I don't know how accum of the dicts works, but it seems to be done somewhere else.

julia> gradient(d -> d["one"]*2 + d["one"]*3, Dict("one"=>1, "two"=>2))
(a, b) = (Dict{Any, Any}("one" => 5.0), Dict{Any, Any}("one" => 5.0))
(Dict{Any, Any}("one" => 5.0),)

where I've added @show a, b in that accum method. So it looks like it's been accumulated somewhere earlier already?

This is the issue from the Slack thread: JuliaDiff/ChainRules.jl#662.

Note that testing that issue with this PR requires the Molly master branch.

So it looks like it's been accumulated somewhere earlier already?

That's correct. Accumulation for Dicts (and mutable structs) is pretty weird in Zygote because of the need to support setindex!/setfield! on them. In this case, the accumulation is happening in

We could probably add that special case of === do nothing to ChainRulesCore.
Its certainly not a valid thing that should normally happen AFAIK -- accumulating a gradient against itself.
I feel like it is a bug that Zygote doesn't then return nothing in the place it has already done the accumulation but that's a different issue

Ideally CR wouldn't have to make any changes here, will have to look into the PR + original issue in more depth though.

I feel like it is a bug that Zygote doesn't then return nothing in the place it has already done the accumulation but that's a different issue

It should be doing this. If there's any rule where it's not then I think that should be called a bug.

The z2d conversion looks good. All that's left other than tests is the other end in wrap_chainrules_output, I take it?

It should be doing this. If there's any rule where it's not then I think that should be called a bug.

Here it doesn't seem to

The z2d conversion looks good. All that's left other than tests is the other end in wrap_chainrules_output, I take it?

Oh true, I forgot that.

Ah, you are right and I was mixing up @adjoint and rrule return conventions again. Making those pullbacks return nothing would be an interesting experiment, though AFAICT they don't cause any issues right now?

After testing locally with the line in #1288 (comment) returning nothing for the Dict itself,

Pairs: Test Failed at ~/.julia/dev/Zygote/test/features.jl:586
  Expression: gradient((d->begin
                (Dict(:x => d[:x]))[:x]
            end), d) == (Dict(:x => 1),)
   Evaluated: (nothing,) == (Dict(:x => 1),)

Were there more tests taking differentiating wrt a Dict argument, there would likely be more failures. But, quelle surprise, Zygote has just the one.

Note that this is just for Dicts. Doing the same for (literal_)getindex on mutable types breaks a whole lot more tests, but that can be a discussion for another day.

For reference, this is the test that fails when one removes the accum overload linked up top:
https://github.com/FluxML/Zygote.jl/blob/v0.6.47/test/interface.jl#L246-L254

Again, the only reason there's a single failure is because there is only one test for nested diff with implicit params. Is there perhaps a way for us to constrain this accum call so that it's only valid in that circumstance?