  We establish a no inner-horizon theorem for black holes with charged scalar
hairs. Considering a general gravitational theory with a charged scalar field,
we prove that there exists no inner Cauchy horizon for both spherical and
planar black holes with non-trivial scalar hair. The hairy black holes approach
to a spacelike singularity at late interior time. This result is independent of
the form of scalar potentials as well as the asymptotic boundary of spacetimes.
We prove that the geometry near the singularity takes a universal Kasner form
when the kinetic term of the scalar hair dominates, while novel behaviors
different from the Kasner form are uncovered when the scalar potential become
important to the background. For the hyperbolic horizon case, we show that
hairy black hole can only has at most one inner horizon, and a concrete example
with an inner horizon is presented. All these features are also valid for the
Einstein gravity coupled with neutral scalars.
