[clang] [clang-tools-extra] [Clang] Add a builtin that deduplicate types into a pack (PR #106730)

[Erich Keane via cfe-commits]

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@@ -6654,6 +6658,56 @@ 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;
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erichkeane wrote:

Who owns these arguments? Are they part of the containing declaration?  How do we know which set are ours?  

It IS a touch confusing we're using the type-bits instead of an arrayref here, but justified :(  (Nothing to do with this sentence, just lamenting...).

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