type:hassubtypeof

[gaymeowing]

Proposes adding a new method to the type userdata for subtype checking.

Rendered

[aatxe]

Haven't read this yet, but just wanted to say that this is, in general, a reasonable idea and something we'd like to consider, for sure. The devil is in the details, but this was originally part of the plan and got cut to scope down the MVP for user-defined type functions.

[gaymeowing]

Don't have an idea on what the description for the musthave methods should be, so its just the same as the has methods.

[gaymeowing]

Making this not a draft as its much more well described now, plus to hopefully have someone contribute a description for the musthave methods.

[gaymeowing]

Would an exact method be a good idea to include aswell? Although It'd be basically rendered useless by generic constraints. But It'd basically not allow unions when the provided subtype isnt a union.
ie

type meow = {
     hi: string
}

type mrrp = {
     hi: "hello"
}

type meowers = meow | mrrp

print(meowers:exacthassubtypeof(meow)) -- false
print(mrrp:exacthassubtypeof(meow)) -- true

[vegorov-rbx]

You seem to be too focused on hard-coding some behaviors exactly targeting unions, which leads to the problems of structural subtyping.
Luau actually has moved to use semantic subtyping (more at: https://corp.roblox.com/newsroom/2022/11/semantic-subtyping-luau)

So to me, having anything aside from hassubtypeof doesn't make sense (even hassubtype - there is no notion of a 'union' or 'intersection' as a standalone type).
I was waiting for the other team members to notice this and didn't comment initially. Would be surprised if they disagree. (they are probably just busy)

[gaymeowing]

You seem to be too focused on hard-coding some behaviors exactly targeting unions, which leads to the problems of structural subtyping. Luau actually has moved to use semantic subtyping (more at: https://corp.roblox.com/newsroom/2022/11/semantic-subtyping-luau)

So to me, having anything aside from hassubtypeof doesn't make sense (even hassubtype - there is no notion of a 'union' or 'intersection' as a standalone type). I was waiting for the other team members to notice this and didn't comment initially. Would be surprised if they disagree. (they are probably just busy)

Yeah, the rest would be better solved with generic constraints admittedly.

[gaymeowing]

Removed everything else and just left hassubtypeof