  We present Hubble Space Telescope imaging of a pre-explosion counterpart to
SN 2019yvr obtained 2.6 years before its explosion as a type Ib supernova (SN
Ib). Aligning to a post-explosion Gemini-S/GSAOI image, we demonstrate that
there is a single source consistent with being the SN 2019yvr progenitor
system, the second SN Ib progenitor candidate after iPTF13bvn. We also analyzed
pre-explosion Spitzer/IRAC imaging, but we do not detect any counterparts at
the SN location. SN 2019yvr was highly reddened, and comparing its spectra and
photometry to those of other, less extinguished SNe Ib we derive
$E(B-V)=0.51\substack{+0.27\\-0.16}$ mag for SN 2019yvr. Correcting photometry
of the pre-explosion source for dust reddening, we determine that this source
is consistent with a $\log(L/L_{\odot}) = 5.3 \pm 0.2$ and $T_{\mathrm{eff}} =
6800\substack{+400\\-200}$ K star. This relatively cool photospheric
temperature implies a radius of 320$\substack{+30\\-50} R_{\odot}$, much larger
than expectations for SN Ib progenitor stars with trace amounts of hydrogen but
in agreement with previously identified SN IIb progenitor systems. The
photometry of the system is also consistent with binary star models that
undergo common envelope evolution, leading to a primary star hydrogen envelope
mass that is mostly depleted but seemingly in conflict with the SN Ib
classification of SN 2019yvr. SN 2019yvr had signatures of strong circumstellar
interaction in late-time ($>$150 day) spectra and imaging, and so we consider
eruptive mass loss and common envelope evolution scenarios that explain the SN
Ib spectroscopic class, pre-explosion counterpart, and dense circumstellar
material. We also hypothesize that the apparent inflation could be caused by a
quasi-photosphere formed in an extended, low-density envelope or circumstellar
matter around the primary star.
