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

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

首先通过 language server 运行 issue 中的 Lua 示例，并检查产生 need-check-nil 诊断的类型检查路径。跟踪 isEmpty 函数的结果如何影响对 input 的收窄，然后验证 input:lower() 不会产生该诊断，同时其他位置的 nil 检查仍然正确。