Warn duplicate definitions in macros (#7037)

Closes #6637

Author: ggcampinho

lib/elixir/src/elixir_def.erl

@@ -96,6 +96,10 @@ store_definition(Kind, CheckClauses, Call, Body, Pos) ->
   %% extract meta information like file and context.
   {_, Meta, _} = Call,
   DoCheckClauses = (not lists:keymember(context, 1, Meta)) andalso (CheckClauses),
+  Generated = case lists:keyfind(generated, 1, Meta) of
+    {generated, true} -> ?generated([]);
+    _ -> []
+  end,
 
   %% Check if there is a file information in the definition.
   %% If so, we assume this come from another source and
@@ -114,7 +118,6 @@ store_definition(Kind, CheckClauses, Call, Body, Pos) ->
   LinifyArgs   = elixir_quote:linify(Line, Key, Args),
   LinifyGuards = elixir_quote:linify(Line, Key, Guards),
   LinifyBody   = elixir_quote:linify(Line, Key, Body),
-  Generated    = case DoCheckClauses of true -> []; false -> ?generated([]) end,
 
   {EL, MetaLocation} =
     case retrieve_location(Location, ?key(E, module)) of

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

@@ -487,6 +487,27 @@ defmodule Kernel.WarningTest do
     purge Sample
   end
 
+  test "generated clause not match" do
+    assert capture_err(fn ->
+             Code.eval_string("""
+             defmodule Sample do
+               defmacro __using__(_) do
+                 quote do
+                   def hello, do: nil
+                   def hello, do: nil
+                 end
+               end
+             end
+             defmodule UseSample do
+               use Sample
+             end
+             """)
+           end) =~ "this clause cannot match because a previous clause at line 10 always matches"
+  after
+    purge(Sample)
+    purge(UseSample)
+  end
+
   test "clause with defaults should be first" do
     assert capture_err(fn ->
       Code.eval_string ~S"""