[clang] [Clang] Fix various bugs in alias CTAD transform (PR #132061)

[Younan Zhang via cfe-commits]

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@@ -1077,7 +1077,11 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
   // !!NOTE: DeduceResults respects the sequence of template parameters of
   // the deduction guide f.
   for (unsigned Index = 0; Index < DeduceResults.size(); ++Index) {
-    if (const auto &D = DeduceResults[Index]; !D.isNull()) // Deduced
+    const auto &D = DeduceResults[Index];
+    bool NonDeduced =
+        D.isNull() || (D.getKind() == TemplateArgument::Pack &&
+                       D.pack_size() == 1 && D.pack_begin()->isNull());
+    if (!NonDeduced)
----------------
zyn0217 wrote:

Added explanations.

https://github.com/llvm/llvm-project/pull/132061
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