[clang] Reapply "[Clang] Improve diagnostics for expansion length mismatch" (PR #121044)

[via cfe-commits]

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@@ -749,132 +759,124 @@ ExprResult Sema::CheckPackExpansion(Expr *Pattern, SourceLocation EllipsisLoc,
     PackExpansionExpr(Context.DependentTy, Pattern, EllipsisLoc, NumExpansions);
 }
 
+static bool IsUnexpandedPackExpansion(const TemplateArgument &TA) {
+  if (!TA.isPackExpansion())
+    return false;
+
+  if (TA.getKind() == TemplateArgument::Type)
+    return !TA.getAsType()->getAs<PackExpansionType>()->getNumExpansions();
+
+  if (TA.getKind() == TemplateArgument::Expression)
+    return !cast<PackExpansionExpr>(TA.getAsExpr())->getNumExpansions();
+
+  return !TA.getNumTemplateExpansions();
+}
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cor3ntin wrote:

Can you leave a comment here? This is a bit confusing.
Actually, I think having it as a lambda, like in the current code, might be clearer.

It's not just that they are unexpended, they are also unresolvable at that point.

https://github.com/llvm/llvm-project/pull/121044