Fix misleading warning when implementing a callback of undefined behaviour

lib/elixir/lib/module/behaviour.ex

@@ -181,8 +181,11 @@ defmodule Module.Behaviour do
       behaviour not in behaviours ->
         {:error, {:behaviour_not_declared, behaviour}}
 
-      true ->
+      not behaviour_defined?(callbacks, behaviour) ->
         {:error, {:behaviour_not_defined, behaviour, callbacks}}
+
+      true ->
+        {:error, {:callback_not_defined, behaviour, callbacks}}
     end
   end
 
@@ -215,6 +218,18 @@ defmodule Module.Behaviour do
     end
   end
 
+  # Determines whether there is at least one callback defined for the given behaviour.
+  # If not, that means that the behaviour has not been defined.
+  defp behaviour_defined?(callbacks, behaviour) do
+    callbacks
+    |> Map.values()
+    |> List.flatten()
+    |> Enum.any?(fn
+      {_kind, ^behaviour, _optional?} -> true
+      {_kind, _behaviour, _optional?} -> false
+    end)
+  end
+
   defp warn_missing_impls(%{callbacks: callbacks} = context, _impl_contexts, _defs)
        when map_size(callbacks) == 0 do
     context
@@ -391,6 +406,19 @@ defmodule Module.Behaviour do
   end
 
   defp format_warning({:behaviour_not_defined, callback, kind, behaviour, callbacks}) do
+    [
+      "got \"@impl ",
+      inspect(behaviour),
+      "\" for ",
+      format_definition(kind, callback),
+      " but behaviour ",
+      inspect(behaviour),
+      " does not exist",
+      known_callbacks(callbacks)
+    ]
+  end
+
+  defp format_warning({:callback_not_defined, callback, kind, behaviour, callbacks}) do
     [
       "got \"@impl ",
       inspect(behaviour),

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

@@ -122,7 +122,7 @@ defmodule Kernel.ImplTest do
     end
   end
 
-  test "warns for undefined value" do
+  test "warns for undefined behaviour" do
     assert capture_err(fn ->
              Code.eval_string("""
              defmodule Kernel.ImplTest.ImplAttributes do
@@ -133,7 +133,8 @@ defmodule Kernel.ImplTest do
              end
              """)
            end) =~
-             "got \"@impl :abc\" for function foo/0 but this behaviour does not specify such callback. There are no known callbacks"
+             "got \"@impl :abc\" for function foo/0 but behaviour :abc does not exist. " <>
+               "There are no known callbacks, please specify the proper @behaviour and make sure it defines callbacks"
   end
 
   test "warns for callbacks without impl and @impl has been set before" do