fix implementation

src/compiler/interface.jl

@@ -114,10 +114,11 @@ sensitivity(y::AbstractArray) = error("Output is an array, so the gradient is no
 sensitivity(y) = error("Output should be scalar; gradients are not defined for output $(repr(y))")
 
 # Preserves output as tuple when gradients are collapsed
-_project_nothings(::NTuple{N}, ::Nothing) where {N} = ntuple(_ -> nothing, N)
-_project_nothings(x::Tuple, dx::Tuple) = map(x, dx) do _x, _dx
-  return _dx === nothing ? _project(_x, _dx) : _dx
-end
+_project_sentinel(::NTuple{N}, ::Nothing) where {N} = ntuple(_ -> nothing, N)
+_project_sentinel(x::Tuple, dx::Tuple) = map(_project_sentinel, x, dx)
+_project_sentinel(::Any, ::NoTangent) = nothing
+_project_sentinel(::Any, ::ZeroTangent) = nothing
+_project_sentinel(::Any, ::Nothing) = nothing
 
 """
     gradient(f, args...)
@@ -148,7 +149,7 @@ julia> gradient([7, 11], 0, 1) do x, y, d
 function gradient(f, args...)
   y, back = pullback(f, args...)
   grad = back(sensitivity(y))
-  return _project_nothings(args, grad)
+  return _project_sentinel(args, grad)
 end
 
 # Base.adjoint(f::Function) = x -> gradient(f, x)[1]  # piracy!
@@ -214,7 +215,7 @@ function withgradient(f, args...)
   else
     back(sensitivity(y))
   end
-  results = _project_nothings(args, grad)
+  results = _project_sentinel(args, grad)
   (val=y, grad=results)
 end
 
@@ -475,7 +476,7 @@ function pullback(f, ps::Params)
 end
 
 # No conversion required here
-_project_nothings(_, dx::Grads) = dx
+_project_sentinel(_, dx::Grads) = dx
 
 # Code Reflection