Mention patterns are not allowed in function heads

Closes #14496.

Author: josevalim

lib/elixir/src/elixir_def.erl

@@ -506,7 +506,7 @@ format_error({invalid_def, Kind, NameAndArgs}) ->
   io_lib:format("invalid syntax in ~ts ~ts", [Kind, 'Elixir.Macro':to_string(NameAndArgs)]);
 
 format_error(invalid_args_for_function_head) ->
-  "only variables and \\\\ are allowed as arguments in function head.\n"
+  "patterns are not allowed in function head, only variables and default arguments (using \\\\)\n"
   "\n"
   "If you did not intend to define a function head, make sure your function "
   "definition has the proper syntax by wrapping the arguments in parentheses "

lib/elixir/test/elixir/kernel/errors_test.exs

@@ -870,7 +870,10 @@ defmodule Kernel.ErrorsTest do
 
   test "invalid args for function head" do
     assert_compile_error(
-      ["nofile:2:7: ", "only variables and \\\\ are allowed as arguments in function head."],
+      [
+        "nofile:2:7: ",
+        "patterns are not allowed in function head, only variables and default arguments (using \\\\)"
+      ],
       ~c"""
       defmodule Kernel.ErrorsTest.InvalidArgsForBodylessClause do
         def foo(nil)