Fix @impl attribute for default arguments (#6595)

`Module.compile_impl/6` was ignoring the default attributes
when compiling the impl, now it checks the count of default
attributes and adds an impl for each one.

Fixes #6550

Author: ggcampinho

lib/elixir/lib/module.ex

@@ -1192,15 +1192,28 @@ defmodule Module do
     case :ets.take(table, :impl) do
       [{:impl, value, _, _}] ->
         impls = :ets.lookup_element(table, {:elixir, :impls}, 2)
-        impl = {{name, length(args)}, kind, line, file, value}
-        :ets.insert(table, {{:elixir, :impls}, [impl | impls]})
+        {total, defaults} = args_count(args, 0, 0)
+
+        impl = for arity <- total..(total - defaults), into: impls do
+          {{name, arity}, kind, line, file, value}
+        end
+
+        :ets.insert(table, {{:elixir, :impls}, impl})
       [] ->
         :ok
     end
 
     :ok
   end
 
+  defp args_count([{:\\, _, _} | tail], total, defaults) do
+    args_count(tail, total + 1, defaults + 1)
+  end
+  defp args_count([_head | tail], total, defaults) do
+    args_count(tail, total + 1, defaults)
+  end
+  defp args_count([], total, defaults), do: {total, defaults}
+
   @doc false
   def check_behaviours_and_impls(env, table, all_definitions, overridable_pairs) do
     behaviours = :ets.lookup_element(table, :behaviour, 2)

lib/elixir/test/elixir/kernel/impl_test.exs

@@ -17,7 +17,11 @@ defmodule Kernel.ImplTest do
   end
 
   defmodule Behaviour do
-    @callback foo :: any
+    @callback foo() :: any
+  end
+
+  defmodule BehaviourWithArgument do
+    @callback foo(any) :: any
   end
 
   defmodule MacroBehaviour do
@@ -285,6 +289,20 @@ defmodule Kernel.ImplTest do
     end) =~ "got @impl Kernel.ImplTest.MacroBehaviour for def bar/0 but the given behaviour was not declared with @behaviour"
   end
 
+  test "does not warn for @impl when using default arguments" do
+    assert capture_err(fn ->
+      Code.eval_string ~S"""
+      defmodule Kernel.ImplTest.ImplAttributes do
+        @behaviour Kernel.ImplTest.Behaviour
+        @behaviour Kernel.ImplTest.BehaviourWithArgument
+
+        @impl true
+        def foo(args \\ []), do: args
+      end
+      """
+    end) == ""
+  end
+
   test "does not warn for no @impl when overriding callback" do
     assert capture_err(fn ->
       Code.eval_string """