[BUG] write_when_maxdim_exceeds not working for TDVP

[mwauters92]

Description of bug

When the flag write_when_maxdim_exceeds is activated in tdvp, the code returns an error, I suspect when it tries to read the file saved on the disk.
As a side comment, when setting "outpulevel=2", the code stops at the println line because "sweeps" is not defined (alternating_update.jl:64)

Minimal code demonstrating the bug or unexpected behavior

If applicable, provide a minimal code that can be run to demonstrate the bug or unexpected behavior.

If you are unable to construct a minimal code that demonstrates the bug or unexpected behavior, provide detailed steps for how to reproduce the behavior you are seeing.

Minimal runnable code

using ITensorMPS: MPO, OpSum, random_mps, siteinds, tdvp

let
    n = 10
    s = siteinds("S=1/2", n)

    function heisenberg(n)
        os = OpSum()
        for j in 1:(n - 1)
            os += 0.5, "S+", j, "S-", j + 1
            os += 0.5, "S-", j, "S+", j + 1
            os += "Sz", j, "Sz", j + 1
        end
        return os
    end

    H = MPO(heisenberg(n), s)
    ψ = random_mps(s, "↑"; linkdims = 10)

 
    ϕ = tdvp(
        H,
        -20.0,
        ψ;
        time_step = -1.0,
        maxdim = 30,
        cutoff = 1.0e-10,
        normalize = true,
        reverse_step = false,
        outputlevel = 1,
        write_when_maxdim_exceeds = 10
    )

    return nothing
end

Expected output or behavior

I expect "write_when_maxdim_exceed" to work in the same way it does for dmrg. I worked in the past with the c++ version of ITensor and writing temporary files on disk during TDVP runs caused no problem

Actual output or behavior

It returns the following error

Output of minimal runnable code

ERROR: LoadError: UndefRefError: access to undefined reference
Stacktrace:
  [1] checkfile
    @ ~/.julia/packages/SerializedElementArrays/cdFxy/src/SerializedElementArrays.jl:64 [inlined]
  [2] getindex(d::SerializedElementArrays.SerializedElementArray{ITensors.ITensor, 1}, I::Int64)
    @ SerializedElementArrays ~/.julia/packages/SerializedElementArrays/cdFxy/src/SerializedElementArrays.jl:71
  [3] iterate
    @ ./abstractarray.jl:1235 [inlined]
  [4] iterate
    @ ./abstractarray.jl:1233 [inlined]
  [5] copyto_unaliased!(deststyle::IndexLinear, dest::Vector{ITensors.ITensor}, srcstyle::IndexCartesian, src::SerializedElementArrays.SerializedElementArray{ITensors.ITensor, 1})
    @ Base ./abstractarray.jl:1095
  [6] copyto!(dest::Vector{ITensors.ITensor}, src::SerializedElementArrays.SerializedElementArray{ITensors.ITensor, 1})
    @ Base ./abstractarray.jl:1070
  [7] copymutable(a::SerializedElementArrays.SerializedElementArray{ITensors.ITensor, 1})
    @ Base ./abstractarray.jl:1201
  [8] copy
    @ ./abstractarray.jl:1144 [inlined]
  [9] copy(P::ITensorMPS.DiskProjMPO)
    @ ITensorMPS ~/.julia/packages/ITensorMPS/TTT5d/src/abstractprojmpo/diskprojmpo.jl:36
 [10] sub_sweep_update(direction::Base.Order.ForwardOrdering, reduced_operator::ITensorMPS.DiskProjMPO, state::ITensorMPS.MPS; updater::Function, updater_kwargs::@NamedTuple{}, which_decomp::Nothing, svd_alg::Nothing, sweep::Int64, current_time::Float64, time_step::Float64, nsite::Int64, reverse_step::Bool, normalize::Bool, observer!::ITensorMPS.EmptyObserver, outputlevel::Int64, maxdim::Int64, mindim::Int64, cutoff::Float64, noise::Bool)
    @ ITensorMPS ~/.julia/packages/ITensorMPS/TTT5d/src/solvers/sweep_update.jl:82
 [11] sweep_update(order::ITensorMPS.TDVPOrder{2, Base.Order.ForwardOrdering()}, reduced_operator::ITensorMPS.DiskProjMPO, state::ITensorMPS.MPS; current_time::Float64, time_step::Float64, kwargs::@Kwargs{updater::typeof(ITensorMPS.exponentiate_updater), updater_kwargs::@NamedTuple{}, nsite::Int64, reverse_step::Bool, sweep::Int64, observer!::ITensorMPS.EmptyObserver, normalize::Bool, outputlevel::Int64, maxdim::Int64, mindim::Int64, cutoff::Float64, noise::Bool})
    @ ITensorMPS ~/.julia/packages/ITensorMPS/TTT5d/src/solvers/sweep_update.jl:25
 [12] macro expansion
    @ ~/.julia/packages/ITensorMPS/TTT5d/src/solvers/alternating_update.jl:70 [inlined]
 [13] macro expansion
    @ ./timing.jl:461 [inlined]
 [14] alternating_update(operator::MPO, init::ITensorMPS.MPS; updater::Function, updater_kwargs::@NamedTuple{}, nsweeps::Int64, checkdone::Returns{Bool}, write_when_maxdim_exceeds::Int64, nsite::Int64, reverse_step::Bool, time_start::Float64, time_step::Float64, order::Int64, observer!::ITensorMPS.EmptyObserver, sweep_observer!::ITensorMPS.EmptyObserver, outputlevel::Int64, normalize::Bool, maxdim::Int64, mindim::Int64, cutoff::Float64, noise::Bool)
    @ ITensorMPS ~/.julia/packages/ITensorMPS/TTT5d/src/solvers/alternating_update.jl:69
 [15] alternating_update
    @ ~/.julia/packages/ITensorMPS/TTT5d/src/solvers/alternating_update.jl:23 [inlined]
 [16] #tdvp#924
    @ ~/.julia/packages/ITensorMPS/TTT5d/src/solvers/tdvp.jl:88 [inlined]
 [17] top-level scope
    @ ~/jjdebug/01_tdvp.jl:22
 [18] include(mod::Module, _path::String)
    @ Base ./Base.jl:306
 [19] exec_options(opts::Base.JLOptions)
    @ Base ./client.jl:317
 [20] _start()
    @ Base ./client.jl:550

Version information

• Output from versioninfo():

julia> versioninfo()
Julia Version 1.12.4
Commit 01a2eadb047 (2026-01-06 16:56 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 12 × 12th Gen Intel(R) Core(TM) i7-1250U
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, alderlake)
  GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 12 virtual cores)

• Output from using Pkg; Pkg.status("ITensorMPS"):

julia> using Pkg; Pkg.status("ITensorMPS")
Status `~/.julia/environments/v1.12/Project.toml`
  [0d1a4710] ITensorMPS v0.3.28```