  SN 2017dio shows both spectral characteristics of a type-Ic supernova (SN)
and signs of a hydrogen-rich circumstellar medium (CSM). Prominent, narrow
emission lines of H and He are superposed on the continuum. Subsequent
evolution revealed that the SN ejecta are interacting with the CSM. The initial
SN Ic identification was confirmed by removing the CSM interaction component
from the spectrum and comparing with known SNe Ic, and reversely, adding a CSM
interaction component to the spectra of known SNe Ic and comparing them to SN
2017dio. Excellent agreement was obtained with both procedures, reinforcing the
SN Ic classification. The light curve constrains the pre-interaction SN Ic peak
absolute magnitude to be around $M_g = -17.6$ mag. No evidence of significant
extinction is found, ruling out a brighter luminosity required by a SN Ia
classification. These pieces of evidence support the view that SN 2017dio is a
SN Ic, and therefore the first firm case of a SN Ic with signatures of
hydrogen-rich CSM in the early spectrum. The CSM is unlikely to have been
shaped by steady-state stellar winds. The mass loss of the progenitor star must
have been intense, $\dot{M} \sim 0.02$ $(\epsilon_{H\alpha}/0.01)^{-1}$
$(v_\textrm{wind}/500$ km s$^{-1}$) $(v_\textrm{shock}/10 000$ km
s$^{-1})^{-3}$ $M_\odot$~yr$^{-1}$, peaking at a few decades before the SN.
Such a high mass loss rate might have been experienced by the progenitor
through eruptions or binary stripping.
