rrules for getindex seems to be ignored with Zygote

[oschulz]

With

using ChainRulesCore, Zygote

struct Foo <: AbstractArray{Int,0} end

running

myfunc(x::Foo) = throw(ErrorException("This should not be called"))
ChainRulesCore.rrule(::typeof(myfunc), x::Foo) = 99, _ -> (NO_FIELDS, 42)
Zygote.gradient(myfunc, Foo())

yields (42,), as expected. myfunc is bypassed and never called. However, the same with Base.getindex

Base.getindex(x::Foo) = throw(ErrorException("This should not be called"))
ChainRulesCore.rrule(::typeof(Base.getindex), x::Foo) = 99, _ -> (NO_FIELDS, 42)
Zygote.gradient(Base.getindex, Foo())

throws

ERROR: This should not be called

so Zygote seems to ignore that rrule and call Base.getindex.

I'm reporting this here, though I'm not sure if it's a problem in ChainRulesCore or Zygote (or maybe I'm doing something wrong here?).

[oschulz]

This only happens with

struct Foo <: AbstractArray{Int,0} end

When using

struct Foo end

Zygote does not call Base.getindex.

[oschulz]

Also, using Foo <: AbstractArray{Int,0} but ZygoteRules.@adjoint instead of ChainRulesCore.rrule

using Zygote, ZygoteRules

struct Foo <: AbstractArray{Int,0} end

Base.getindex(x::Foo) = throw(ErrorException("This should not be called"))
ZygoteRules.@adjoint Base.getindex(x::Foo) = 99, _ -> (42,)
Zygote.gradient(Base.getindex, Foo())

works as expected.

[oschulz]

Given how lightweight ChainRulesCore is, I guess it's must be a Zygote issue - I'll move it over there. Sorry!

[oschulz]

Moved to FluxML/Zygote.jl#811 .

[willtebbutt]

This is more of a feature than a bug.

With the way that Zygote is currently implemented, any applicable Zygote.@adjoint rule will supercede ChainRulesCore.rrules, even if the typing of the rrule is more specific than that of the Zygote.@adjoint.

This is exactly what's happening here -- Zygote has a Zygote.@adjoint defined for Base.getindex, so overloading rrule won't over-ride that. I'm assuming that the current getindex implementation is very loosely typed, which is bad, so that should be brought up on Zygote.

[oschulz]

Thanks, @willtebbutt!