fix: overloaded_input_type for one-element vector input

Author: ErikQQY

DifferentiationInterface/ext/DifferentiationInterfaceForwardDiffExt/misc.jl

@@ -28,5 +28,5 @@ DI.overloaded_input_type(prep::ForwardDiffTwoArgDerivativePrep) = typeof(prep.co
 DI.overloaded_input_type(prep::ForwardDiffGradientPrep) = typeof(prep.config.duals)
 
 ## Jacobian
-DI.overloaded_input_type(prep::ForwardDiffOneArgJacobianPrep) = typeof(prep.config.duals[2])
-DI.overloaded_input_type(prep::ForwardDiffTwoArgJacobianPrep) = typeof(prep.config.duals[2])
+DI.overloaded_input_type(prep::ForwardDiffOneArgJacobianPrep) = typeof(last(prep.config.duals))
+DI.overloaded_input_type(prep::ForwardDiffTwoArgJacobianPrep) = typeof(last(prep.config.duals))

DifferentiationInterface/test/Back/ForwardDiff/test.jl

@@ -120,6 +120,9 @@ end
     @test DI.overloaded_input_type(
         prepare_jacobian(copyto!, similar(x), sparse_backend, x)
     ) == Vector{ForwardDiff.Dual{ForwardDiff.Tag{typeof(copyto!), Float64}, Float64, 1}}
+    # Jacobian with one-element input
+    @test DI.overloaded_input_type(prepare_jacobian(copy, backend, [1.0])) ==
+        ForwardDiff.Dual{ForwardDiff.Tag{typeof(copy), Float64}, Float64, 1}
 end;
 
 include("benchmark.jl")