Problem with Apply instance

[jwiegley]

The following code demonstrates the problem:

works :: (Apply Eq1 r, Eq v) => Sum r v -> Sum r v -> Bool
works x y = x == y

-- Could not deduce (Apply Eq1 (Const () : r))
fails :: (Apply Eq1 r, Eq v) => Sum r v -> Sum r v -> Bool
fails x y = weaken @_ @_ @(Const ()) x == weaken y

Of course, if I try to provide the missing instance it complains about overlap. So how do I convince it that if Apply Eq1 r, then clearly Apply Eq1 (Const () : r), since there is both Eq1 for Const and Eq for ()?