[clang] [clang-tools-extra] [Clang] Add builtins that deduplicate and sort types (PR #106730)

[Ilya Biryukov via cfe-commits]

================
@@ -6484,6 +6499,57 @@ 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 represents a pack expansion that had to be
+/// delayed,
+/// - TODO: represents a pack expansion represented by 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);
----------------
ilya-biryukov wrote:

Thanks @zyn0217 for capturing the discussion.

FWIW, I have also been in favor of a more direct implementation that is simpler (see #139730), but I ultimately lost this battle (which is fine, as long as this implementation does have a chance to contribute to a new feature in the future). The downside is that my response times got really long as I couldn't find the time to dive deeper into the more complex implementation we need here, but that's on me (and sorry about that).

https://github.com/llvm/llvm-project/pull/106730