  We here present the spectroscopic follow-up observations with VLT/X-shooter
of the Swift long-duration gamma-ray burst GRB 160804A at z = 0.737. Typically,
GRBs are found in low-mass, metal-poor galaxies which constitute the
sub-luminous population of star-forming galaxies. For the host galaxy of the
GRB presented here we derive a stellar mass of $\log(M_*/M_{\odot}) = 9.80\pm
0.07$, a roughly solar metallicity (12+log(O/H) = $8.74\pm 0.12$) based on
emission line diagnostics, and an infrared luminosity of $M_{3.6/(1+z)} =
-21.94$ mag, but find it to be dust-poor ($E(B-V) < 0.05$ mag). This
establishes the galaxy hosting GRB 160804A as one of the most luminous, massive
and metal-rich GRB hosts at z < 1.5. Furthermore, the gas-phase metallicity is
found to be representative of the physical conditions of the gas close to the
explosion site of the burst. The high metallicity of the host galaxy is also
observed in absorption, where we detect several strong FeII transitions as well
as MgII and MgI. While host galaxy absorption features are common in GRB
afterglow spectra, we detect absorption from strong metal lines directly in the
host continuum (at a time when the afterglow was contributing to < 15%).
Finally, we discuss the possibility that the geometry and state of the
absorbing and emitting gas is indicative of a galactic scale outflow expelled
at the final stage of two merging galaxies.
