  About 10% of stars more massive than ${\approx}\,1.5\,\mathrm{M}_\odot$ have
strong, large-scale surface magnetic fields and are being discussed as
progenitors of highly-magnetic white dwarfs and magnetars. The origin of these
fields remains uncertain. Recent 3D magnetohydrodynamical simulations have
shown that strong magnetic fields can be generated in the merger of two massive
stars. Here, we follow the long-term evolution of such a 3D merger product in a
1D stellar evolution code. During a thermal relaxation phase after the
coalescence, the merger product reaches critical surface rotation, sheds mass
and then spins down primarily because of internal mass readjustments. The spin
of the merger product after thermal relaxation is mainly set by the
co-evolution of the star-torus structure left after coalescence. This evolution
is still uncertain, so we also consider magnetic braking and other
angular-momentum-gain and -loss mechanisms that may influence the final spin of
the merged star. Because of core compression and mixing of carbon and nitrogen
in the merger, enhanced nuclear burning drives a transient convective core that
greatly contributes to the rejuvenation of the star. Once the merger product
relaxed back to the main sequence, it continues its evolution similar to that
of a genuine single star of comparable mass. It is a slow rotator that matches
the magnetic blue straggler $\tau$ Sco. Our results show that merging is a
promising mechanism to explain some magnetic massive stars and it may also be
key to understand the origin of the strong magnetic fields of highly-magnetic
white dwarfs and magnetars.
