out of place gradients for collect

src/tracker/scalar.jl

@@ -115,3 +115,7 @@ end
 function back_(c::Call{typeof(collect)}, Δ)
   foreach(back, c.args[1], data(Δ))
 end
+
+function back_(g::Grads, c::Call{typeof(collect)}, Δ)
+  foreach((x, Δ) -> back(g, x, Δ), c.args[1], Δ)
+end

test/tracker.jl

@@ -232,6 +232,11 @@ Tracker.back!(b)
   z = xy[1]*xy[2]
   back!(z)
   @test grad.((x,y)) == (3, 2)
+
+  @test Tracker.gradient(2, 3) do x, y
+    xy = Tracker.collect([x, y])
+    xy[1]*xy[2]
+  end == (3, 2)
 end
 
 # Gradient Hooks