  We construct forecasts for cosmological parameter constraints from weak
gravitational lensing surveys involving the Square Kilometre Array (SKA).
Considering matter content, dark energy and modified gravity parameters, we
show that the first phase of the SKA (SKA1) can be competitive with other Stage
III experiments such as the Dark Energy Survey (DES) and that the full SKA
(SKA2) can potentially form tighter constraints than Stage IV optical weak
lensing experiments, such as those that will be conducted with LSST,
WFIRST-AFTA or Euclid-like facilities. Using weak lensing alone, going from
SKA1 to SKA2 represents improvements by factors of $\sim10$ in matter, $\sim10$
in dark energy and $\sim5$ in modified gravity parameters. We also show, for
the first time, the powerful result that comparably tight constraints (within
$\sim5\%$) for both Stage III and Stage IV experiments, can be gained from
cross-correlating shear maps between the optical and radio wavebands, a process
which can also eliminate a number of potential sources of systematic errors
which can otherwise limit the utility of weak lensing cosmology.
