[clang] [clang-tools-extra] [clang] Concepts: support pack expansions for type constraints (PR #132626)

[Matheus Izvekov via cfe-commits]

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@@ -70,7 +70,14 @@ struct alignas(ConstraintAlignment) AtomicConstraint {
     // We do not actually substitute the parameter mappings into the
     // constraint expressions, therefore the constraint expressions are
     // the originals, and comparing them will suffice.
-    if (ConstraintExpr != Other.ConstraintExpr)
+    if (AC.ConstraintExpr != Other.AC.ConstraintExpr)
+      return false;
+
+    // FIXME: As the normalization cache doesn't take
+    // ArgumentPackSubstitutionIndex into account,
+    // this won't have an effect.
+    if (AC.ArgumentPackSubstitutionIndex !=
+        Other.AC.ArgumentPackSubstitutionIndex)
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mizvekov wrote:

I think in this case the constraint was expanded, the pack index should be part of the mapping.

A different pack index is notionally a different template parameter, or at least another slot where a different template argument could be located, so the different treatment wrt to non-pack parameters doesn't seem consistent to me. 

Though it's possible this doesn't make a difference in practice.
I failed to come up with an example where this would really matter, ie make something ill-formed where it's not, or vice-versa.

https://github.com/llvm/llvm-project/pull/132626