fix #1828

cvxpy/atoms/sign.py

@@ -56,12 +56,12 @@ def is_atom_concave(self) -> bool:
     def is_atom_quasiconvex(self) -> bool:
         """Is the atom quasiconvex?
         """
-        return True
+        return self.args[0].is_scalar()
 
     def is_atom_quasiconcave(self) -> bool:
         """Is the atom quasiconvex?
         """
-        return True
+        return self.args[0].is_scalar()
 
     def is_incr(self, idx) -> bool:
         """Is the composition non-decreasing in argument idx?

cvxpy/tests/test_dqcp.py

@@ -429,6 +429,14 @@ def test_sign(self) -> None:
         cp.sign(variable).value
         self.assertItemsAlmostEqual(value, variable.value)
 
+        # sign is only QCP for univariate input.
+        # See issue #1828
+        x = cp.Variable(2)
+        obj = cp.sum_squares(np.ones(2) - x)
+        constr = [cp.sum(cp.sign(x)) <= 1]
+        prob = cp.Problem(cp.Minimize(obj), constr)
+        assert not prob.is_dqcp()
+
     def test_dist_ratio(self) -> None:
         x = cp.Variable(2)
         a = np.ones(2)