ForwardDiff is broken for differently sized inputs

Nobody has claimed this yet.

julia> using AbstractDifferentiation, ForwardDiff

julia> AD = AbstractDifferentiation
AbstractDifferentiation

julia> ad = AD.ForwardDiffBackend()
AbstractDifferentiation.ForwardDiffBackend{Nothing}()

julia> AD.jacobian(ad, (x, y) -> sum(x .+ y), randn(2), randn(1)) # (!!!) Result should not be tuple of inputs!
(1.0, 2.0)

Environment

julia> versioninfo()
Julia Version 1.8.0
Commit 5544a0fab76 (2022-08-17 13:38 UTC)
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 12 × Intel(R) Core(TM) i7-10710U CPU @ 1.10GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-13.0.1 (ORCJIT, skylake)
  Threads: 1 on 12 virtual cores

(jl_z9VwJo) pkg> st
Status `/tmp/jl_z9VwJo/Project.toml`
  [c29ec348] AbstractDifferentiation v0.4.3
  [f6369f11] ForwardDiff v0.10.32

(jl_z9VwJo) pkg> st --manifest
Status `/tmp/jl_z9VwJo/Manifest.toml`
  [c29ec348] AbstractDifferentiation v0.4.3
  [d360d2e6] ChainRulesCore v1.15.6
  [9e997f8a] ChangesOfVariables v0.1.4
  [bbf7d656] CommonSubexpressions v0.3.0
  [34da2185] Compat v4.3.0
  [163ba53b] DiffResults v1.1.0
  [b552c78f] DiffRules v1.11.1
  [ffbed154] DocStringExtensions v0.9.1
  [e2ba6199] ExprTools v0.1.8
  [f6369f11] ForwardDiff v0.10.32
  [3587e190] InverseFunctions v0.1.8
  [92d709cd] IrrationalConstants v0.1.1
  [692b3bcd] JLLWrappers v1.4.1
  [2ab3a3ac] LogExpFunctions v0.3.18
  [1914dd2f] MacroTools v0.5.10
  [77ba4419] NaNMath v1.0.1
  [21216c6a] Preferences v1.3.0
  [ae029012] Requires v1.3.0
  [276daf66] SpecialFunctions v2.1.7
  [90137ffa] StaticArrays v1.5.9
  [1e83bf80] StaticArraysCore v1.4.0
  [efe28fd5] OpenSpecFun_jll v0.5.5+0
  [0dad84c5] ArgTools v1.1.1
  [56f22d72] Artifacts
  [2a0f44e3] Base64
  [ade2ca70] Dates
  [f43a241f] Downloads v1.6.0
  [7b1f6079] FileWatching
  [b77e0a4c] InteractiveUtils
  [b27032c2] LibCURL v0.6.3
  [76f85450] LibGit2
  [8f399da3] Libdl
  [37e2e46d] LinearAlgebra
  [56ddb016] Logging
  [d6f4376e] Markdown
  [ca575930] NetworkOptions v1.2.0
  [44cfe95a] Pkg v1.8.0
  [de0858da] Printf
  [3fa0cd96] REPL
  [9a3f8284] Random
  [ea8e919c] SHA v0.7.0
  [9e88b42a] Serialization
  [6462fe0b] Sockets
  [2f01184e] SparseArrays
  [10745b16] Statistics
  [fa267f1f] TOML v1.0.0
  [a4e569a6] Tar v1.10.0
  [8dfed614] Test
  [cf7118a7] UUIDs
  [4ec0a83e] Unicode
  [e66e0078] CompilerSupportLibraries_jll v0.5.2+0
  [deac9b47] LibCURL_jll v7.84.0+0
  [29816b5a] LibSSH2_jll v1.10.2+0
  [c8ffd9c3] MbedTLS_jll v2.28.0+0
  [14a3606d] MozillaCACerts_jll v2022.2.1
  [4536629a] OpenBLAS_jll v0.3.20+0
  [05823500] OpenLibm_jll v0.8.1+0
  [83775a58] Zlib_jll v1.2.12+3
  [8e850b90] libblastrampoline_jll v5.1.1+0
  [8e850ede] nghttp2_jll v1.48.0+0
  [3f19e933] p7zip_jll v17.4.0+0

Start by running the Julia reproducer using AD.ForwardDiffBackend() and AD.jacobian with the differently sized inputs shown. Trace the jacobian entry point and the ForwardDiffBackend path to determine why the result is the tuple of inputs; done means this example no longer returns that incorrect tuple result.

Written by the indexing model from the issue text.

Description

Contributor guide

First steps

Research direction

Assessment