  We enumerate all possible types of spacetime causal structures that can
appear in static, spherically symmetric configurations of a self-gravitating,
real, nonlinear, minimally coupled scalar field \phi in general relativity,
with an arbitrary potential V(\phi), not necessarily positive-definite. It is
shown that a variable scalar field adds nothing to the list of possible
structures with a constant \phi field, namely, Minkowski (or AdS),
Schwarzschild, de Sitter and Schwarzschild - de Sitter. It follows, in
particular, that, whatever is V(\phi), this theory does not admit regular black
holes with flat or AdS asymptotics. It is concluded that the only possible
globally regular, asymptotically flat solutions are solitons with a regular
center, without horizons and with at least partly negative potentials V(\phi).
Extension of the results to more general field models is discussed.
