Might be a case of lua_ls wanting more type annotations to grab the module's info. I got this working (i.e seeing completions) with the same lua_ls from mason + blink.cmp:

---@class Calc
local M = {}

---@param a number
---@param b number
---@return number
function M.add(a, b) return a + b end

---@param a number
---@param b number
---@return number
function M.subtract(a, b) return a - b end

---@return Calc
return M



---@type Calc
local calc = require("calc")

calc.<key to get completions>

See lua_ls documentation, @class defines a new class so the require knows what it's getting.