Metatable on Metatables

meta.lua

local x = { value = 42 }
local index = {
  __index = function(t, k)
    print(k)
    if k == "__add" then
      return function(l, r)
        return { value = l.value + r.value }
      end
    end
  end
}

setmetatable(index, index)
setmetatable(x, index)

index.__add -- works fine, gives the right add
x.__add -- works fine, gives the right add
x + x -- fails, presumably the metatable lookup uses primitive indexing and skips meta indexing