Avoid false warnings on private clauses that raise (return none())

Author: josevalim

lib/elixir/lib/module/types.ex

@@ -231,10 +231,13 @@ defmodule Module.Types do
         not Keyword.get(meta, :from_super, false),
         reduce: context do
       context ->
-        {_kind, _inferred, mapping} = Map.fetch!(context.local_sigs, fun_arity)
+        {_kind, info, mapping} = Map.fetch!(context.local_sigs, fun_arity)
 
         clauses_indexes =
-          for type_index <- pending, {clause_index, ^type_index} <- mapping, do: clause_index
+          for type_index <- pending,
+              not skip_unused_clause?(info, type_index),
+              {clause_index, ^type_index} <- mapping,
+              do: clause_index
 
         Enum.reduce(clauses_indexes, context, fn clause_index, context ->
           {meta, _args, _guards, _body} = Enum.fetch!(clauses, clause_index)
@@ -244,6 +247,20 @@ defmodule Module.Types do
     end
   end
 
+  defp skip_unused_clause?(info, type_index) do
+    case info do
+      # If an inferred clause returns an empty type, then the reverse arrow
+      # will never propagate its domain up, which may lead to the clause never
+      # being invoked.
+      {:infer, _, inferred} ->
+        {_args_types, return} = Enum.fetch!(inferred, type_index)
+        Descr.empty?(return)
+
+      _ ->
+        false
+    end
+  end
+
   defp local_handler(_meta, fun_arity, stack, context, finder) do
     case context.local_sigs do
       %{^fun_arity => {kind, inferred, _mapping}} ->

lib/elixir/test/elixir/module/types/integration_test.exs

@@ -361,20 +361,22 @@ defmodule Module.Types.IntegrationTest do
       assert_warnings(files, warnings)
     end
 
-    test "unused generated private clauses" do
+    test "unused overridable private clauses" do
       files = %{
         "a.ex" => """
         defmodule A do
           use B
-          def public(x), do: private(List.to_tuple(x))
+          def public(x), do: private(x)
+          defp private(x), do: super(List.to_tuple(x))
         end
         """,
         "b.ex" => """
         defmodule B do
           defmacro __using__(_) do
-            quote generated: true do
+            quote do
               defp private({:ok, ok}), do: ok
               defp private(:error), do: :error
+              defoverridable private: 1
             end
           end
         end
@@ -384,22 +386,28 @@ defmodule Module.Types.IntegrationTest do
       assert_no_warnings(files)
     end
 
-    test "unused overridable private clauses" do
+    test "unused private clauses without warnings" do
       files = %{
         "a.ex" => """
         defmodule A do
           use B
-          def public(x), do: private(x)
-          defp private(x), do: super(List.to_tuple(x))
+
+          # Not all clauses are invoked, but do not warn since they are generated
+          def public1(x), do: generated(List.to_tuple(x))
+
+          # Avoid false positives caused by inference
+          def public2(x), do: (:ok = raising_private(x))
+
+          defp raising_private(true), do: :ok
+          defp raising_private(false), do: raise "oops"
         end
         """,
         "b.ex" => """
         defmodule B do
           defmacro __using__(_) do
-            quote do
-              defp private({:ok, ok}), do: ok
-              defp private(:error), do: :error
-              defoverridable private: 1
+            quote generated: true do
+              defp generated({:ok, ok}), do: ok
+              defp generated(:error), do: :error
             end
           end
         end