  We report a $5.3\sigma$ detection of the gravitational lensing effect of
cosmic voids from the Baryon Oscillation Spectroscopic (BOSS) Data Release 12
seen in the $Planck$ 2018 cosmic microwave background (CMB) lensing convergence
map. To make this detection, we introduce new optimal techniques for void
stacking and filtering of the CMB maps, such as binning voids by a combination
of their observed galaxy density and size to separate those with distinctive
lensing signatures. We calibrate theoretical expectations for the void-lensing
signal using mock catalogs generated in a suite of 108 full-sky lensing
simulations from Takahashi et al. (2017). Relative to these templates, we
measure the lensing amplitude parameter in the data to be $A_L=1.10 \pm 0.21$
using a matched-filter stacking technique, and confirm it using an alternative
Wiener filtering method. We demonstrate that the result is robust against
thermal Sunyaev-Zel'dovich contamination and other sources of systematics. We
use the lensing measurements to test the relationship between the matter and
galaxy distributions within voids, and show that the assumption of linear bias
with a value consistent with galaxy clustering results is discrepant with
observation at $\sim 3\sigma$; we explain why such a result is consistent with
simulations and previous results, and is expected as a consequence of void
selection effects. We forecast the potential for void-CMB lensing measurements
in future data from the Advanced ACT, Simons Observatory and CMB-S4
experiments, showing that, for the same number of voids, the achievable
precision improves by a factor of more than two compared to $Planck$.
