Fix misleading warning when implementing a callback of undefined behaviour

lib/elixir/lib/module/behaviour.ex

@@ -181,8 +181,16 @@ defmodule Module.Behaviour do
       behaviour not in behaviours ->
         {:error, {:behaviour_not_declared, behaviour}}
 
+      not Code.ensure_loaded?(behaviour) ->
+        # Module does not exist, but we have already warned about it.
+        {:ok, []}
+
+      not behaviour_defined?(callbacks, behaviour) ->
+        # Module does not define behaviour, but we have already warned about it.
+        {:ok, []}
+
       true ->
-        {:error, {:behaviour_not_defined, behaviour, callbacks}}
+        {:error, {:callback_not_defined, behaviour, callbacks}}
     end
   end
 
@@ -215,6 +223,18 @@ 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 +410,17 @@ defmodule Module.Behaviour do
     ]
   end
 
-  defp format_warning({:behaviour_not_defined, callback, kind, behaviour, callbacks}) do
+  defp format_warning({:callback_not_defined, callback, kind, behaviour, callbacks}) do
+    behaviour_string = inspect(behaviour)
+
     [
       "got \"@impl ",
-      inspect(behaviour),
+      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 +122,44 @@ defmodule Kernel.ImplTest do
     end
   end
 
-  test "warns for undefined value" do
-    assert capture_err(fn ->
-             Code.eval_string("""
-             defmodule Kernel.ImplTest.ImplAttributes do
-               @behaviour :abc
+  test "warns about undefined module, but does not warn at @impl line" do
+    capture_err =
+      capture_err(fn ->
+        Code.eval_string("""
+        defmodule Kernel.ImplTest.ImplAttributes do
+          @behaviour 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"
+          @impl Abc
+          def foo(), do: :ok
+        end
+        """)
+      end)
+
+    assert capture_err =~
+             "@behaviour Abc does not exist (in module Kernel.ImplTest.ImplAttributes)"
+
+    refute capture_err =~
+             "got \"@impl Abc\""
+  end
+
+  test "warns about undefined behaviour, but does not warn at @impl line" do
+    capture_err =
+      capture_err(fn ->
+        Code.eval_string("""
+        defmodule Kernel.ImplTest.ImplAttributes do
+          @behaviour Enum
+
+          @impl Enum
+          def foo(), do: :ok
+        end
+        """)
+      end)
+
+    assert capture_err =~
+             "module Enum is not a behaviour (in module Kernel.ImplTest.ImplAttributes)"
+
+    refute capture_err =~
+             "got \"@impl Abc\""
   end
 
   test "warns for callbacks without impl and @impl has been set before" do
@@ -315,7 +341,7 @@ 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 +354,7 @@ 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 +367,7 @@ 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 +381,7 @@ 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 +535,7 @@ 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