Reconstruction from Tangent fails

[mcabbott]

id(x) = x

function ChainRules.rrule(::typeof(id), x)
    y = x
    function back(dy)
        dx = ProjectTo(x)(@show dy)
        (NoTangent(), @show dx)
    end
    y, back
end

test_rrule(id, 3.4)  # fine

test_rrule(id, Ref(3.4))

#=

dy = Tangent{Base.RefValue{Float64}}(x = 4.35,)
test_rrule: id on Base.RefValue{Float64}: Error During Test at /Users/me/.julia/packages/ChainRulesTestUtils/8380y/src/testers.jl:191
  Got exception outside of a @test
  ArgumentError: type does not have a definite number of fields
  Stacktrace:
    [1] fieldcount(t::Any)
      @ Base ./reflection.jl:764
    [2] construct(#unused#::Type{Ref}, fields::NamedTuple{(:x,), Tuple{Float64}})
      @ ChainRulesCore ~/.julia/packages/ChainRulesCore/Voykb/src/differentials/composite.jl:213
    [3] ProjectTo
      @ ~/.julia/packages/ChainRulesCore/Voykb/src/projection.jl:78 [inlined]
    [4] (::var"#back#9"{Base.RefValue{Float64}})(dy::Tangent{Base.RefValue{Float64}, NamedTuple{(:x,), Tuple{Float64}}})

=#

Maybe the short version is something like this:

x = 1:3.0
dx = Tangent{typeof(x)}(ref=NoTangent(),step=0.1,len=NoTangent(),offset=NoTangent())
ProjectTo(x)(dx)
#=
ERROR: ArgumentError: Named tuple names do not match.
Stacktrace:
 [1] map(f::Function, nt::NamedTuple{(:element, :axes), Tuple{ProjectTo{Float64, NamedTuple{(), Tuple{}}}, Tuple{Base.OneTo{Int64}}}}, nts::NamedTuple{(:ref, :step, :len, :offset), Tuple{NoTangent, Float64, NoTangent, NoTangent}})
=#

y = 1.0+im;
dy = Tangent{typeof(y)}(; re=NoTangent(), im=0.1)
ProjectTo(y)(dy)
#=
ERROR: ArgumentError: Named tuple names do not match.
Stacktrace:
 [1] map(f::Function, nt::NamedTuple{(), Tuple{}}, nts::NamedTuple{(:re, :im), Tuple{NoTangent, Float64}})
=#