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

[via cfe-commits]

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@@ -756,125 +766,123 @@ bool Sema::CheckParameterPacksForExpansion(
     bool &RetainExpansion, std::optional<unsigned> &NumExpansions) {
   ShouldExpand = true;
   RetainExpansion = false;
-  std::pair<IdentifierInfo *, SourceLocation> FirstPack;
-  bool HaveFirstPack = false;
-  std::optional<unsigned> NumPartialExpansions;
-  SourceLocation PartiallySubstitutedPackLoc;
+  std::pair<const IdentifierInfo *, SourceLocation> FirstPack;
+  std::optional<std::pair<unsigned, SourceLocation>> PartialExpansion;
+  std::optional<unsigned> CurNumExpansions, CurMaximumOfLeastExpansions;
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cor3ntin wrote:

Can we add comments?

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