[UIElements] Implicit type casting not supported when implemented in generic class in 2018.2

Steps to Reproduce:
1. Create a new project and import attached package.
2. Observe the following errors in the console:
The type `UnityEditor.Experimental.UIElements.IntegerField' cannot be used as type parameter `T' in the generic type or method `UIElementsExamples.E07_EditorControls.AddTestControl<T,U>(T, System.Func<U,string>)'. There is no implicit reference conversion from `UnityEditor.Experimental.UIElements.IntegerField' to `UnityEngine.Experimental.UIElements.INotifyValueChanged<long>'

Occuring On: 2018.2.0a3, 2018.2.0a8

Working fine On: 2018.2.0a2, 2018.1.0b13

Notes:
- Appears Always
- Appears on both OSX and Windows

Work Around:

For Example, AddTestControl<IntegerField, long>(new IntegerField(), (v) => v.ToString());

Replace 'IntegerField' with 'LongField'