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

[Matheus Izvekov via cfe-commits]

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@@ -1072,12 +1072,25 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
       AliasRhsTemplateArgs, TDeduceInfo, DeduceResults,
       /*NumberOfArgumentsMustMatch=*/false);
 
+  static auto IsNonDeducedArgument = [&](const DeducedTemplateArgument &TA) {
+    // The following cases indicate the template argument is non-deducible:
+    //   1. The result is null. E.g. When it comes from a default template
+    //   argument that doesn't appear in the alias declaration.
+    //   2. The template parameter is a pack and that cannot be deduced from
+    //   the arguments within the alias declaration.
+    // Non-deducible template parameters will persist in the transformed
+    // deduction guide.
+    return TA.isNull() || (TA.getKind() == TemplateArgument::Pack &&
+                           TA.pack_size() == 1 && TA.pack_begin()->isNull());
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mizvekov wrote:

I find the special case here a bit odd.

We do have the concept of a partially deduced pack.
Why does that blow up here? Why doesn't that problem extend to wholly undeduced packs of size greater than one?

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