test_rrule(sum, iszero, ...) errors

Error is:

julia> using ChainRules, ChainRulesTestUtils

julia> test_rrule(sum, iszero, randn(5))
test_rrule: sum on typeof(iszero),Vector{Float64}: Error During Test at /home/oxinabox/.julia/packages/ChainRulesTestUtils/f5cNH/src/testers.jl:191
  Got exception outside of a @test
  DimensionMismatch("second dimension of A, 1, does not match length of x, 0")
  Stacktrace:
    [1] gemv!(y::Vector{Float64}, tA::Char, A::Matrix{Float64}, x::Vector{Float64}, α::Bool, β::Bool)
      @ LinearAlgebra /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/matmul.jl:530
    [2] mul!
      @ /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/matmul.jl:97 [inlined]
    [3] mul!
      @ /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/matmul.jl:275 [inlined]
    [4] *(transA::LinearAlgebra.Transpose{Float64, Matrix{Float64}}, x::Vector{Bool})
      @ LinearAlgebra /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/matmul.jl:87
    [5] _j′vp(fdm::FiniteDifferences.AdaptedFiniteDifferenceMethod{5, 1, FiniteDifferences.UnadaptedFiniteDifferenceMethod{7, 5}}, f::Function, ȳ::Vector{Bool}, x::Vector{Float64})
      @ FiniteDifferences ~/.julia/packages/FiniteDifferences/W3rQO/src/grad.jl:80
    [6] j′vp(fdm::FiniteDifferences.AdaptedFiniteDifferenceMethod{5, 1, FiniteDifferences.UnadaptedFiniteDifferenceMethod{7, 5}}, f::Function, ȳ::NoTangent, x::Vector{Float64})
      @ FiniteDifferences ~/.julia/packages/FiniteDifferences/W3rQO/src/grad.jl:73
    [7] _make_j′vp_call(fdm::Any, f::Any, ȳ::Any, xs::Any, ignores::Any)
      @ ChainRulesTestUtils ~/.julia/packages/ChainRulesTestUtils/f5cNH/src/finite_difference_calls.jl:51
    [8] macro expansion
      @ ~/.julia/packages/ChainRulesTestUtils/f5cNH/src/testers.jl:222 [inlined]
    [9] macro expansion
      @ /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1151 [inlined]
   [10] test_rrule(::RuleConfig, ::Any, ::Any, ::Vararg{Any, N} where N; output_tangent::Any, check_thunked_output_tangent::Any, fdm::Any, rrule_f::Any, check_inferred::Bool, fkwargs::NamedTuple, rtol::Real, atol::Real, kwargs::Any)
      @ ChainRulesTestUtils ~/.julia/packages/ChainRulesTestUtils/f5cNH/src/testers.jl:194
   [11] test_rrule
      @ ~/.julia/packages/ChainRulesTestUtils/f5cNH/src/testers.jl:186 [inlined]
   [12] #test_rrule#47
      @ ~/.julia/packages/ChainRulesTestUtils/f5cNH/src/testers.jl:168 [inlined]
   [13] test_rrule(::Any, ::Any, ::Any)
      @ ChainRulesTestUtils ~/.julia/packages/ChainRulesTestUtils/f5cNH/src/testers.jl:167

I assume it is something to do with the fact that the everything including the output, has the determined tangent of NoTangent()

Reproduce the failure with test_rrule(sum, iszero, randn(5)) and start in src/testers.jl around lines 168–222, following the call into finite_difference_calls.jl. Trace how the NoTangent output is passed to the finite-difference path and identify the expected handling. Done means this case no longer raises DimensionMismatch and is covered by an appropriate regression test.