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julia> nums = (i^2 for i in Iterators.filter(_->rand(Bool), 1:100));

julia> Base.haslength(nums)
false

julia> stack(nums)'
1×44 LinearAlgebra.Adjoint{Int64,Array{Int64,1}}:
 1  4  9  49  81  144  196  324  361  400  484  529  …  6241  6561  7396  7744  7921  8649  10000

julia> vecs = ([i,i^2] for i in Iterators.filter(_->rand(Bool), 1:100));

julia> stack(vecs)
2×54 Array{Int64,2}:
 2   7   9   10   11   13   14   16   17   18   20  …    90    92    94    95    96    98    100
 4  49  81  100  121  169  196  256  289  324  400     8100  8464  8836  9025  9216  9604  10000

julia> tups = ((i,i^2) for i in Iterators.filter(_->rand(Bool), 1:100));

julia> stack(tups)
ERROR: MethodError: no method matching vec(::Tuple{Int64,Int64})

julia> stack((i,i^2) for i in  1:100) # with a length, no problem
2×100 Array{Int64,2}:
 1  2  3   4   5   6   7   8   9   10   11   12  …    94    95    96    97    98    99    100
 1  4  9  16  25  36  49  64  81  100  121  144     8836  9025  9216  9409  9604  9801  10000

julia> using Zygote, LazyStack

julia> gradient(x -> sum(stack([x,x'])), rand(2,2)) # vector of arrays
([2.0 2.0; 2.0 2.0],)

julia> gradient(x -> sum(stack(x,x')), rand(2,2)) # tuple of arrays
([2.0 2.0; 2.0 2.0],)

julia> gradient(x -> sum(stack(x' for x in [x,x])), rand(2,2)) # generator
ERROR: not yet!

using Zygote, SliceMap
gradient(x -> sum(hcat(x...) * sum(x)), [rand(2), rand(2)]) # errors, as in linked issue
gradient(x -> sum(reduce(hcat, x) * sum(x)), [rand(2), rand(2)]) # ok!

using LazyStack
gradient(x -> sum(stack(x) * sum(x)), [rand(2), rand(2)]) # BoundsError: attempt to access (1,) at index [2]
gradient(x -> sum(stack(x...) * sum(x)), [rand(2), rand(2)]) 
gradient(x -> sum(collect(stack(x)) * sum(x)), [rand(2), rand(2)]) # MethodError: no method matching view(::NamedTuple{(:slices,),Tuple{Array{SubArray
gradient(x -> sum(collect(stack(x...)) * sum(x)), [rand(2), rand(2)]) # Mutating arrays is not supported

rr = rand(2,2); vv = rand(2);
gradient(x -> sum(rr * stack(x)), [rand(2), rand(2)])[1][1] # ok
gradient(x -> sum(rr * stack(x...)), [rand(2), rand(2)])[1][1] # ok

gradient(x -> sum(stack(x) * rr), [rand(2), rand(2)])[1][1] # ok
gradient(x -> sum(stack(x) * vv), [rand(2), rand(2)])[1][1] # BoundsError: attempt to access (1,) at index [2]
gradient(x -> sum(stack(x...) * vv), [rand(2), rand(2)])[1][1] # error!

xx = [rand(2), rand(2)];
stack(xx) * vv # BoundsError: attempt to access (1,) at index [2]
stack(xx)[1] # same

julia> using LazyStack

julia> lazystack([1,2,3])
3×1 lazystack(::Tuple{Vector{Int64}}) with eltype Int64:
 1
 2
 3

julia> Base.stack([1,2,3])  # correctly regards numbers as zero-dim
3-element Vector{Int64}:
 1
 2
 3