplateParameterList::Create(
+      C, SourceLocation(), SourceLocation(), Args, SourceLocation(), nullptr);
+
+  // template <class... Args> class BaseTemplate
+  auto *BaseTemplate = TemplateTemplateParmDecl::Create(
+      C, DC, SourceLocation(), /*Depth=*/0, /*Position=*/0,
+      /*ParameterPack=*/false, /*Id=*/nullptr,
+      /*Typename=*/false, BaseTemplateList);
+
+  // class TypeMember
+  auto *TypeMember =
+      TemplateTypeParmDecl::Create(C, DC, SourceLocation(), SourceLocation(),
+                                   /*Depth=*/1, /*Position=*/0, /*Id=*/nullptr,
+                                   /*Typename=*/false, /*ParameterPack=*/false);
+
+  // <class TypeMember>
+  auto *HasTypeMemberList =
+      TemplateParameterList::Create(C, SourceLocation(), SourceLocation(),
+                                    TypeMember, SourceLocation(), nullptr);
+
+  // template <class TypeMember> class HasTypeMember
+  auto *HasTypeMember = TemplateTemplateParmDecl::Create(
+      C, DC, SourceLocation(), /*Depth=*/0, /*Position=*/1,
+      /*ParameterPack=*/false, /*Id=*/nullptr,
+      /*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, SourceLocation(), SourceLocation(), /*Depth=*/0, /*Position=*/3,
+      /*Id=*/nullptr, /*Typename=*/false, /*ParameterPack=*/true);
+
+  // template <template <class... Args> class BaseTemplate,
+  //   template <class TypeMember> class HasTypeMember, class HasNoTypeMember,
+  //   class... Ts>
+  return TemplateParameterList::Create(
+      C, SourceLocation(), SourceLocation(),
+      {BaseTemplate, HasTypeMember, HasNoTypeMember, Ts}, SourceLocation(),
+      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__builtin_common_type:
+    return createBuiltinCommonTypeList(C, DC);
   }
 
   llvm_unreachable("unhandled BuiltinTemplateKind!");

clang/lib/Lex/PPMacroExpansion.cpp

@@ -1836,6 +1836,7 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
               // Report builtin templates as being builtins.
               .Case("__make_integer_seq", getLangOpts().CPlusPlus)
               .Case("__type_pack_element", getLangOpts().CPlusPlus)
+              .Case("__builtin_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/SemaLookup.cpp

@@ -928,10 +928,15 @@ bool Sema::LookupBuiltin(LookupResult &R) {
         if (II == getASTContext().getMakeIntegerSeqName()) {
           R.addDecl(getASTContext().getMakeIntegerSeqDecl());
           return true;
-        } else if (II == getASTContext().getTypePackElementName()) {
+        }
+        if (II == getASTContext().getTypePackElementName()) {
           R.addDecl(getASTContext().getTypePackElementDecl());
           return true;
         }
+        if (II == getASTContext().getBuiltinCommonTypeName()) {
+          R.addDecl(getASTContext().getBuiltinCommonTypeDecl());
+          return true;
+        }
       }
 
       // Check if this is an OpenCL Builtin, and if so, insert its overloads.