local lanes = require "lanes".configure{ with_timers = true, verbose_errors = true} -- with timers enabled

local function foo()
	local lanes = lanes -- lanes as upvalue
end

local g = lanes.gen( "*", { name = 'auto', error_trace_level = "extended"}, foo)

-- this should raise an error as lanes.timer_lane is a Lane (a non-deep full userdata)
local res, err = pcall( g)
assert(res == false and type(err) == "string", "got " .. tostring(res) .. " " .. tostring(err))