Before revision:  A key challenge to learning such risk-invariant predictors is shortcut learning, or the tendency for models to rely on spurious correlations in practice, even when a predictor based on shift-invariant features could achieve optimal i.i.d generalization in principle .
After revision:  A key challenge to learning such risk-invariant predictors is shortcut learning, where a robust predictor can achieve optimal i.i.d generalization in principle .