Allow NotImplemented tangents for things that have a correct tangent of NoTangent

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesTestUtils"
 uuid = "cdddcdb0-9152-4a09-a978-84456f9df70a"
-version = "1.2.2"
+version = "1.2.3"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/testers.jl

@@ -230,7 +230,14 @@ function test_rrule(
                     "The pullback in the rrule should use NoTangent()" *
                     " rather than ZeroTangent() for non-perturbable arguments.",
                 )
-                @test ad_cotangent isa NoTangent  # we said it wasn't differentiable.
+                if ad_cotangent isa ChainRulesCore.NotImplemented
+                    # this situation can occur if a cotangent is not implemented and
+                    # the default `rand_tangent` is `NoTangent`:
+                    # https://github.com/JuliaDiff/ChainRulesTestUtils.jl/issues/217
+                    @test_broken ad_cotangent isa NoTangent
+                else
+                    @test ad_cotangent isa NoTangent  # we said it wasn't differentiable.
+                end
             else
                 ad_cotangent isa AbstractThunk && check_inferred && _test_inferred(unthunk, ad_cotangent)
 

test/testers.jl

@@ -644,6 +644,16 @@ struct MySpecialConfig <: RuleConfig{Union{MySpecialTrait}} end
         @scalar_rule f_notimplemented(x, y) (@not_implemented(""), 1) (1, -1)
         test_frule(f_notimplemented, randn(), randn())
         test_rrule(f_notimplemented, randn(), randn())
+
+        f_notimplemented2(f, x) = 2 * f(x)
+        function ChainRulesCore.rrule(::typeof(f_notimplemented2), f::typeof(identity), x)
+            Ω = f_notimplemented2(f, x)
+            function f_notimplemented2_pullback(Ω̄)
+                return NoTangent(), @not_implemented("TODO: implement this!"), 2 * Ω̄
+            end
+            return Ω, f_notimplemented2_pullback
+        end
+        test_rrule(f_notimplemented2, identity, randn())
     end
 
     @testset "custom rrule_f" begin