[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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@@ -3331,6 +3343,65 @@ checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
     QualType HasNoTypeMember = Converted[2].getAsType();
     return HasNoTypeMember;
   }
+  case BTK__builtin_dedup_pack: {
+    assert(Converted.size() == 1 && "__builtin_dedup_pack should be given "
+                                    "a parameter pack");
+    TemplateArgument Ts = Converted[0];
+    // Delay the computation until we can compute the final result. We choose
+    // not to remove the duplicates upfront before substitution to keep the code
+    // simple.
+    if (Ts.isDependent())
+      return QualType();
+    assert(Ts.getKind() == clang::TemplateArgument::Pack);
+    llvm::SmallVector<TemplateArgument> OutArgs;
+    llvm::SmallDenseSet<QualType> Seen;
+    // Synthesize a new template argument list, removing duplicates.
+    for (auto T : Ts.getPackAsArray()) {
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erichkeane wrote:

I suspect it is a 'holding it wrong'?  Though I probably MEANT to suggest `SetVector`?  Does that make it easier?

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