*Its only Debug-time error, but a very weird one.

Problem:

While running with Debugger attached and calling a method, exposed in separate Portable library, returning ConfiguredTaskAwaitable, we get InvalidProgramException.

Repro:

Having 2 projects:

1. PortableClassLibrary (supporting .Net 4.5; Windows Store; Windows Phone 8) with 1 class:

   public class Weird
   {
       public static ConfiguredTaskAwaitable GetConfiguredTaskAwaitable()
       {
           return new ConfiguredTaskAwaitable();
       }
   }
   

2. ConsoleApplication with code:

   static void Main(string[] args)
   {
       Weird.GetConfiguredTaskAwaitable();
   }
   

Notes:

1. replacing ConfiguredTaskAwaitable with ConfiguredTaskAwaitable<T> (a generic version) fixes this strange issue
2. consuming this method form WP8 or Win8 app under Debugger works fine.

Currently it causes problems cause I cant run my Unit Tests under Debugger. I'm forced to change my "ObjectUnderTest" implementation to return generic ConfiguredTaskAwaitable<T>, which is fine for the real project, but still is only a workaround.

The Question is:

does anybody knows the reason of this error? It definitely related to Portable Class Library magic.