Add :erlang.--/2 parametric TODO and cover static non-empty subtraction

Author: lukaszsamson

lib/elixir/lib/module/types/apply.ex

@@ -1153,6 +1153,8 @@ defmodule Module.Types.Apply do
   end
 
   defp remote_apply(:erlang, :--, _info, [left, right], stack) do
+    # TODO: remove once we add parametric types, this will just be:
+    # list(a), list(term()) -> list(a)
     case {list_of(left), list_of(right)} do
       {{_, list_of}, {_, _}} ->
         result = if list_of, do: list(list_of), else: empty_list()

lib/elixir/test/elixir/module/types/expr_test.exs

@@ -176,6 +176,7 @@ defmodule Module.Types.ExprTest do
 
     test "--" do
       assert typecheck!([x], [1, 2, 3] -- x) == list(integer())
+      assert typecheck!([1] -- [1]) == list(integer())
       assert typecheck!([x], [] -- x) == empty_list()
 
       assert typeerror!([x], [1, 2, 3] -- String.to_integer(x)) |> strip_ansi() =~