From d25e05d9eed5cde043a609bf6aca63bc545ee6b5 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Thu, 27 Sep 2018 10:40:44 +0200
Subject: [PATCH] evaluate both 2-ary DiffRules only when needed

---
 src/tracker/scalar.jl | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/tracker/scalar.jl b/src/tracker/scalar.jl
index 81ccb9a3..1b6098fb 100644
--- a/src/tracker/scalar.jl
+++ b/src/tracker/scalar.jl
@@ -63,7 +63,9 @@ for (M, f, arity) in DiffRules.diffrules()
   da, db = DiffRules.diffrule(M, f, :a, :b)
   f = :($M.$f)
   @eval begin
-    @grad $f(a::Real, b::Real) = $f(data(a), data(b)), Δ -> (Δ * $da, Δ * $db)
+    @grad $f(a::TrackedReal, b::TrackedReal) = $f(data(a), data(b)), Δ -> (Δ * $da, Δ * $db)
+    @grad $f(a::TrackedReal, b::Real) = $f(data(a), b), Δ -> (Δ * $da, zero(b))
+    @grad $f(a::Real, b::TrackedReal) = $f(a, data(b)), Δ -> (zero(a), Δ * $db)
     $f(a::TrackedReal, b::TrackedReal)  = track($f, a, b)
     $f(a::TrackedReal, b::Real) = track($f, a, b)
     $f(a::Real, b::TrackedReal) = track($f, a, b)