fix crash when unable to get stackframe arity

Author: lukaszsamson

apps/elixir_ls_debugger/lib/debugger/stacktrace.ex

@@ -21,7 +21,11 @@ defmodule ElixirLS.Debugger.Stacktrace do
 
     def name(%__MODULE__{function: function} = frame) when not is_nil(function) do
       {f, a} = frame.function
-      "#{inspect(frame.module)}.#{f}/#{a}"
+
+      case a do
+        :undefined -> "#{inspect(frame.module)}.#{f}/?"
+        _ -> "#{inspect(frame.module)}.#{f}/#{a}"
+      end
     end
 
     def name(%__MODULE__{} = frame) do
@@ -40,7 +44,7 @@ defmodule ElixirLS.Debugger.Stacktrace do
         first_frame = %Frame{
           level: level,
           module: module,
-          function: {function, Enum.count(args)},
+          function: {function, get_arity(args)},
           args: args,
           file: get_file(module),
           # vscode raises invalid request when line is nil
@@ -62,7 +66,7 @@ defmodule ElixirLS.Debugger.Stacktrace do
                 %Frame{
                   level: level,
                   module: mod,
-                  function: {function, Enum.count(args)},
+                  function: {function, get_arity(args)},
                   args: args,
                   file: get_file(mod),
                   # vscode raises invalid request when line is nil
@@ -107,4 +111,7 @@ defmodule ElixirLS.Debugger.Stacktrace do
   def get_file(module) do
     Path.expand(to_string(ModuleInfoCache.get(module)[:compile][:source]))
   end
+
+  defp get_arity(:undefined), do: :undefined
+  defp get_arity(args), do: Enum.count(args)
 end