  Type Ia supernovae (SNe Ia) arise from the thermonuclear explosion of
carbon-oxygen white dwarfs. Though the uniformity of their light curves makes
them powerful cosmological distance indicators, long-standing issues remain
regarding their progenitors and explosion mechanisms. Recent detection of the
early ultraviolet pulse of a peculiar subluminous SN Ia has been claimed as new
evidence for the companion-ejecta interaction through the single-degenerate
channel. Here, we report the discovery of a prominent but red optical flash at
$\sim$ 0.5 days after the explosion of a SN Ia which shows hybrid features of
different SN Ia sub-classes: a light curve typical of normal-brightness SNe Ia,
but with strong titanium absorptions, commonly seen in the spectra of
subluminous ones. We argue that the early flash of such a hybrid SN Ia is
different from predictions of previously suggested scenarios such as the
companion-ejecta interaction. Instead it can be naturally explained by a SN
explosion triggered by a detonation of a thin helium shell either on a
near-Chandrasekhar-mass white dwarf ($\gtrsim$ 1.3 M$_{\odot}$) with low-yield
$^{56}$Ni or on a sub-Chandrasekhar-mass white dwarf ($\sim$ 1.0 M$_{\odot}$)
merging with a less massive white dwarf. This finding provides compelling
evidence that one branch of the previously proposed explosion models, the
helium-ignition scenario, does exist in nature, and such a scenario may account
for explosions of white dwarfs in a wider mass range in contrast to what was
previously supposed.
