Fix issymmetric (JuliaGraphs#265)

* Fix issymmetric

* Add @ traitfn

* Fix @ traitfn

* Wrong value for undirected

* Replace src and dst with reverse

Author: gdalle

src/operators.jl

@@ -509,7 +509,19 @@ sparse(g::AbstractGraph) = adjacency_matrix(g)
 
 length(g::AbstractGraph) = widen(nv(g)) * widen(nv(g))
 ndims(g::AbstractGraph) = 2
-issymmetric(g::AbstractGraph) = !is_directed(g)
+
+@traitfn function issymmetric(g::AG) where {AG <: AbstractGraph; !IsDirected{AG}}
+    return true
+end
+
+@traitfn function issymmetric(g::AG) where {AG <: AbstractGraph; IsDirected{AG}}
+    for e in edges(g)
+        if !has_edge(g, reverse(e))
+            return false
+        end
+    end
+    return true
+end
 
 """
     cartesian_product(g, h)

test/operators.jl

@@ -203,6 +203,15 @@
         @test @inferred(!issymmetric(g))
     end
 
+    gx = SimpleDiGraph(4)
+    add_edge!(gx, 1, 2)
+    add_edge!(gx, 2, 1)
+    add_edge!(gx, 1, 3)
+    add_edge!(gx, 3, 1)
+    @testset "Matrix operations: $g" for g in testdigraphs(gx)
+        @test @inferred(issymmetric(g))
+    end
+
     nx = 20
     ny = 21
     @testset "Cartesian Product / Crosspath: $g" for g in testlargegraphs(path_graph(ny))