Hello everyone,

I am trying to call Base.Iterators.product on a number of arguments which is not fixed in advance, i.e. to call the following function on integer vectors of arbitrary length.

```
function indices_below(Λ_max::Union{AbstractArray{U, 1}, Tuple}) where U <: Integer
return Base.Iterators.product((0:Λi_max for Λi_max in Λ_max)...)
end
```

This function does the job, but it performs some allocations:

```
using BenchmarkTools
@btime indices_below($(5,4,6))
403.880 ns (9 allocations: 480 bytes)
Base.Iterators.ProductIterator{Tuple{UnitRange{Int64},UnitRange{Int64},UnitRange{Int64}}}((0:5, 0:4, 0:6))
```

However, calling Base.Iterators.product directly on the desired UnitRanges produces no allocation:

```
@btime Base.Iterators.product(0:5, 0:4, 0:6)
1.888 ns (0 allocations: 0 bytes)
Base.Iterators.ProductIterator{Tuple{UnitRange{Int64},UnitRange{Int64},UnitRange{Int64}}}((0:5, 0:4, 0:6))
```

Do you have any suggestion on how to improve the performance of this function ?

Many thanks