  Current data from neutrino oscillation experiments are in good agreement with
$\delta =-\frac{\pi}{2}$ and $\theta_{23}^{} = \frac{\pi}{4}$. We define the
notion of "constrained maximal CP violation" (CMCPV) for predicting these
features and study their origin in flavor symmetry. We derive the
parametrization-independent solution of CMCPV and give a set of equivalent
definitions for it. We further present a theorem on how the CMCPV can be
realized. This theorem takes advantage of residual symmetries in the neutrino
and charged lepton mass matrices, and states that, up to a few minor
exceptions, $(|\delta |, \theta_{23}) =(\frac{\pi}{2}, \frac{\pi}{4})$ is
generated when those symmetries are real. The often considered $\mu$-$\tau$
reflection symmetry, as well as specific discrete subgroups of O(3), are
special cases of our theorem.
