Press ctr-P here and see strange text: (int x, __UNKNOWN_TYPE__ ???)
and highlighted error here says that "Argument type 'int' is not assignable to parameter type '__UNKNOWN_TYPE__' ".

It will be better to see for example : (int x, unknown)
And the message smth about: "Signature of method Foo(int x, unknown) is not completed".

public class Bar
{
  public void Foo(int x, ){}
  public void Test()
  {
     Foo(1, 2{caret});
  }
}

Issue was closed