Built-in types can be inherited from, but that results in strange semantics

[purplesyringa]

local record A
    is userdata
end

local x: any
print(x is A) -- compiles to type(x) == "table", not type(x) == "userdata"

Similarly for is number, is string, etc. Sounds like that should be a compile error, since those are not interfaces. (Maybe number technically is?)