[Clang] Add a builtin that deduplicate types into a pack

clang-tools-extra/clangd/unittests/FindTargetTests.cpp

@@ -731,6 +731,12 @@ TEST_F(TargetDeclTest, BuiltinTemplates) {
     using type_pack_element = [[__type_pack_element]]<N, Pack...>;
   )cpp";
   EXPECT_DECLS("TemplateSpecializationTypeLoc", );
+
+  Code = R"cpp(
+    template <template <class...> class Templ, class... Types>
+    using dedup_types = Templ<[[__builtin_dedup_pack]]<Types...>...>;
+  )cpp";
+  EXPECT_DECLS("TemplateSpecializationTypeLoc", );
 }
 
 TEST_F(TargetDeclTest, MemberOfTemplate) {

clang/docs/LanguageExtensions.rst

@@ -1791,6 +1791,37 @@ __make_integer_seq
 
 This alias returns ``IntSeq`` instantiated with ``IntSeqT = T``and ``Ints`` being the pack ``0, ..., N - 1``.
 
+__builtin_dedup_pack
+--------------------
+
+.. code-block:: c++
+
+  template <class... Ts>
+  using __builtin_dedup_pack = ...;
+
+This alias takes a template parameter pack ``Ts`` and produces a new unexpanded pack containing the unique types
+from ``Ts``, with the order of the first occurrence of each type preserved.
+It is useful in template metaprogramming to normalize type lists.
+
+The resulting pack can be expanded in contexts like template argument lists or base specifiers.
+
+**Example of Use**:
+
+.. code-block:: c++
+
+  template <typename...> struct TypeList;
+
+  // The resulting type is TypeList<int, double, char>
+  template <typename ...ExtraTypes>
+  using MyTypeList = TypeList<__builtin_dedup_pack<int, double, int, char, double, ExtraTypes...>...>;
+
+**Limitations**:
+
+* This builtin can only be used inside a template.
+* The resulting pack is currently only supported for expansion in template argument lists and base specifiers.
+* This builtin cannot be assigned to a template template parameter.
+
+
 Type Trait Primitives
 =====================
 

clang/docs/ReleaseNotes.rst

@@ -142,6 +142,21 @@ Non-comprehensive list of changes in this release
   correct method to check for these features is to test for the ``__PTRAUTH__``
   macro.
 
+- Added a new builtin, ``__builtin_dedup_pack``, to remove duplicate types from a parameter pack.
+  This feature is particularly useful in template metaprogramming for normalizing type lists.
+  The builtin produces a new, unexpanded parameter pack that can be used in contexts like template
+  argument lists or base specifiers.
+
+  .. code-block:: c++
+
+    template <typename...> struct TypeList;
+
+    // The resulting type is TypeList<int, double, char>
+    using MyTypeList = TypeList<__builtin_dedup_pack<int, double, int, char, double>...>;
+
+  Currently, the use of ``__builtin_dedup_pack`` is limited to template arguments and base
+  specifiers, it also must be used within a template context.
+
 
 New Compiler Flags
 ------------------

clang/include/clang/AST/ASTContext.h

@@ -230,6 +230,8 @@ class ASTContext : public RefCountedBase<ASTContext> {
     SubstTemplateTypeParmTypes;
   mutable llvm::FoldingSet<SubstTemplateTypeParmPackType>
     SubstTemplateTypeParmPackTypes;
+  mutable llvm::FoldingSet<SubstBuiltinTemplatePackType>
+      SubstBuiltinTemplatePackTypes;
   mutable llvm::ContextualFoldingSet<TemplateSpecializationType, ASTContext&>
     TemplateSpecializationTypes;
   mutable llvm::FoldingSet<ParenType> ParenTypes{GeneralTypesLog2InitSize};
@@ -1895,6 +1897,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   QualType getSubstTemplateTypeParmPackType(Decl *AssociatedDecl,
                                             unsigned Index, bool Final,
                                             const TemplateArgument &ArgPack);
+  QualType getSubstBuiltinTemplatePack(const TemplateArgument &ArgPack);
 
   QualType
   getTemplateTypeParmType(unsigned Depth, unsigned Index,

clang/include/clang/AST/DeclTemplate.h

@@ -1796,7 +1796,10 @@ class BuiltinTemplateDecl : public TemplateDecl {
   }
 
   BuiltinTemplateKind getBuiltinTemplateKind() const { return BTK; }
+
+  bool isPackProducingBuiltinTemplate() const;
 };
+bool isPackProducingBuiltinTemplateName(TemplateName N);
 
 /// Provides information about an explicit instantiation of a variable or class
 /// template.

