overload type narrowing for returns

How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

Which OS are you using?

Linux

What is the issue affecting?

Type Checking, Completion

Expected Behaviour

I am not sure this is a duplicate issue considering how common it probably is, but just to make sure.
Possibly a duplicate of https://github.com/LuaLS/lua-language-server/issues/1583.
This is similar to https://github.com/LuaLS/lua-language-server/issues/1146, but with returns instead of params.

It is a common pattern in Lua that on errors you might instead return a nil value to imply failure, along side an error message.

---@return table
---@overload fun(): nil, string
local function fetchX() end

local resource, fail = fetchX()
if not resource then
  print(fail:sub(1, 3)) -- fail type here is expected to be "string"
end

Actual Behaviour

You get a warning on fail that it might possibly be nil. Because its type is string|nil.

---@return table
---@overload fun(): nil, string
local function fetchX() end

local resource, fail = fetchX()
if not resource then
  print(fail:sub(1, 3) )-- fail type here is actually "string|nil"
end

Reproduction steps

1. Copy the setup code
2. follow along with the comments

Additional Notes

I believe narrowing down the types in this case is still possible? It seems to me like https://github.com/LuaLS/lua-language-server/issues/1583 was discussing the possibility of adding tuples, but shouldn't this be doable even without tuples support?

Log File

No response

まず issue にある overload-return の例を再現し、条件付き narrowing に対する language server の型チェックと補完の挙動を調べます。payload にはリポジトリのファイルやテストが記載されていないため、変更を加える前に、関連する overload と return-type の解析エントリーポイントを特定してください。resource が nil であることを確認した後に fail が string に絞り込まれ、既存の警告が出ない状態になれば完了です。