`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

Start by reproducing the need-check-nil diagnostic in the Visual Studio Code extension with the Lua example in the issue, then trace the diagnostic's nil-check propagation logic. Done means the example no longer warns on item.key when the earlier check through x already excludes item from being nil.