Warn when defimp is called for consolidated protocol

Conflicts:
	lib/elixir/lib/protocol.ex

Author: lexmag

lib/elixir/lib/protocol.ex

@@ -540,6 +540,7 @@ defmodule Protocol do
           "the dict[key] syntax, please implement the Dict behaviour instead"
       else
         Protocol.assert_protocol!(protocol)
+        Protocol.__ensure_defimpl__(protocol, for, __ENV__)
       end
 
       defmodule name do
@@ -580,6 +581,7 @@ defmodule Protocol do
   defp derive(protocol, for, struct, opts, env) do
     extra = ", cannot derive #{inspect protocol} for #{inspect for}"
     assert_protocol!(protocol, extra)
+    __ensure_defimpl__(protocol, for, env)
     assert_impl!(protocol, Any, extra)
 
     # Clean up variables from eval context
@@ -608,6 +610,17 @@ defmodule Protocol do
     end)
   end
 
+  @doc false
+  def __ensure_defimpl__(protocol, for, env) do
+    if Protocol.consolidated?(protocol) do
+      message =
+        "the #{inspect protocol} protocol has already been consolidated" <>
+        ", an implementation for #{inspect for} has no effect"
+      :elixir_errors.warn(env.line, env.file, message)
+    end
+    :ok
+  end
+
   @doc false
   def __spec__?(module, name, arity) do
     signature = {name, arity}

lib/elixir/test/elixir/protocol_test.exs

@@ -319,6 +319,17 @@ defmodule Protocol.ConsolidationTest do
     refute Protocol.consolidated?(Enumerable)
   end
 
+  test "consolidation prevents new implementations" do
+    assert ExUnit.CaptureIO.capture_io(:stderr, fn ->
+      defimpl WithAny, for: Integer do
+        def ok(_any), do: :ok
+      end
+    end) =~ ~r"warning: the .+WithAny protocol has already been consolidated"
+  after
+    :code.purge(WithAny.Atom)
+    :code.delete(WithAny.Atom)
+  end
+
   test "consolidated implementations without any" do
     assert is_nil Sample.impl_for(:foo)
     assert is_nil Sample.impl_for(fn(x) -> x end)