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

[Matheus Izvekov via cfe-commits]

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@@ -1650,6 +1665,23 @@ namespace {
       return inherited::TransformTemplateArgument(Input, Output, Uneval);
     }
 
+    std::optional<unsigned> ComputeSizeOfPackExprWithoutSubstitution(
+        ArrayRef<TemplateArgument> PackArgs) {
+      // Don't do this when rewriting template parameters for CTAD:
+      //   1) The heuristic needs the unpacked Subst* nodes to figure out the
+      //   expanded size, but this never applies since Subst* nodes are not
+      //   created in rewrite scenarios.
+      //
+      //   2) The heuristic substitutes into the pattern with pack expansion
+      //   suppressed, which does not meet the requirements for argument
+      //   rewriting when template arguments include a non-pack matching against
+      //   a pack, particularly when rewriting an alias CTAD.
+      if (TemplateArgs.isRewrite())
+        return std::nullopt;
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mizvekov wrote:

Instead of special casing this on rewrite, can we do that whenever we have dependent template arguments?

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