If you are here because you tried to implement HasPointScheduleTestParams for some type `TestBlockWith Foo` and got an overlapping instance warning, the `a ~ ()` constraint is only here to get your attention. The tests should remain as block polymorphic as possible, so maybe there should be a class for the types a that can appear in `TestBlockWith a`. But we in the past do not know what that class should look like! So the choice is yours: if what you need from this class can be made polymorphic in a, consider adding a class. If not, specialize this instance.