[ConstraintSystem] Form a special one type binding set for pack expansion variables

Pack expansion type variable can only ever have one binding
which is handled by \c resolvePackExpansion.

There is no need to iterate over other bindings here because
there is no use for contextual types (unlike closures that can
propagate contextual information into the body).

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -7207,6 +7207,21 @@ TypeVarBindingProducer::TypeVarBindingProducer(BindingSet &bindings)
     return;
   }
 
+  // Pack expansion type variable can only ever have one binding
+  // which is handled by \c resolvePackExpansion.
+  //
+  // There is no need to iterate over other bindings here because
+  // there is no use for contextual types (unlike closures that can
+  // propagate contextual information into the body).
+  if (TypeVar->getImpl().isPackExpansion()) {
+    for (const auto &entry : bindings.Defaults) {
+      auto *constraint = entry.second;
+      Bindings.push_back(getDefaultBinding(constraint));
+    }
+
+    return;
+  }
+
   // A binding to `Any` which should always be considered as a last resort.
   Optional<Binding> Any;