`need-check-nil` doesn't recognize nil check propagation

How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

Which OS are you using?

MacOS

What is the issue affecting?

Diagnostics/Syntax Checking

Expected Behaviour

Ideally, need-check-nil would infer a nil check has been completed if I've already done that nil check earlier as part of a larger check. Specifically, the statement if not (a and a.b) then will properly register as a nil check on a, but assigning the inner expression to a local var prevents LLS from detecting it as a nil check.

Actual Behaviour

LLS warns need-check-nil when a variable cannot possibly be nil, if the check was masked via another variable.

Reproduction steps

---@return table?
local function stuff()
    return nil
end

local item = stuff()
local x = item and item.x
if not x then -- Works: `if not (item and item.x) then`
   -- This includes case where `item` is `nil` 
else
    local subitem = item.key -- Error on `item` (need-check-nil)
end

Additional Notes

No response

Log File

No response

首先使用 issue 中的 Lua 示例，在 Visual Studio Code 扩展中复现 need-check-nil 诊断，然后跟踪该诊断的 nil-check 传播逻辑。当之前通过 x 进行的检查已经排除了 item 为 nil 的可能性时，该示例不再对 item.key 发出警告，即表示完成。