[clang] [Sema] Substitute parameter packs when deduced from function arguments (PR #79371)

[Gábor Spaits via cfe-commits]

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@@ -858,6 +859,27 @@ class PackDeductionScope {
       Info.PendingDeducedPacks[Pack.Index] = Pack.Outer;
   }
 
+  // Return the size of the saved packs if all of them has the same size.
+  std::optional<unsigned> getSavedPackSizeIfAllEqual() const {
+    if (Packs.size() == 0 ||
+        Packs[0].Saved.getKind() != clang::TemplateArgument::Pack)
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spaits wrote:

The reason I added them is because I wanted to be careful. What if someone constructs a `PackDeductionScope` with a non pack `TemplateArgument`.

Should I replace them with insertions?

https://github.com/llvm/llvm-project/pull/79371