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

[Ilya Biryukov via cfe-commits]

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@@ -1666,6 +1685,21 @@ 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());
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ilya-biryukov wrote:

> The issue I see in this case here is that you are just dropping the TemplateSpecializationType, why can't you just build a new one here to wrap over the transformed underlying type?

`TemplateSpecializationType` looks like the right sugar for the whole pack, but after we pick a single element from it I don't think it models what we want correctly. I.e. `dedup<int, double> == argpack{int, double}`, but **not equal** to the individual `int` or `double` after we've picked them.
So I decided to stick with the lack of sugar rather than pick the incorrect one (from that perspective).

Or do you feel `TemplateSpecializationType` is a good fit here?

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