`idx_to_dim` type unstable

[roflmaostc]

Body::Any
1 ─ %1 = NDTools.ndims(idx_arr)::Core.Const(2)
│   %2 = (%1 + 1)::Core.Const(3)
│   %3 = (#self#)(idx_arr, %2)::Any
└──      return %3

[RainerHeintzmann]

Do you mean:
@code_warntype idx_to_dim([(x,y) for x in 1:3, y in 1:3]) ?
I only get two yellow warnings:
Locals @_3::Union{Nothing, Tuple{CartesianIndex{2}, CartesianIndex{2}}} val::Nothing out_arr::Array{Int64, 3} @_6::Union{Nothing, Tuple{Tuple{Int64, Int64}, Tuple{Int64, Int64}}} I::CartesianIndex{2}
The rest is blue.

[RainerHeintzmann]

The current PR makes it fully type stable (all blue).