Warn on unused shadowed aliases

lib/elixir/lib/kernel/lexical_tracker.ex

@@ -123,7 +123,14 @@ defmodule Kernel.LexicalTracker do
 
   @doc false
   def handle_call(:unused_aliases, _from, state) do
-    {:reply, Enum.sort(state.aliases), state}
+    unused_aliases =
+      Enum.map(state.aliases, fn
+        {{:shadowed, module}, meta} -> {module, meta}
+        {module, meta} when is_atom(module) -> {module, meta}
+      end)
+      |> Enum.sort()
+
+    {:reply, unused_aliases, state}
   end
 
   def handle_call(:unused_imports, _from, state) do
@@ -220,6 +227,12 @@ defmodule Kernel.LexicalTracker do
 
   def handle_cast({:add_alias, module, meta, warn}, state) do
     if warn do
+      state =
+        case state do
+          %{aliases: %{^module => meta}} -> put_in(state.aliases[{:shadowed, module}], meta)
+          _ -> state
+        end
+
       {:noreply, put_in(state.aliases[module], meta)}
     else
       {:noreply, state}

lib/elixir/lib/macro/env.ex

@@ -447,7 +447,7 @@ defmodule Macro.Env do
 
   ## Examples
 
-      iex> alias List, as: MyList
+      iex> alias List, as: MyList, warn: false
       iex> Macro.Env.expand_alias(__ENV__, [], [:MyList])
       {:alias, List}
       iex> Macro.Env.expand_alias(__ENV__, [], [:MyList, :Nested])
@@ -456,7 +456,7 @@ defmodule Macro.Env do
   If there is no alias or the alias starts with `Elixir.`
   (which disables aliasing), then `:error` is returned:
 
-      iex> alias List, as: MyList
+      iex> alias List, as: MyList, warn: false
       iex> Macro.Env.expand_alias(__ENV__, [], [:Elixir, MyList])
       :error
       iex> Macro.Env.expand_alias(__ENV__, [], [:AnotherList])

lib/elixir/test/elixir/kernel/alias_test.exs

@@ -34,6 +34,12 @@ defmodule Kernel.AliasTest do
     assert Billing == :"Elixir.MyApp.Billing"
   end
 
+  test "overriding parent alias with child alias" do
+    alias MyApp.Billing
+    alias Billing.Billing
+    assert Billing == :"Elixir.MyApp.Billing.Billing"
+  end
+
   test "lexical" do
     if true_fun() do
       alias OMG, as: List, warn: false

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -894,6 +894,22 @@ defmodule Kernel.WarningTest do
     purge(Sample)
   end
 
+  test "unused alias due to shadowing" do
+    assert_warn_compile(
+      ["nofile:2:3", "unused alias Baz"],
+      """
+      defmodule Sample do
+        alias Foo.Baz
+        alias Bar.Baz
+
+        def baz, do: Baz
+      end
+      """
+    )
+  after
+    purge(Sample)
+  end
+
   test "unused inside dynamic module" do
     import List, only: [flatten: 1], warn: false