add test for #551

Author: mcabbott

test/HessianTest.jl

@@ -157,4 +157,11 @@ for T in (StaticArrays.SArray, StaticArrays.MArray)
     @test DiffResults.hessian(sresult3) == DiffResults.hessian(result)
 end
 
+@testset "branches in dot" begin
+    # https://github.com/JuliaDiff/ForwardDiff.jl/issues/551
+    H = [1 2 3; 4 5 6; 7 8 9];
+    @test ForwardDiff.hessian(x->dot(x,H,x), fill(0.00001, 3)) ≈ [2 6 10; 6 10 14; 10 14 18]
+    @test ForwardDiff.hessian(x->dot(x,H,x), zeros(3)) ≈ [2 6 10; 6 10 14; 10 14 18]
+end
+
 end # module