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

[Ilya Biryukov via cfe-commits]

================
@@ -5154,6 +5177,84 @@ bool TreeTransform<Derived>::TransformTemplateArguments(
 
 }
 
+template <typename Derived>
+bool TreeTransform<Derived>::PreparePackForExpansion(TemplateArgumentLoc In,
+                                                     bool Uneval,
+                                                     TemplateArgumentLoc &Out,
+                                                     UnexpandedInfo &Info) {
+  auto ComputeInfo = [this](TemplateArgumentLoc Arg,
+                            bool IsLateExpansionAttempt, UnexpandedInfo &Info,
+                            TemplateArgumentLoc &Pattern) {
+    assert(Arg.getArgument().isPackExpansion());
+    // We have a pack expansion, for which we will be substituting into the
+    // pattern.
+    Pattern = getSema().getTemplateArgumentPackExpansionPattern(
+        Arg, Info.Ellipsis, Info.OrigNumExpansions);
+    SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+    getSema().collectUnexpandedParameterPacks(Pattern, Unexpanded);
+    if (IsLateExpansionAttempt) {
+      // Request expansion only when there is an opportunity to expand a pack
+      // that required a substituion first.
+      bool SawPackTypes =
+          llvm::any_of(Unexpanded, [](UnexpandedParameterPack P) {
+            return P.first.dyn_cast<const SubstBuiltinTemplatePackType *>();
+          });
+      if (!SawPackTypes) {
+        Info.Expand = false;
+        return false;
+      }
+    }
+    assert(!Unexpanded.empty() && "Pack expansion without parameter packs?");
+
+    // Determine whether the set of unexpanded parameter packs can and
+    // should be expanded.
+    Info.Expand = true;
+    Info.RetainExpansion = false;
+    Info.NumExpansions = Info.OrigNumExpansions;
+    return getDerived().TryExpandParameterPacks(
+        Info.Ellipsis, Pattern.getSourceRange(), Unexpanded, false, Info.Expand,
+        Info.RetainExpansion, Info.NumExpansions);
+  };
+
+  TemplateArgumentLoc Pattern;
+  if (ComputeInfo(In, false, Info, Pattern))
+    return true;
+
+  if (Info.Expand) {
+    Out = Pattern;
+    return false;
+  }
+
+  // The transform has determined that we should perform a simple
+  // transformation on the pack expansion, producing another pack
+  // expansion.
+  TemplateArgumentLoc OutPattern;
+  std::optional<Sema::ArgPackSubstIndexRAII> SubstIndex(
+      std::in_place, getSema(), std::nullopt);
+  if (getDerived().TransformTemplateArgument(Pattern, OutPattern, Uneval))
+    return true;
+
+  Out = getDerived().RebuildPackExpansion(OutPattern, Info.Ellipsis,
+                                          Info.NumExpansions);
+  if (Out.getArgument().isNull())
+    return true;
+  SubstIndex.reset();
+
+  if (!OutPattern.getArgument().containsUnexpandedParameterPack())
+    return false;
+
+  // Some packs will learn their length after substitution.
+  // We may need to request their expansion.
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ilya-biryukov wrote:

I wrote a pretty long comment, hope it clarifies things.
PTAL

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