class T::Private::Methods::DeclBuilder

def checked(level)
  check_live!
  if !decl.checked.equal?(ARG_NOT_PROVIDED)
    raise BuilderError.new("You can't call .checked multiple times in a signature.")
  end
  if level == :never && !decl.on_failure.equal?(ARG_NOT_PROVIDED)
    raise BuilderError.new("You can't use .checked(:#{level}) with .on_failure because .on_failure will have no effect.")
  end
  if !T::Private::RuntimeLevels::LEVELS.include?(level)
    raise BuilderError.new("Invalid `checked` level '#{level}'. Use one of: #{T::Private::RuntimeLevels::LEVELS}.")
  end
  decl.checked = level
  self
end