Fix arg iteration

trans/transpiler/phases/emit_cpp/block.py

@@ -61,7 +61,7 @@ class BlockVisitor(NodeVisitor):
             yield f"{node.name}_inner::"
         yield "operator()"
         yield "("
-        padded_defaults = [None] * node.type.optional_at + node.args.defaults
+        padded_defaults = [None] * (node.type.optional_at or len(node.args.args)) + node.args.defaults
         args_iter = zip(node.args.args, node.type.parameters, padded_defaults)
         if skip_first_arg:
             next(args_iter)