Unreachable reached on Julia v1.11.3 - v1.11.6

[zuhengxu]

I triggered Unreachable reached at 0x7f8c369ccb6b when using an autodiff tool Mooncake.jl v0.4.124 to diff through a nested struct. Please see the detailed description of the bug in TuringLang/NormalizingFlows.jl#53 (comment) and a discussion in chalk-lab/Mooncake.jl#661.

Initially I thought the bug was introduced by the build_fdata function implemented in Mooncake.jl v0.4.124. But it seems like this just revealed the symptom of the issue.

version info

julia> versioninfo()
Julia Version 1.11.3
Commit d63adeda50d (2025-01-21 19:42 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 24 × AMD Ryzen 9 5900X 12-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 20 default, 0 interactive, 10 GC (on 24 virtual cores)

This bug can be reproduced on several different machine up to julia version v1.11.6.

[zuhengxu]

not sure if this is the same as #58986.