[clang] [clang-tools-extra] [clang] support pack expansions for trailing requires clauses (PR #133190)

[Erich Keane via cfe-commits]

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@@ -78,6 +78,22 @@ class UnresolvedSetImpl;
 class VarTemplateDecl;
 enum class ImplicitParamKind;
 
+// Holds a constraint expression along with a pack expansion index, if
+// expanded.
+struct AssociatedConstraint {
+  const Expr *ConstraintExpr = nullptr;
+  int ArgumentPackSubstitutionIndex = -1;
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erichkeane wrote:

I was thinking about it... I'd LOVE instead (though this is a 'more work' thing :D so nothing to do here unless you'd like to ) if we just  had a `std::optional` like type to represent all of these substitution/pack indexes.  The difference here is that the `optional` type would just use a single value as its `unset` state instead of a separate value, so we don't pay the size cost.

Additionally, we could make all of these unsigned instead of signed ints, so presumably we get a little bit of (unusable for other reasons!) range back.  

Also/also: serialization/etc all ends up being self-solved since they can have their own serialization/etc.

https://github.com/llvm/llvm-project/pull/133190