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -492,6 +492,8 @@ template <typename Derived> class RecursiveASTVisitor {
   bool TraverseTemplateArgumentLocsHelper(const TemplateArgumentLoc *TAL,
                                           unsigned Count);
   bool TraverseArrayTypeLocHelper(ArrayTypeLoc TL);
+  bool TraverseSubstPackTypeHelper(SubstPackType *T);
+  bool TraverseSubstPackTypeLocHelper(SubstPackTypeLoc TL);
   bool TraverseRecordHelper(RecordDecl *D);
   bool TraverseCXXRecordHelper(CXXRecordDecl *D);
   bool TraverseDeclaratorHelper(DeclaratorDecl *D);
@@ -1138,9 +1140,10 @@ DEF_TRAVERSE_TYPE(TemplateTypeParmType, {})
 DEF_TRAVERSE_TYPE(SubstTemplateTypeParmType, {
   TRY_TO(TraverseType(T->getReplacementType()));
 })
-DEF_TRAVERSE_TYPE(SubstTemplateTypeParmPackType, {
-  TRY_TO(TraverseTemplateArgument(T->getArgumentPack()));
-})
+DEF_TRAVERSE_TYPE(SubstTemplateTypeParmPackType,
+                  { TRY_TO(TraverseSubstPackTypeHelper(T)); })
+DEF_TRAVERSE_TYPE(SubstBuiltinTemplatePackType,
+                  { TRY_TO(TraverseSubstPackTypeHelper(T)); })
 
 DEF_TRAVERSE_TYPE(AttributedType,
                   { TRY_TO(TraverseType(T->getModifiedType())); })
@@ -1481,9 +1484,26 @@ DEF_TRAVERSE_TYPELOC(TemplateTypeParmType, {})
 DEF_TRAVERSE_TYPELOC(SubstTemplateTypeParmType, {
   TRY_TO(TraverseType(TL.getTypePtr()->getReplacementType()));
 })
-DEF_TRAVERSE_TYPELOC(SubstTemplateTypeParmPackType, {
+
+template <typename Derived>
+bool RecursiveASTVisitor<Derived>::TraverseSubstPackTypeLocHelper(
+    SubstPackTypeLoc TL) {
   TRY_TO(TraverseTemplateArgument(TL.getTypePtr()->getArgumentPack()));
-})
+  return true;
+}
+
+template <typename Derived>
+bool RecursiveASTVisitor<Derived>::TraverseSubstPackTypeHelper(
+    SubstPackType *T) {
+  TRY_TO(TraverseTemplateArgument(T->getArgumentPack()));
+  return true;
+}
+
+DEF_TRAVERSE_TYPELOC(SubstTemplateTypeParmPackType,
+                     { TRY_TO(TraverseSubstPackTypeLocHelper(TL)); })
+
+DEF_TRAVERSE_TYPELOC(SubstBuiltinTemplatePackType,
+                     { TRY_TO(TraverseSubstPackTypeLocHelper(TL)); })
 
 DEF_TRAVERSE_TYPELOC(ParenType, { TRY_TO(TraverseTypeLoc(TL.getInnerLoc())); })
 

clang/include/clang/AST/Type.h

@@ -2210,20 +2210,24 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     unsigned PackIndex : 15;
   };
 
-  class SubstTemplateTypeParmPackTypeBitfields {
+  class SubstPackTypeBitfields {
+    friend class SubstPackType;
     friend class SubstTemplateTypeParmPackType;
 
     LLVM_PREFERRED_TYPE(TypeBitfields)
     unsigned : NumTypeBits;
 
-    // The index of the template parameter this substitution represents.
-    unsigned Index : 16;
-
     /// The number of template arguments in \c Arguments, which is
     /// expected to be able to hold at least 1024 according to [implimits].
     /// However as this limit is somewhat easy to hit with template
     /// metaprogramming we'd prefer to keep it as large as possible.
     unsigned NumArgs : 16;
+
+    // The index of the template parameter this substitution represents.
+    // Only used by SubstTemplateTypeParmPackType. We keep it in the same
+    // class to avoid dealing with complexities of bitfields that go over
+    // the size of `unsigned`.
+    unsigned SubstTemplTypeParmPackIndex : 16;
   };
 
   class TemplateSpecializationTypeBitfields {
@@ -2340,7 +2344,7 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     VectorTypeBitfields VectorTypeBits;
     TemplateTypeParmTypeBitfields TemplateTypeParmTypeBits;
     SubstTemplateTypeParmTypeBitfields SubstTemplateTypeParmTypeBits;
-    SubstTemplateTypeParmPackTypeBitfields SubstTemplateTypeParmPackTypeBits;
+    SubstPackTypeBitfields SubstPackTypeBits;
     TemplateSpecializationTypeBitfields TemplateSpecializationTypeBits;
     DependentTemplateSpecializationTypeBitfields
       DependentTemplateSpecializationTypeBits;
@@ -6980,6 +6984,56 @@ class SubstTemplateTypeParmType final
   }
 };
 
