Remove the .+ 0 hack

Ref: JuliaDiff/ChainRulesCore.jl#90

Author: YingboMa

Project.toml

@@ -10,7 +10,7 @@ Requires = "ae029012-a4dd-5104-9daa-d747884805df"
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 
 [compat]
-ChainRulesCore = "0.5"
+ChainRulesCore = "0.5.1"
 FiniteDifferences = "^0.7"
 Reexport = "0.2"
 Requires = "0.5.2, 1"

test/test_util.jl

@@ -69,7 +69,6 @@ function frule_test(f, xẋs::Tuple{Any, Any}...; rtol=1e-9, atol=1e-9, fdm=_fdm
     xs, ẋs = collect(zip(xẋs...))
     dself = Zero()
     Ω, dΩ_ad = ChainRules.frule(f, xs..., dself, ẋs...)
-    dΩ_ad = dΩ_ad .+ 0
     @test f(xs...) == Ω
 
     # Correctness testing via finite differencing.