`need-check-nil` when nil check is done in a function

[Rathoz]

How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

Which OS are you using?

Windows

What is the issue affecting?

Type Checking

Expected Behaviour

No type checking errors

Actual Behaviour

Need check nil.Lua Diagnostics.(need-check-nil) on the :lower()

Reproduction steps

Simplified case with the issue

---@param str string?
---@return boolean
local function isEmpty(str)
	return str == nil or str == ''
end

---@param input string?
---@return string?
local function myFunc(input)
	if isEmpty(input) then
		return
	end
	return input:lower()
end

Additional Notes

No response

Log File

No response

[RomanSpector]

I also often use this feature and do not want to give it up.

--- - **string** == "" -> **`true`**
--- - **number** -> **`false`**
--- - **boolean** -> **`false`**
--- - **nil** -> **`true`**
--- - **function** -> **`false`**
--- - **#table == 0** or **table:count() == 0** -> **`true`**
---@overload fun(value: string) : boolean
---@overload fun(value: number) : boolean
---@overload fun(value: nil) : boolean
---@overload fun(value: boolean) : boolean
---@overload fun(value: function) : boolean
---@overload fun(value: table) : boolean
function Core.IsEmpty(value) end

[carsakiller]

Add ---@cast input -nil above return input:lower() and the type checking will work as desired:

Fix

---@param str string?
---@return boolean
local function isEmpty(str)
	return str == nil or str == ''
end

---@param input string?
---@return string?
local function myFunc(input)
	if isEmpty(input) then
		return
	end
    ---@cast input -nil
	return input:lower()
end

The server is not able to follow the function and confirm that input is not nil.

[Rathoz]

If the isEmpty annotations are changed from

---@param str string?
---@return boolean
local function isEmpty(str)
	return str == nil or str == ''
end

to

---@param str string
---@return boolean
---@overload fun(str: nil) : true
local function isEmpty(str)
	return str == nil or str == ''
end

Would that make it easier to add the functionality to the LSP? Adding a cast after every call to an isEmpty would uglify the code quite a lot.