Dependent return value types for externally-declared functions

[PhoenixWhitefire]

I'm trying to implement support for Luau's Analyzer via Luau LSP for Visual Studio Code in my runtime. I've gotten most of the way there, and one of the main things left is adding a function which returns a type dependent on it's input, similar to Instance.new in Roblox. I tried looking through the globalTypes.d.luau, but I couldn't figure out how it worked.

Essentially:

local animal = Object.createWithComponent("Deer")
-- type of `animal` should be "Object & Deer", where `Object` and `Deer` are already defined external types
-- `animal` should have the combined members of `Object` and `Deer`

Currently, I just have the Object library defined as:

declare Object: {
  createWithComponent: (( Component: string ) -> ( Object ))
}

[qwitwa]

https://devforum.roblox.com/t/how-to-trick-luau-typechecking-into-returning-instance-classes-based-on-function-arguments/1787282/25

[PhoenixWhitefire]

I've seen that already. However, I'm not sure how to convert it to the syntax of the type definition files that the Typechecker uses. Also, this method seems to be bugged at the moment, unless I'm doing it wrong:

[PhoenixWhitefire]

After making a GitHub Issue about the "bug", it appears to no longer be supported and the DevForum post seems to have been relying on an unintentional behavior:

luau-lang/luau#2044 (comment)

[qwitwa]

Ah, thanks for the reply, this is useful for me to know trying to understand the type system. If you're not already in the Roblox OSS Discord a lot of the language devs are there, I've found that the best way to get technical questions about what is allowed in the typesystem answered, at least in lieu of more rigorous documentation coming to the luau website - I believe improving the documentation story is a goal for this quarter, and I'm trying, as an outsider, to collect up all of these confusing sharp edges so I can write a bit of a guide.