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

[Ilya Biryukov via cfe-commits]

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@@ -1700,6 +1717,21 @@ namespace {
       return inherited::TransformTemplateArgument(Input, Output, Uneval);
     }
 
+    using TreeTransform::TransformTemplateSpecializationType;
+    QualType
+    TransformTemplateSpecializationType(TypeLocBuilder &TLB,
+                                        TemplateSpecializationTypeLoc TL) {
+      auto *T = TL.getTypePtr();
+      if (!getSema().ArgPackSubstIndex || !T->isSugared() ||
+          !isPackProducingBuiltinTemplateName(T->getTemplateName()))
+        return TreeTransform::TransformTemplateSpecializationType(TLB, TL);
+      // Look through sugar to get to the SubstBuiltinTemplatePackType that we
+      // need to substitute into.
+      QualType R = TransformType(T->desugar());
+      TLB.pushTrivial(getSema().getASTContext(), R, TL.getBeginLoc());
+      return R;
+    }
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ilya-biryukov wrote:

> But I'd think you be able to build a new AST Node which is basically a stripped down base class of PackIndexType which would be a good fit for this, and that's not as much work as building something from scratch.

Extracting a shared base definitely seems like a good, left a FIXME for now, will make sure to follow up on that.

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