Add unhappy path test

Author: oxinabox

test/check_result.jl

@@ -97,6 +97,14 @@ end
             @test fails(() -> test_approx(@thunk(10 * [[1.0], [2.0]]), [[1.0], [2.0]]))
 
             @test fails(() -> test_approx(@not_implemented("a"), @not_implemented("b")))
+
+            # should fail, not ambig error
+            @test fails() do 
+                test_approx(
+                    Tangent{Tuple{Float64,Float64}}(1.0, 2.0),
+                    @thunk(Tangent{Tuple{Float64,Float64}}(2.0, 2.0)),
+                )
+            end
         end
         @testset "type negative" begin
             @test fails() do  # these have different primals so should not be equal