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

Comienza reproduciendo el ejemplo de retorno de sobrecarga del issue e inspecciona el comportamiento del language server en la comprobación de tipos y la finalización para el estrechamiento condicional. El payload no menciona archivos ni pruebas del repositorio, así que localiza los puntos de entrada relevantes para el análisis de sobrecargas y tipos de retorno antes de hacer cambios. Se considera terminado cuando fail se estrecha a string después de comprobar que resource es nil, sin la advertencia existente.