Infer function parameter types when overriding the same-named class function in an instance of that class

[tomlau10]

resolves #2158, #2569 (comment)

The problem

Sometimes we will want to define interface for overriding in instance object of that class, and retain the param types defined in the interface while overriding. But currently no way to do so as described in #2158 and #2569 (comment)

---@class A
local A = {}

---@return A
function A.new() end

---@param x number
function A:user_callback(x) end

local a = A.new()
function a:user_callback(x) end -- no type infer for `x`, also throwing `duplicate-set-field` warning

Proposed change

Detailed explanation here: #2569 (comment)

Expected Result

---@class A
local A = {}

---@return A
function A.new() end

---@param x number
function A:user_callback(x) end

local a = A.new()
function a:user_callback(x)
    -- x -> number
    -- and no more `duplicate-set-field` warning :)
end

Additional notes

• The way that I modified duplicate-set-field diagnostic is allowing an instance variable to override any of its class function/method.
  i.e. all function/methods from a class can be overriden in variable of that type, without any duplicate-set-field warning.
• This auto infer only works in a class variable / instance variable pair ✅
  i.e. it doesn't work when the namespace is just a global variable ❌

--- file a
global_namespace = {}

---@param x number
function global_namespace.user_callback(x) end

--- file b
function global_namespace.user_callback(x)
    -- this will not work, because this is not in `class/type` variable pair
end

• I believe a @override will be definitely needed for that case, as otherwise no way to distinguish between setting duplicate field or really doing override 😕

---

中文版

• 支持在 instance variable 下 override 其 class function 時做 auto param type infer
• 同時忽略在這種 use case 下的 duplicate-set-field warning
• 這個只針對 class/type variable 組合下的 override 場景 (例子見上邊)
  • 並不處理 define 成 global namespace 後再做 override 的使用場景
  • 該種 use case 似必需等待 @override 語法支持了，否則不能區分是出現 duplicate 還是真的想 override

[sumneko]

Thank you!