Test the number of outputs in frule and rrule are correct

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesTestUtils"
 uuid = "cdddcdb0-9152-4a09-a978-84456f9df70a"
-version = "0.7.1"
+version = "0.7.2"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/testers.jl

@@ -110,7 +110,8 @@ function test_frule(
         end
         res = frule((NoTangent(), deepcopy(ẋs)...), f, deepcopy(xs)...; deepcopy(fkwargs)...)
         res === nothing && throw(MethodError(frule, typeof((f, xs...))))
-        res isa Tuple || error("The frule should return (y, ∂y), not $res.")
+        msg = "The frule should return (y, ∂y), not $res."
+        @test_msg msg res isa Tuple && length(res) == 2
         Ω_ad, dΩ_ad = res
         Ω = f(deepcopy(xs)...; deepcopy(fkwargs)...)
         test_approx(Ω_ad, Ω; isapprox_kwargs...)
@@ -191,6 +192,9 @@ function test_rrule(
         ∂self = ∂s[1]
         x̄s_ad = ∂s[2:end]
         @test ∂self === NoTangent()  # No internal fields
+        msg = "The pullback must return `(∂self, ∂args...)`, where `length(∂args) == " *
+            "length(args)`, and `args` are the arguments of the primal function."
+        @test_msg msg length(x̄s_ad) == length(inputs)
 
         # Correctness testing via finite differencing.
         # TODO: remove Nothing when https://github.com/JuliaDiff/ChainRulesTestUtils.jl/issues/113

test/testers.jl

@@ -495,6 +495,24 @@ end
             @test fails(() -> test_frule(my_identity2, 2.2))
             @test fails(() -> test_rrule(my_identity2, 2.2))
         end
+
+        @testset "wrong number of outputs #167" begin
+            foo(x, y) = x + 2y
+
+            function ChainRulesCore.frule((_, ẋ, ẏ), ::typeof(foo), x, y)
+                return foo(x, y), ẋ + 2ẏ, NoTangent() # extra derivative
+                #return foo(x, y), ẋ + 2ẏ # correct expression
+            end
+
+            function ChainRulesCore.rrule(::typeof(foo), x, y)
+                foo_pullback(dz) = NoTangent(), dz # missing derivative
+                #foo_pullback(dz) = NoTangent(), dz, 2dz # correct expression
+                return foo(x,y), foo_pullback
+            end
+
+            @test fails(() -> test_frule(foo, 2.1, 2.1))
+            @test fails(() -> test_rrule(foo, 21.0, 32.0))
+        end
     end
 
     @testset "Tuple primal that is not equal to differential backing" begin