Merge pull request JuliaDiff#74 from YingboMa/myb/hypot

Fix hypot frule

Author: oxinabox

src/rulesets/Base/base.jl

@@ -62,7 +62,7 @@
 @scalar_rule(cotd(x), -(π / oftype(x, 180)) * (1 + Ω^2))
 @scalar_rule(sech(x), -tanh(x) * Ω)
 @scalar_rule(csch(x), -coth(x) * Ω)
-@scalar_rule(hypot(x, y), (y / Ω, x / Ω))
+@scalar_rule(hypot(x, y), (x / Ω, y / Ω))
 @scalar_rule(sincos(x), @setup((sinx, cosx) = Ω), cosx, -sinx)
 @scalar_rule(atan(y, x), @setup(u = hypot(x, y)), (x / u, y / u))
 @scalar_rule(max(x, y), @setup(gt = x > y), (gt, !gt))

test/rulesets/Base/base.jl

@@ -83,18 +83,18 @@ end
             x, y = rand(2)
             h, dxy = frule(hypot, x, y)
 
-            @test extern(dxy(One(), Zero())) === y / h
-            @test extern(dxy(Zero(), One())) === x / h
+            @test extern(dxy(One(), Zero())) === x / h
+            @test extern(dxy(Zero(), One())) === y / h
 
             cx, cy = cast((One(), Zero())), cast((Zero(), One()))
             dx, dy = extern(dxy(cx, cy))
-            @test dx === y / h
-            @test dy === x / h
+            @test dx === x / h
+            @test dy === y / h
 
             cx, cy = cast((rand(), Zero())), cast((Zero(), rand()))
             dx, dy = extern(dxy(cx, cy))
-            @test dx === y / h * cx.value[1]
-            @test dy === x / h * cy.value[2]
+            @test dx === x / h * cx.value[1]
+            @test dy === y / h * cy.value[2]
         end
     end
     @testset "identity" begin