Warn on zero arity callbacks inside protocols (#11519)

Author: thiamsantos

lib/elixir/lib/protocol.ex

@@ -723,14 +723,14 @@ defmodule Protocol do
 
   defp callback_ast_to_fa({kind, {:"::", meta, [{name, _, args}, _return]}, _pos})
        when kind in [:callback, :macrocallback] do
-    [{{name, length(args)}, meta}]
+    [{{name, length(List.wrap(args))}, meta}]
   end
 
   defp callback_ast_to_fa(
          {kind, {:when, _, [{:"::", meta, [{name, _, args}, _return]}, _vars]}, _pos}
        )
        when kind in [:callback, :macrocallback] do
-    [{{name, length(args)}, meta}]
+    [{{name, length(List.wrap(args))}, meta}]
   end
 
   defp callback_ast_to_fa({kind, _, _pos}) when kind in [:callback, :macrocallback] do

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

@@ -1163,9 +1163,11 @@ defmodule Kernel.WarningTest do
 
             def without_specs(term, options \\ [])
 
+            @callback foo :: {:ok, term}
             @callback foo(term) :: {:ok, term}
             @callback foo(term, keyword) :: {:ok, term, keyword}
 
+            @callback foo_when :: {:ok, x} when x: term
             @callback foo_when(x) :: {:ok, x} when x: term
             @callback foo_when(x, opts) :: {:ok, x, opts} when x: term, opts: keyword
 
@@ -1178,12 +1180,18 @@ defmodule Kernel.WarningTest do
         """)
       end)
 
+    assert message =~
+             "cannot define @callback foo/0 inside protocol, use def/1 to outline your protocol definition\n  nofile:1"
+
     assert message =~
              "cannot define @callback foo/1 inside protocol, use def/1 to outline your protocol definition\n  nofile:1"
 
     assert message =~
              "cannot define @callback foo/2 inside protocol, use def/1 to outline your protocol definition\n  nofile:1"
 
+    assert message =~
+             "cannot define @callback foo_when/0 inside protocol, use def/1 to outline your protocol definition\n  nofile:1"
+
     assert message =~
              "cannot define @callback foo_when/1 inside protocol, use def/1 to outline your protocol definition\n  nofile:1"