[Clang] Add __builtin_common_reference

clang/docs/LanguageExtensions.rst

@@ -1697,6 +1697,23 @@ 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
 ---------------------
 

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;
 }
 
@@ -56,6 +56,32 @@ def __builtin_common_type : CPlusPlusBuiltinTemplate<
    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 : CPlusPlusBuiltinTemplate<
+            [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 : CPlusPlusBuiltinTemplate<[Class<>]>;
+    }
+  }
+}
+
 // template <uint32_t Opcode,
 //           uint32_t Size,
 //           uint32_t Alignment,

clang/include/clang/Sema/Sema.h

@@ -15188,6 +15188,17 @@ 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,
@@ -15202,6 +15213,9 @@ class Sema final : public SemaBase {
   QualType BuiltinChangeSignedness(QualType BaseType, UTTKind UKind,
                                    SourceLocation Loc);
 
+  bool BuiltinIsConvertible(QualType From, QualType To, SourceLocation Loc,
+                            bool CheckNothrow = false);
+
   bool BuiltinIsBaseOf(SourceLocation RhsTLoc, QualType LhsT, QualType RhsT);
 
   /// Ensure that the type T is a literal type.