  We study a two-dimensional fermionic square lattice, which supports the
existence of two-dimensional Weyl semimetal, quantum anomalous Hall effect, and
$2\pi$-flux topological semimetal in different parameter ranges. We show that
the band degenerate points of the two-dimensional Weyl semimetal and
$2\pi$-flux topological semimetal are protected by two distinct novel hidden
symmetries, which both corresponds to antiunitary composite operations. When
these hidden symmetries are broken, a gap opens between the conduction and
valence bands, turning the system into a insulator. With appropriate
parameters, a quantum anomalous Hall effect emerges. The degenerate point at
the boundary between the quantum anomalous Hall insulator and trivial band
insulator is also protected by the hidden symmetry.