+/// Represents the result of substituting a set of types as a template argument
+/// that needs to be expanded later.
+///
+/// These types are always dependent and produced depending on the situations:
+/// - SubstTemplateTypeParmPack is an expansion that had to be delayed,
+/// - SubstBuiltinTemplatePackType is an expansion from a builtin.
+class SubstPackType : public Type, public llvm::FoldingSetNode {
+  friend class ASTContext;
+
+  /// A pointer to the set of template arguments that this
+  /// parameter pack is instantiated with.
+  const TemplateArgument *Arguments;
+
+protected:
+  SubstPackType(TypeClass Derived, QualType Canon,
+                const TemplateArgument &ArgPack);
+
+public:
+  unsigned getNumArgs() const { return SubstPackTypeBits.NumArgs; }
+
+  TemplateArgument getArgumentPack() const;
+
+  void Profile(llvm::FoldingSetNodeID &ID);
+  static void Profile(llvm::FoldingSetNodeID &ID,
+                      const TemplateArgument &ArgPack);
+
+  static bool classof(const Type *T) {
+    return T->getTypeClass() == SubstTemplateTypeParmPack ||
+           T->getTypeClass() == SubstBuiltinTemplatePack;
+  }
+};
+
+/// Represents the result of substituting a builtin template as a pack.
+class SubstBuiltinTemplatePackType : public SubstPackType {
+  friend class ASTContext;
+
+  SubstBuiltinTemplatePackType(QualType Canon, const TemplateArgument &ArgPack);
+
+public:
+  bool isSugared() const { return false; }
+  QualType desugar() const { return QualType(this, 0); }
+
+  /// Mark that we reuse the Profile. We do not introduce new fields.
+  using SubstPackType::Profile;
+
+  static bool classof(const Type *T) {
+    return T->getTypeClass() == SubstBuiltinTemplatePack;
+  }
+};
+
 /// Represents the result of substituting a set of types for a template
 /// type parameter pack.
 ///
@@ -6992,7 +7046,7 @@ class SubstTemplateTypeParmType final
 /// that pack expansion (e.g., when all template parameters have corresponding
 /// arguments), this type will be replaced with the \c SubstTemplateTypeParmType
 /// at the current pack substitution index.
-class SubstTemplateTypeParmPackType : public Type, public llvm::FoldingSetNode {
+class SubstTemplateTypeParmPackType : public SubstPackType {
   friend class ASTContext;
 
   /// A pointer to the set of template arguments that this
@@ -7018,21 +7072,17 @@ class SubstTemplateTypeParmPackType : public Type, public llvm::FoldingSetNode {
 
   /// Returns the index of the replaced parameter in the associated declaration.
   /// This should match the result of `getReplacedParameter()->getIndex()`.
-  unsigned getIndex() const { return SubstTemplateTypeParmPackTypeBits.Index; }
+  unsigned getIndex() const {
+    return SubstPackTypeBits.SubstTemplTypeParmPackIndex;
+  }
 
   // This substitution will be Final, which means the substitution will be fully
   // sugared: it doesn't need to be resugared later.
   bool getFinal() const;
 
-  unsigned getNumArgs() const {
-    return SubstTemplateTypeParmPackTypeBits.NumArgs;
-  }
-
   bool isSugared() const { return false; }
   QualType desugar() const { return QualType(this, 0); }
 
-  TemplateArgument getArgumentPack() const;
-
   void Profile(llvm::FoldingSetNodeID &ID);
   static void Profile(llvm::FoldingSetNodeID &ID, const Decl *AssociatedDecl,
                       unsigned Index, bool Final,
@@ -7267,9 +7317,7 @@ class TemplateSpecializationType : public TypeWithKeyword,
             TemplateSpecializationTypeBits.NumArgs};
   }
 
-  bool isSugared() const {
-    return !isDependentType() || isCurrentInstantiation() || isTypeAlias();
-  }
+  bool isSugared() const;
 
   QualType desugar() const {
     return isTypeAlias() ? getAliasedType() : getCanonicalTypeInternal();

clang/include/clang/AST/TypeLoc.h

@@ -989,12 +989,22 @@ class SubstTemplateTypeParmTypeLoc :
                                      SubstTemplateTypeParmType> {
 };
 
