fix crash when suggesting specs on protocol defs with default args

Author: lukaszsamson

apps/language_server/lib/language_server/providers/code_lens/type_spec/contract_translator.ex

@@ -144,10 +144,16 @@ defmodule ElixirLS.LanguageServer.Providers.CodeLens.TypeSpec.ContractTranslator
     cond do
       Code.ensure_loaded?(mod) and function_exported?(mod, :__protocol__, 1) ->
         # defprotocol
-        # defs in defprotocol do not have when and have at least 1 arg
-        {:"::", [], [{:foo, [], [_ | rest_args]}, res]} = ast
-        # first arg in defprotocol defs is always of type t
-        {:"::", [], [{:foo, [], [{:t, [], []} | rest_args]}, res]}
+        case ast do
+          {:"::", [], [{:foo, [], [_ | rest_args]}, res]} ->
+            # ordinary defs in defprotocol do not have when and have at least 1 arg
+            # first arg in defprotocol defs is always of type t
+            {:"::", [], [{:foo, [], [{:t, [], []} | rest_args]}, res]}
+
+          {:"::", [], [{:foo, [], []}, _]} ->
+            # def with default arg
+            ast
+        end
 
       Code.ensure_loaded?(mod) and function_exported?(mod, :__impl__, 1) ->
         # defimpl