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

[via cfe-commits]

================
@@ -888,31 +896,47 @@ bool Sema::CheckParameterPacksForExpansion(
       // Pack comes from another template parameter. 'S<int>' is first
       // instantiated, expanding the outer pack 'Outer' to <int>. The alias
       // declaration is accordingly substituted, leaving the template arguments
-      // as unexpanded
-      // '<Pack...>'.
+      // as unexpanded '<Pack...>'.
       //
       // Since we have no idea of the size of '<Pack...>' until its expansion,
       // we shouldn't assume its pack size for validation. However if we are
       // certain that there are extra arguments beyond unexpanded packs, in
       // which case the pack size is already larger than the previous expansion,
       // we can complain that before instantiation.
-      unsigned LeastNewPackSize = NewPackSize - PendingPackExpansionSize;
-      if (PendingPackExpansionSize && LeastNewPackSize <= *NumExpansions) {
+      if (PendingPackExpansionSize && LeastNewPackSize <= *CurNumExpansions) {
         ShouldExpand = false;
         continue;
       }
       // C++0x [temp.variadic]p5:
       //   All of the parameter packs expanded by a pack expansion shall have
       //   the same number of arguments specified.
-      if (HaveFirstPack)
-        Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict)
-            << FirstPack.first << Name << *NumExpansions
-            << (LeastNewPackSize != NewPackSize) << LeastNewPackSize
-            << SourceRange(FirstPack.second) << SourceRange(ParmPack.second);
-      else
-        Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict_multilevel)
-            << Name << *NumExpansions << (LeastNewPackSize != NewPackSize)
-            << LeastNewPackSize << SourceRange(ParmPack.second);
+      Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict)
+          << FirstPack.first << Name << *CurNumExpansions
+          << (LeastNewPackSize != NewPackSize) << LeastNewPackSize
+          << SourceRange(FirstPack.second) << SourceRange(Loc);
+      return true;
+    }
+  }
+
+  // We have tried our best in the for loop to find out which outer pack
+  // expansion has a different length than the current one (by checking those
+  // Subst* nodes). However, if we fail to determine that, we'll have to
+  // complain the difference in a vague manner.
+  if (NumExpansions && CurNumExpansions &&
+      *NumExpansions != *CurNumExpansions) {
+    // If the current pack expansion contains any unresolved packs, and since
+    // they might eventually expand to match the outer expansion, we don't
+    // complain it too early.
----------------
cor3ntin wrote:

```suggestion
    // diagnose it now.
```

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