-  /// Wrapper for substituted template type parameters.
-class SubstTemplateTypeParmPackTypeLoc :
-    public InheritingConcreteTypeLoc<TypeSpecTypeLoc,
-                                     SubstTemplateTypeParmPackTypeLoc,
-                                     SubstTemplateTypeParmPackType> {
-};
+/// Abstract type representing delayed type pack expansions.
+class SubstPackTypeLoc
+    : public InheritingConcreteTypeLoc<TypeSpecTypeLoc, SubstPackTypeLoc,
+                                       SubstPackType> {};
+
+/// Wrapper for substituted template type parameters.
+class SubstTemplateTypeParmPackTypeLoc
+    : public InheritingConcreteTypeLoc<SubstPackTypeLoc,
+                                       SubstTemplateTypeParmPackTypeLoc,
+                                       SubstTemplateTypeParmPackType> {};
+
+/// Wrapper for substituted template type parameters.
+class SubstBuiltinTemplatePackTypeLoc
+    : public InheritingConcreteTypeLoc<SubstPackTypeLoc,
+                                       SubstBuiltinTemplatePackTypeLoc,
+                                       SubstBuiltinTemplatePackType> {};
 
 struct AttributedLocInfo {
   const Attr *TypeAttr;

clang/include/clang/AST/TypeProperties.td

@@ -820,26 +820,33 @@ let Class = PackExpansionType in {
   }]>;
 }
 
+let Class = SubstPackType in {
+  def : Property<"replacementPack", TemplateArgument> {
+    let Read = [{ node->getArgumentPack() }];
+  }
+}
+
 let Class = SubstTemplateTypeParmPackType in {
   def : Property<"associatedDecl", DeclRef> {
     let Read = [{ node->getAssociatedDecl() }];
   }
   def : Property<"Index", UInt32> {
     let Read = [{ node->getIndex() }];
   }
-  def : Property<"Final", Bool> {
-    let Read = [{ node->getFinal() }];
-  }
-  def : Property<"replacementPack", TemplateArgument> {
-    let Read = [{ node->getArgumentPack() }];
-  }
+  def : Property<"Final", Bool> { let Read = [{ node->getFinal() }]; }
 
   def : Creator<[{
     return ctx.getSubstTemplateTypeParmPackType(
                         associatedDecl, Index, Final, replacementPack);
   }]>;
 }
 
+let Class = SubstBuiltinTemplatePackType in {
+  def : Creator<[{
+    return ctx.getSubstBuiltinTemplatePack(replacementPack);
+  }]>;
+}
+
 let Class = BuiltinType in {
   def : Property<"kind", BuiltinTypeKind> {
     let Read = [{ node->getKind() }];

clang/include/clang/Basic/BuiltinTemplates.td

@@ -62,3 +62,7 @@ def __builtin_common_type : CPlusPlusBuiltinTemplate<
 //           typename ...Operands>
 def __hlsl_spirv_type : HLSLBuiltinTemplate<
 [Uint32T, Uint32T, Uint32T, Class<"Operands", /*is_variadic=*/1>]>;
+
+// template <class ...Args>
+def __builtin_dedup_pack
+    : CPlusPlusBuiltinTemplate<[Class<"Args", /*is_variadic=*/1>]>;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6074,6 +6074,13 @@ def warn_cxx23_pack_indexing : Warning<
 def err_pack_outside_template : Error<
   "pack declaration outside of template">;
 
+def err_builtin_pack_outside_template
+    : Error<"%0 cannot be used outside of template">;
+
+def err_unsupported_builtin_template_pack_expansion
+    : Error<"expansions of %0 are not supported here. Only expansions in "
+            "template arguments and class bases are supported">;
+
 def err_fold_expression_packs_both_sides : Error<
   "binary fold expression has unexpanded parameter packs in both operands">;
 def err_fold_expression_empty : Error<

clang/include/clang/Basic/TypeNodes.td

@@ -97,7 +97,9 @@ def HLSLAttributedResourceType : TypeNode<Type>;
 def HLSLInlineSpirvType : TypeNode<Type>;
 def TemplateTypeParmType : TypeNode<Type>, AlwaysDependent, LeafType;
 def SubstTemplateTypeParmType : TypeNode<Type>, NeverCanonical;
-def SubstTemplateTypeParmPackType : TypeNode<Type>, AlwaysDependent;
+def SubstPackType : TypeNode<Type, 1>;
+def SubstTemplateTypeParmPackType : TypeNode<SubstPackType>, AlwaysDependent;
+def SubstBuiltinTemplatePackType : TypeNode<SubstPackType>, AlwaysDependent;
 def TemplateSpecializationType : TypeNode<Type>, NeverCanonicalUnlessDependent;
 def DeducedType : TypeNode<Type, 1>;
 def AutoType : TypeNode<DeducedType>;