  We examine the equilibrium properties of lattice bosons with attractive
on-site interactions in the presence of a three-body hard-core constraint that
stabilizes the system against collapse and gives rise to a dimer superfluid
phase formed by virtual hopping processes of boson pairs. Employing quantum
Monte Carlo simulations, the ground state phase diagram of this system on the
square lattice is analyzed. In particular, we study the quantum phase
transition between the atomic and dimer superfluid regime and analyze the
nature of the superfluid-insulator transitions. Evidence is provided for the
existence of a tricritical point along the saturation transition line, where
the transition changes from being first-order to a continuous transition of the
dilute bose gas of holes. The Berzinskii-Kosterlitz-Thouless transition from
the dimer superfluid to the normal fluid is found to be consistent with an
anomalous stiffness jump, as expected from the unbinding of half-vortices.
