inboundstest.jl

julia> struct DupVector{T} <: AbstractVector{T}
           A::Vector{T}
       end

julia> Base.size(dv::DupVector) = size(dv.A) .* 2

julia> @inline function Base.getindex(dv::DupVector, idx)
           @boundscheck checkbounds(dv, idx)
           dv.A[div(idx+1, 2)]
       end

julia> testindex(v, i) = @inbounds return v[i]
testindex (generic function with 1 method)

julia> dv = DupVector([3, 7, 6, 1])
8-element DupVector{Int64}:
 3
 3
 7
 7
 6
 6
 1
 1

julia> testindex(dv, 9)
0