Copy callback typespecs

lib/ex_doc/retriever.ex

@@ -239,18 +239,9 @@ defmodule ExDoc.Retriever do
     function = actual_def(name, arity, type)
     line = find_actual_line(module_info.abst_code, function, :function) || doc_line
 
-    doc = docstring(doc, name, arity, Map.fetch(module_info.impls, {name, arity}))
-
-    specs = module_info.specs
-            |> Map.get(function, [])
-            |> Enum.map(&Typespec.spec_to_ast(name, &1))
-
-    specs =
-      if type == :defmacro do
-        Enum.map(specs, &remove_first_macro_arg/1)
-      else
-        specs
-      end
+    impl = Map.fetch(module_info.impls, {name, arity})
+    doc = docstring(doc, name, arity, impl)
+    specs = get_specs(module_info, type, function, name, arity, impl)
 
     annotations =
       case {type, name, arity} do
@@ -297,6 +288,31 @@ defmodule ExDoc.Retriever do
     end
   end
 
+  defp get_specs(module_info, type, function, name, arity, impl) do
+    specs =
+      module_info.specs
+      |> Map.get(function, [])
+      |> Enum.map(&Typespec.spec_to_ast(name, &1))
+
+    specs =
+      if type == :defmacro do
+        Enum.map(specs, &remove_first_macro_arg/1)
+      else
+        specs
+      end
+
+    copy_callback_specs(specs, name, arity, impl)
+  end
+
+  defp copy_callback_specs([], name, arity, {:ok, behaviour}) do
+    callbacks = Kernel.Typespec.beam_callbacks(behaviour)
+    {_, specs} = List.keyfind(callbacks, {name, arity}, 0)
+    Enum.map(specs, &Typespec.spec_to_ast(name, &1))
+  end
+  defp copy_callback_specs(specs, _, _, _) do
+    specs
+  end
+
   defp docstring(nil, name, arity, {:ok, behaviour}) do
     info = "Callback implementation for `c:#{inspect behaviour}.#{name}/#{arity}`."
     callback_docs = Code.get_docs(behaviour, :callback_docs) || []

test/ex_doc/retriever_test.exs

@@ -230,10 +230,7 @@ defmodule ExDoc.RetrieverTest do
   end
 
   test "undocumented callback implementations get default doc" do
-    [module_node] =
-      ["CustomBehaviourOne", "CustomBehaviourTwo", "CustomBehaviourImpl"]
-      |> docs_from_files()
-      |> Enum.filter(&match?(%ExDoc.ModuleNode{id: "CustomBehaviourImpl"}, &1))
+    [module_node] = docs_from_files ["CustomBehaviourImpl"]
     docs = module_node.docs
     assert Enum.map(docs, &(&1.id)) == ["bye/1", "greet/1", "hello/1"]
     assert Enum.at(docs, 0).doc ==
@@ -245,6 +242,17 @@ defmodule ExDoc.RetrieverTest do
       "Callback implementation for `c:CustomBehaviourOne.hello/1`."
   end
 
+  test "callback implementations get specs from behaviour" do
+    [module_node] = docs_from_files ["CustomBehaviourImpl"]
+
+    [_, greet, hello] = module_node.docs
+    assert greet.name == :greet
+    assert Macro.to_string(greet.specs) == "[greet(integer() | String.t()) :: integer()]"
+
+    assert hello.name == :hello
+    assert Macro.to_string(hello.specs) == "[hello(non_neg_integer()) :: non_neg_integer()]"
+  end
+
   ## PROTOCOLS
 
   test "docs_from_files properly tag protocols" do

test/fixtures/behaviour.ex

@@ -20,7 +20,9 @@ defmodule CustomBehaviourImpl do
   @behaviour CustomBehaviourOne
   @behaviour CustomBehaviourTwo
 
+  @spec hello(non_neg_integer()) :: non_neg_integer()
   def hello(i), do: i
+
   def greet(i), do: i
 
   @doc "A doc for this so it doesn't use 'Callback implementation for'"