Warn if non-protocol functions are defined in protocol modules

Previous approaches tried removing macros but they could always
be added back. Therefore we now check for non-protocol functions
in a before compile callback.

Author: josevalim

lib/elixir/lib/list/chars.ex

@@ -19,12 +19,6 @@ defprotocol List.Chars do
   """
   @spec to_charlist(t) :: charlist
   def to_charlist(term)
-
-  @doc false
-  @deprecated "Use List.Chars.to_charlist/1 instead"
-  Kernel.def to_char_list(term) do
-    __MODULE__.to_charlist(term)
-  end
 end
 
 defimpl List.Chars, for: Atom do

lib/elixir/lib/module/types.ex

@@ -80,7 +80,7 @@ defmodule Module.Types do
           kind in [:def, :defmacro],
           reduce: {[], context()} do
         {types, context} ->
-          # Optimized version of finder, since we already the definition
+          # Optimized version of finder, since we already have the definition
           finder = fn _ ->
             default_domain(infer_mode(kind, infer_signatures?), def, fun_arity, impl)
           end

lib/elixir/lib/protocol.ex

@@ -787,8 +787,7 @@ defmodule Protocol do
         @before_compile Protocol
 
         # We don't allow function definition inside protocols
-        import Kernel,
-          except: [def: 1, def: 2, defdelegate: 2, defguard: 1, defguardp: 1]
+        import Kernel, except: [def: 1, def: 2]
 
         # Import the new `def` that is used by protocols
         import Protocol, only: [def: 1]
@@ -851,6 +850,21 @@ defmodule Protocol do
       )
     end
 
+    extra =
+      ((Module.definitions_in(env.module, :def) ++ Module.definitions_in(env.module, :defmacro)) --
+         functions) --
+        [impl_for: 1, impl_for!: 1, __protocol__: 1, __deriving__: 2, __deriving__: 3]
+
+    # TODO: Make an error on Elixir v2.0
+    if extra != [] do
+      warn(
+        "protocols can only define functions without implementation via def/1, found: " <>
+          Enum.map_join(extra, ", ", fn {name, arity} -> "#{name}/#{arity}" end),
+        env,
+        nil
+      )
+    end
+
     callback_metas = callback_metas(env.module, :callback)
     callbacks = :maps.keys(callback_metas)
 

lib/elixir/test/elixir/fixtures/consolidation/sample.ex

@@ -4,9 +4,4 @@ defprotocol Protocol.ConsolidationTest.Sample do
   @deprecated "Reason"
   @spec ok(t) :: boolean
   def ok(term)
-
-  # Not a protocol function. While this is not "officially" supported,
-  # it does happen in practice, so we need to make sure we preserve
-  # its signature.
-  Kernel.def(regular_fun(term), do: term + 1)
 end

lib/elixir/test/elixir/protocol/consolidation_test.exs

@@ -238,12 +238,6 @@ defmodule Protocol.ConsolidationTest do
       assert %{{:ok, 1} => %{sig: {:strong, nil, clauses}}} = exports
       assert clauses == [{[none()], dynamic()}]
     end
-
-    test "handles regular function definitions" do
-      exports = exports(sample_binary())
-
-      assert %{{:regular_fun, 1} => %{sig: :none}} = exports
-    end
   end
 
   test "consolidation errors on missing BEAM files" do