Fix misleading warning when implementing a callback of undefined behaviour

lib/elixir/lib/module/behaviour.ex

@@ -181,8 +181,14 @@ defmodule Module.Behaviour do
       behaviour not in behaviours ->
         {:error, {:behaviour_not_declared, behaviour}}
 
+      not Code.ensure_loaded?(behaviour) ->
+        {:error, {:module_not_defined, behaviour}}
+
+      not behaviour_defined?(callbacks, behaviour) ->
+        {:error, {:behaviour_not_defined, behaviour}}
+
       true ->
-        {:error, {:behaviour_not_defined, behaviour, callbacks}}
+        {:error, {:callback_not_defined, behaviour, callbacks}}
     end
   end
 
@@ -215,6 +221,18 @@ 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
@@ -390,13 +408,45 @@ defmodule Module.Behaviour do
     ]
   end
 
-  defp format_warning({:behaviour_not_defined, callback, kind, behaviour, callbacks}) do
+  defp format_warning({:module_not_defined, callback, kind, behaviour}) do
+    behaviour_string = inspect(behaviour)
+
     [
       "got \"@impl ",
-      inspect(behaviour),
+      behaviour_string,
+      "\" for ",
+      format_definition(kind, callback),
+      " but module ",
+      behaviour_string,
+      " does not exist"
+    ]
+  end
+
+  defp format_warning({:behaviour_not_defined, callback, kind, behaviour}) do
+    behaviour_string = inspect(behaviour)
+
+    [
+      "got \"@impl ",
+      behaviour_string,
+      "\" for ",
+      format_definition(kind, callback),
+      " but behaviour ",
+      behaviour_string,
+      " does not exist"
+    ]
+  end
+
+  defp format_warning({:callback_not_defined, callback, kind, behaviour, callbacks}) do
+    behaviour_string = inspect(behaviour)
+
+    [
+      "got \"@impl ",
+      behaviour_string,
       "\" for ",
       format_definition(kind, callback),
-      " but this behaviour does not specify such callback",
+      " but ",
+      behaviour_string,
+      " does not specify such callback",
       known_callbacks(callbacks)
     ]
   end

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

@@ -122,18 +122,32 @@ defmodule Kernel.ImplTest do
     end
   end
 
-  test "warns for undefined value" do
+  test "warns about undefined module" do
     assert capture_err(fn ->
              Code.eval_string("""
              defmodule Kernel.ImplTest.ImplAttributes do
-               @behaviour :abc
+               @behaviour Abc
 
-               @impl :abc
+               @impl Abc
                def foo(), do: :ok
              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 module Abc does not exist"
+  end
+
+  test "warns about undefined behaviour" do
+    assert capture_err(fn ->
+             Code.eval_string("""
+             defmodule Kernel.ImplTest.ImplAttributes do
+               @behaviour Enum
+
+               @impl Enum
+               def foo(), do: :ok
+             end
+             """)
+           end) =~
+             "got \"@impl Enum\" for function foo/0 but behaviour Enum does not exist"
   end
 
   test "warns for callbacks without impl and @impl has been set before" do
@@ -315,7 +329,7 @@ defmodule Kernel.ImplTest do
              end
              """)
            end) =~
-             "got \"@impl Kernel.ImplTest.Behaviour\" for function bar/0 but this behaviour does not specify such callback"
+             "got \"@impl Kernel.ImplTest.Behaviour\" for function bar/0 but Kernel.ImplTest.Behaviour does not specify such callback"
   end
 
   test "warns for @impl module with macro callback name not in behaviour" do
@@ -328,7 +342,7 @@ defmodule Kernel.ImplTest do
              end
              """)
            end) =~
-             "got \"@impl Kernel.ImplTest.MacroBehaviour\" for macro foo/0 but this behaviour does not specify such callback"
+             "got \"@impl Kernel.ImplTest.MacroBehaviour\" for macro foo/0 but Kernel.ImplTest.MacroBehaviour does not specify such callback"
   end
 
   test "warns for @impl module with macro callback kind not in behaviour" do
@@ -341,7 +355,7 @@ defmodule Kernel.ImplTest do
              end
              """)
            end) =~
-             "got \"@impl Kernel.ImplTest.MacroBehaviour\" for function foo/0 but this behaviour does not specify such callback"
+             "got \"@impl Kernel.ImplTest.MacroBehaviour\" for function foo/0 but Kernel.ImplTest.MacroBehaviour does not specify such callback"
   end
 
   test "warns for @impl module and callback belongs to another known module" do
@@ -355,7 +369,7 @@ defmodule Kernel.ImplTest do
              end
              """)
            end) =~
-             "got \"@impl Kernel.ImplTest.Behaviour\" for function bar/0 but this behaviour does not specify such callback"
+             "got \"@impl Kernel.ImplTest.Behaviour\" for function bar/0 but Kernel.ImplTest.Behaviour does not specify such callback"
   end
 
   test "warns for @impl module and callback belongs to another unknown module" do
@@ -509,7 +523,7 @@ defmodule Kernel.ImplTest do
              end
              """)
            end) =~
-             "got \"@impl Kernel.ImplTest.MacroBehaviour\" for function foo/0 but this behaviour does not specify such callback"
+             "got \"@impl Kernel.ImplTest.MacroBehaviour\" for function foo/0 but Kernel.ImplTest.MacroBehaviour does not specify such callback"
   end
 
   test "warns only for non-generated functions in non-generated @impl" do