  The LHCb Collaboration has recently reported the discovery of direct CP
violation in combined $D^0 \to K^+ K^-$ and $D^0 \to \pi^+ \pi^-$ decay modes
at the $5.3 \sigma$ level. Assuming U-spin symmetry (i.e., $d \leftrightarrow
s$ interchange symmetry) for the strong-interaction parts of these two
channels, we find that their corresponding direct CP-violating asymmetries are
${\cal A}^{}_{\rm CP} (K^+ K^-) \simeq \left(-7.7 \pm 1.5\right) \times
10^{-4}$ and ${\cal A}^{}_{\rm CP} (\pi^+ \pi^-) \simeq \left(7.7 \pm
1.5\right) \times 10^{-4}$. The CP-forbidden transition $e^+ e^- \to
D^0\bar{D}^0 \to \left(K^+ K^-\right)^{}_D \left(\pi^+ \pi^-\right)^{}_D$ on
the $\psi(3770)$ resonance is therefore expected to have a rate of ${\cal
O}(10^{-10})$ or smaller under U-spin symmetry, and it can be observed at a
high-luminosity super-$\tau$-charm factory if at least $10^{10}$ pairs of
coherent $D^0$ and $\bar{D}^0$ events are accumulated.
