how to make this typecheck?

[velvet jay]

i have a small UI library that mostly returns setmetatable(SomeTable, { __index = SomeWidget (extends GuiBase) }). how do i make a type Widget that accepts both regular widgets (i.e. GuiBase) and the table-with-metatable?

example:

type UIButton = setmetatable<{
  Label: TextLabel,
}, { __index: ImageButton }>

function createButton(label: string): UIButton
  local self = Instance.new "ImageButton"
  local wrapper = {}

  self.Image = "blah blah blah"
  -- blah blah blah

  wrapper.Label = Instance.new "TextLabel"
  wrapper.Label.Parent = self

  return setmetatable(wrapper, { __index = self })
end

-- help with this pls
type Widget = GuiBase | setmetatable<{}, { __index: GuiBase }>

function addPadding(pad: number, child: Widget): UIPadding
  local self = Instance.new "UIPadding"
  -- blah blah blah
  return self
end

local b = createButton "Click on me"
local padded = addPadding(6, b) -- typechecker fails because UIButton cant be converted to Widget :/

and no, "just dont have --!strict" or "type Widget = any" is not a solution

[velvet jay]

actually i'm pretty sure this is impossible without covariance/contravariance in generics or any :/

[somber iron]

type Widget = GuiBase & {
   Label: TextLabel
}

[velvet jay]

i want it to match any "regular ui widget or has ui widget in __index"
like the UIButton above, this thing below, etc

-- just an example
type UIScrollingList = setmetatable<{
  Layout: UIListLayout,
  CanvasSize: number,
}, { __index = ScrollingFrame }>

[somber iron]

It should be possible with union

[velvet jay]

so Widget = UIButton | UIScrollingList | ...

[somber iron]

Well yea

While looking ugly I dont think you have any other methods

Unless you manually make the UIBase

Also the type checking is yet to be able to actually predict what type would be returned from methods/functions

So you are at best making only some of the properties visible or all of them