class RBS::AST::TypeParam

def self.validate(type_params)
  optionals = type_params.filter {|param| param.default_type }
  optional_param_names = optionals.map(&:name).sort
  optionals.filter! do |param|
    default_type = param.default_type or raise
    optional_param_names.any? { default_type.free_variables.include?(_1) }
  end
  unless optionals.empty?
    optionals
  end
end