I have stumbled upon a strange behavior of table.sort(), in which the comparing function is invoked by passing the same array element in both parameters.

Consider the code below. I want to sort the array from highest to lowest value of the property v and, in case they are equal (and since table.sort() algorithm is not stable as of the docs), I want to sort them by the original indices of the elements.

-- The array to sort
local arr = {
  {id="A", v = 1},
  {id="B", v = 1},
  {id="C", v = 0},
  {id="D", v = 1},
  {id="E", v = 1}
}

-- A map containing the original index of the elements in the array: element => originalIndex
local original_indices = {}
for index, elem in ipairs(arr) do
  original_indices[elem] = index
end

-- Sort the array
table.sort(arr, 
  function(a, b) 
    assert(a ~= b, "Comparing the same element in the array!")
    
    -- Compare the value
    if a.v > b.v then
      return true
    elseif a.v < b.v then
      return false
    end
    
    -- Values are equal. Compare original indices, which should always
    -- decide the final order.
    local ia = original_indices[a]
    local ib = original_indices[b]
    
    if ia < ib then
      return true
    elseif ia > ib then
      return false
    end
    
    error("BUG! Comparing the same element in the array!") 
  end
)

The expected result should be:

{
  {id="C", v = 0},
  {id="A", v = 1},
  {id="B", v = 1},
  {id="D", v = 1},
  {id="E", v = 1}
}

But, instead, I get an error because Lua is invoking the sorting function by passing the same element in both parameters, which should not.

Am I missing something? How can I get the expected result?

You can play around with the code here.