// (8,14): warning CS8620: Nullability of reference types in argument of type 'A' doesn't match target type 'I<object?>' for parameter 'y' in 'void C.F(I<object> x, I<object?> y)'.
// (18,8): warning CS8631: The type 'A?' cannot be used as type parameter 'T' in the generic type or method 'B4<T>'. Nullability of type argument 'A?' doesn't match constraint type 'A'.
// (32,8): warning CS8631: The type 'A?' cannot be used as type parameter 'T' in the generic type or method 'B4<T>'. Nullability of type argument 'A?' doesn't match constraint type 'A'.
// (18,8): warning CS8631: The type 'A<object?>' cannot be used as type parameter 'T' in the generic type or method 'B4<T>'. Nullability of type argument 'A<object?>' doesn't match constraint type 'A<object>'.
// (27,8): warning CS8631: The type 'A<object>' cannot be used as type parameter 'T' in the generic type or method 'B1<T>'. Nullability of type argument 'A<object>' doesn't match constraint type 'A<object?>'.
// (32,8): warning CS8631: The type 'A<object?>' cannot be used as type parameter 'T' in the generic type or method 'B4<T>'. Nullability of type argument 'A<object?>' doesn't match constraint type 'A<object>'.
// (18,8): warning CS8631: The type 'A<object?>' cannot be used as type parameter 'T' in the generic type or method 'B4<T>'. Nullability of type argument 'A<object?>' doesn't match constraint type 'A<object>'.