A strange compiler error during the testing of my own GroupBy method (to work around the missing GroupBy in Linq). Attached is a zip file containing a Visual studio 2013 solution and a small example. Build it to see the error.
Now open the MainPage.xaml.cs and uncomment line 36 and comment line 38. It now compiles and runs fine. In my opinion the two lines are identical with respect to resolving types T and TKey (the call is the same in the rest of the code).