Zygote ignores ChainRulesCore.rrule for Base.getindex on AbstractArray types

[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.

This only happens with

struct Foo <: AbstractArray{Int,0} end

When using

struct Foo end

Zygote does not call Base.getindex.

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.