  It is generally believed that long-duration gamma-ray bursts (GRBs) are
associated with massive star core-collapse, whereas short-duration GRBs are
associated with mergers of compact star binaries. However, growing observations
have suggested that oddball GRBs do exist, and multiple criteria (prompt
emission properties, supernova/kilonova associations, and host galaxy
properties) rather than burst duration only are needed to classify GRBs
physically. A previously reported long-duration burst, GRB 060614, could be
viewed as a short GRB with extended emission if it were observed at a larger
distance and was associated with a kilonova-like feature. As a result, it
belongs to the Type-I (compact star merger) GRB category and is likely of the
binary neutron star merger origin. Here we report a peculiar long-duration
gamma-ray burst, GRB 211211A, whose prompt emission properties in many aspects
differ from all known Type-I GRBs, yet its multi-band observations suggest a
non-massive-star origin. In particular, significant excess emission in both
optical and near-infrared wavelengths has been discovered, which resembles
kilonova emission as observed in some Type-I GRBs. These observations point
towards a new progenitor type of GRBs. A scenario invoking a white
dwarf-neutron star merger with a post-merger magnetar engine provides a
self-consistent interpretation for all the observations, including prompt
gamma-rays, early X-ray afterglow, as well as the engine-fed kilonova emission.
