Paper:
Learning theory tells us that more data is better when minimizing the generalization error of identically distributed training and test sets. However, when training and test distribution differ, this distribution shift can have a significant effect. With a novel perspective on function transfer learning, we are able to lower bound the change of performance when transferring from training to test set with the Wasserstein distance between the embedded training and test set distribution. We find that there is a trade-off affecting performance between how invariant a function is to changes in training and test distribution and how large this shift in distribution is.

Review:
The authors consider the relation between Frechet distance of training and test distribution and the generalization gap. The authors derive the lower bound for the difference of loss function w.r.t. training and test set by the Wasserstein distance between embedding training and test set distribution. Empirically, the authors illustrate a strong correlation between test performance and the distance in distributions between training and test set.
The motivation to find the relation between generalization gap and the Frechet distance of training and test distribution is sound. However, I am not sure that the lower bound as in Equation (1) is enough. I am curious that one can derive the upper bound for the relation or not. The finding about choosing a training data distribution should be close to the test data distribution seems quite trivial in some sense. I am not clear about its important since it is quite popular that the distribution shift affects the performance and many learning approach assumes same distribution for training and test data. Overall I feel that the contribution may be quite weak, and I lean on the negative side.
Below are some of my concerns:
1) About the lower-bound in Equation (1), it seems unclear to me that when the W_2(p1, p2) = 0, we can inference any information about the test performance (It seems quite trivial for this case, the left hand side time is greater than or equal 0?) In my opinion, the upper-bound is more important which one can inference much information about the difference of generalization gap.
2) In the proof of Theorem 1, it is quite hard to follow with the current notation, for the integral in (i), (ii) as well as in the proof using the intermediate value theorem, which variables are used? I am confused which one is variable, which one is constants in those integrals.
3) In page 5, at the interpretation (1), for W2(p1, p2) = 0, the learned function fits training distribution perfectly and is not ill-conditioned ==> why one can deduce that the test distribution is fit perfectly? What we have in Theorem 1 is the lower-bound only?