`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

Commencez par reproduire le diagnostic need-check-nil dans l’extension Visual Studio Code avec l’exemple Lua de l’issue, puis suivez la logique de propagation du nil-check du diagnostic. C’est terminé lorsque l’exemple n’émet plus d’avertissement sur item.key, car la vérification précédente via x exclut déjà que item soit nil.