Function Prototype Narrowing with nils

[Rathoz]

How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

Which OS are you using?

Windows

What is the issue affecting?

Type Checking

Expected Behaviour

Foo.Bar('Baz', 'Qux') -- **string**
Foo.Bar('Baz', nil) -- nil
Foo.Bar('Baz') -- nil
Foo.Bar(nil, 'Qux') -- nil
Foo.Bar(nil, nil) -- nil
Foo.Bar() -- nil

Actual Behaviour

Foo.Bar('Baz', 'Qux') -- **string|nil**
Foo.Bar('Baz', nil) -- nil
Foo.Bar('Baz') -- nil
Foo.Bar(nil, 'Qux') -- nil
Foo.Bar(nil, nil) -- nil
Foo.Bar() -- nil

Reproduction steps

---@param baz string|number
---@param qux string|number
---@return string
---@overload fun(text: string|number, title: nil):nil
---@overload fun(text: nil, title: string|number):nil
---@overload fun(text: nil, title: nil):nil
function Foo.Bar(baz, qux)
	if not baz or not qux then
		return nil
	end
	return 'SomeString'
end

Additional Notes

It seems to match the 'strings' can be nil

First input example

Last input example

Log File

No response

[tomlau10]

Just did some testings, seems need to add a ? when declaration a function param to be of nil type 😕
Then LuaLS can type narrow the function signature correctly:

---@param baz string|number
---@param qux string|number
---@return string
---@overload fun(text: string|number, title?: nil):nil ### add `?` to `title`
---@overload fun(text?: nil, title: string|number):nil ### add `?` to `text`
---@overload fun(text?: nil, title?: nil):nil ### add `?` to both `text` and `title`
function Foo.Bar(baz, qux)
    if not baz or not qux then
        return nil
    end
    return 'SomeString'
end

local a = Foo.Bar('Baz', 'Qux') -- **string** => it's corrects now
local a = Foo.Bar('Baz', nil) -- nil
local a = Foo.Bar('Baz') -- nil
local a = Foo.Bar(nil, 'Qux') -- nil
local a = Foo.Bar(nil, nil) -- nil
local a = Foo.Bar() -- nil

[Rathoz]

Interesting workaround 🤔