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

[Ilya Biryukov via cfe-commits]

================
@@ -777,12 +844,32 @@ bool Sema::CheckParameterPacksForExpansion(
     IdentifierInfo *Name;
     bool IsVarDeclPack = false;
     FunctionParmPackExpr *BindingPack = nullptr;
+    std::optional<unsigned> NumPrecomputedArguments;
 
-    if (const TemplateTypeParmType *TTP =
-            ParmPack.first.dyn_cast<const TemplateTypeParmType *>()) {
+    if (auto *TTP = ParmPack.first.dyn_cast<const TemplateTypeParmType *>()) {
       Depth = TTP->getDepth();
       Index = TTP->getIndex();
       Name = TTP->getIdentifier();
+    } else if (auto *TST =
+                   ParmPack.first
+                       .dyn_cast<const TemplateSpecializationType *>()) {
+      assert(isPackProducingBuiltinTemplateName(TST->getTemplateName()));
+      // Delay expansion, substitution is required to know the size.
+      ShouldExpand = false;
+      if (!FailOnPackProducingTemplates)
+        continue;
+
+      // It is not yet supported in many positions.
+      Diag(PatternRange.getBegin().isValid() ? PatternRange.getBegin()
+                                             : EllipsisLoc,
+           diag::err_unsupported_builtin_template_pack_position)
+          << TST->getTemplateName();
+      return true;
----------------
ilya-biryukov wrote:

Done.

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