Fix zero-dimensional cone in cones

src/PolyhedralGeometry/PolyhedralFan/properties.jl

@@ -212,8 +212,17 @@ julia> cones(PF, 2)
 """
 function cones(PF::_FanLikeType, cone_dim::Int)
   l = cone_dim - length(lineality_space(PF))
-  l < 1 && return nothing
-  return SubObjectIterator{Cone{_get_scalar_type(PF)}}(
+  t = Cone{_get_scalar_type(PF)}
+  l < 0 && return _empty_subobjectiterator(t, PF)
+  l == 0 && length(lineality_space(PF)) == 0 && return SubObjectIterator{t}(
+    PF, (_, _, _) -> cone(zeros(Int, ambient_dim(PF))), 1, NamedTuple()
+  )
+
+  # The function `lineality_space` returns a ray even in the case where
+  # the lineality space is actually a line.
+  l == 0 && length(lineality_space(PF)) > 0 && return error("Not implemented.")
+
+  return SubObjectIterator{t}(
     PF, _cone_of_dim, size(Polymake.fan.cones_of_dim(pm_object(PF), l), 1), (c_dim=l,)
   )
 end

test/PolyhedralGeometry/polyhedral_fan.jl

@@ -58,11 +58,18 @@
     @test size(cones(F2, 2)) == (2,)
     @test lineality_space(cones(F2, 2)[1]) == [[0, 1, 0]]
     @test rays.(cones(F2, 2)) == [[], []]
-    @test isnothing(cones(F2, 1))
     @test _check_im_perm_rows(ray_indices(cones(F1, 2)), incidence1)
     @test _check_im_perm_rows(incidence_matrix(cones(F1, 2)), incidence1)
     @test _check_im_perm_rows(cones(IncidenceMatrix, F1, 2), incidence1)
 
+    A3 = affine_space(NormalToricVariety, 3)
+    @test length(cones(A3, 1)) == 3
+    @test length(cones(A3, 0)) == 1
+    @test length(cones(A3, -1)) == 0
+    @test cones(A3, 1) isa SubObjectIterator{Cone{QQFieldElem}}
+    @test cones(A3, 0) isa SubObjectIterator{Cone{QQFieldElem}}
+    @test cones(A3, -1) isa SubObjectIterator{Cone{QQFieldElem}}
+
     II = ray_indices(maximal_cones(NFsquare))
     NF0 = polyhedral_fan(II, rays(NFsquare))
     @test n_rays(NF0) == 4