  We study the role of impurities in a two-band superconductor, and elucidate
the nature of the recently predicted transition from s_{+-} state to s_{++}
state induced by interband impurity scattering. Using a Ginzburg-Landau theory,
derived from microscopic equations, we demonstrate that close to T_c this
transition is necessarily a direct one, but deeper in the superconducting state
an intermediate complex state appears. This state has a distinct order
parameter, which breaks the time-reversal symmetry, and is separated from the
s_{+-} and s_{++} states by phase transitions. Based on our results, we suggest
a phase diagram for systems with weak repulsive interband pairing, and discuss
its relevance to iron-based superconductors.
