second derivatives

src/destructure.jl

@@ -128,3 +128,8 @@ end
 _grad!(x, dx::Zero, off, flat::AbstractVector) = nothing
 _grad!(x, dx::Zero, off::Integer, flat::AbstractVector) = nothing  # ambiguity
 
+function ChainRulesCore.rrule(::typeof(_grad!), x, dx, off, flat)
+  println("grad! fwd ", length(flat))
+  _grad_back(dflat) = (NoT, NoT, _rebuild(x, off, unthunk(dflat); walk = _Tangent_biwalk, prune = NoT), NoT, NoT)
+  _grad!(x, dx, off, flat), _grad_back
+end

test/destructure.jl

@@ -57,10 +57,15 @@ end
   @test g6.a isa Vector{Float64}
   @test g6.b == [0+im]
 
-  # Second derivative -- no method matching rrule(::typeof(Optimisers._rebuild), ...?
-  @test_broken gradient([1,2,3]) do v
-    sum(abs2, gradient(m -> sum(abs2, destructure(m)[1]), (v, [4,5,6]))[1][1])
-  end[1] ≈ [8,16,24]
+  @testset "second derivative" begin
+    @test_broken gradient([1,2,3.0]) do v
+      sum(abs2, gradient(m -> sum(abs2, destructure(m)[1]), (v, [4,5,6.0]))[1][1])
+    end[1] ≈ [8,16,24]
+
+    @test_skip gradient([1,2,3.0]) do v
+      sum(gradient(m -> sum(destructure(m)[1]), (v, [4,5,6.0]))[1][1])
+    end
+  end
 end
 
 @testset "gradient of rebuild" begin
@@ -85,10 +90,18 @@ end
   @test gradient(x -> re7(x).b[2][2], rand(3))[1] == [0,0,0]
   @test gradient(x -> re7(x).c[2][1], rand(3))[1] == [0,0,0]
 
-  # Second derivative -- error from _tryaxes(x::Tangent) in Zygote's map rule
-  @test_broken gradient(collect(1:6)) do y
-    sum(abs2, gradient(x -> sum(abs2, re2(x)[1]), y)[1])
-  end[1] ≈ [8,16,24,0,0,0]
+  @testset "second derivative" begin
+    # ERROR: Need an adjoint for constructor ChainRulesCore.Tangent{Any, Tuple{Vector{Float64}, ChainRulesCore.ZeroTangent}}. Gradient is of type Tuple{Vector{Float64}, Vector{Float64}}
+    @test_broken gradient(collect(1:6.0)) do y
+      sum(abs2, gradient(x -> sum(abs2, re2(x)[1]), y)[1])
+    end[1] ≈ [8,16,24,0,0,0]
+    # This fixes it!
+    # Zygote.@adjoint Tangent{T,B}(x::Tuple) where {T,B<:Tuple} = Tangent{T,B}(x), dx -> (dx,)
+    @test_skip gradient(collect(1:6.0)) do y
+      sum(abs2, gradient(x -> sum(abs2, re3(x).z), y)[1])
+    end[1]
+    # Zygote.@adjoint Tangent{T,B}(x::NamedTuple) where {T,B<:NamedTuple} = Tangent{T,B}(x), dx -> (dx,)
+  end
 end
 
 @testset "Flux issue 1826" begin