Version Used:
C# 7 using .net framework 4.5.2
Steps to Reproduce:

1. Create a custom async method builder
   [AsyncMethodBuilder(typeof(CustomMethodBuilder<>))]

Implement AwaitOnCompleted, AwaitUnsafeOnCompleted with a custom interface like
where TAwaiter : IFoo
3.
try to await an awaiter that doesn't implement IFoo

Expected Behavior:
Compile error

Actual Behavior:
---- System.Security.VerificationException : Method Eff.Core.CustomMethodBuilder1[System.Int32].AwaitUnsafeOnCompleted: type argument 'MyTestType1[System.Int32]' violates the constraint of type parameter 'TAwaiter'.