  We present 3-mm observations of the quasar J0015+1842 at z=2.63 with the
NOrthern Extended Millimeter Array (NOEMA). Our data reveals molecular gas,
traced via a Gaussian CO(3-2) line, with a remarkably large velocity dispersion
(FWHM=1010+/-120 km/s) and corresponding to a total molecular mass
MH2~(3.4-17)x10^10 Msun, depending on the adopted CO-to-H2 conversion factor
alphaCO=(0.8-4.0) Msun (km/s pc^2)^-1. Assuming the 3-mm continuum emission is
thermal, we derive a dust mass of the order of Mdust ~5x10^8 Msun. J0015+1842
is located in the molecular gas-rich region in the IR vs CO line luminosity
diagram, in-between the main locus of main-sequence and sub-millimetre galaxies
and that of most other AGNs targeted so far for CO measurements. While the
large velocity dispersion of the CO line suggests a merging system, J0015+1842
is observed to be a regular, only very moderately dust-reddened (Av~0.3-0.4)
type-I quasar from its UV-optical spectrum, from which we infer a mass of the
super-massive black hole be around MBH~6x10^8 Msun. We suggest that J0015+1842
is observed at a galaxy evolutionary stage where a massive merger has brought
significant amounts of gas towards an actively accreting super-massive black
hole (quasar). While the host still contains a large amount of dust and
molecular gas with high velocity dispersion, the quasar has already cleared the
way towards the observer, likely through powerful outflows as recently revealed
by optical observations of the same object. High angular resolution
observations of this and similar systems} should help determining better the
respective importance of evolution and orientation in the appearance of quasars
and their host galaxies and have the potential to investigate early feedback
and star-formation processes in galaxies in their quasar phases.
