Gradient of diff gives StackOverflowError

[jgreener64]

On Enzyme main (3c0871d) and Julia 1.10.3:

using Enzyme
f(x) = sum(diff([0.0 x; 1.0 2x]; dims=2))
autodiff(Forward, f, Duplicated(2.0, 1.0))

ERROR: StackOverflowError:
Stacktrace:
     [1] fwddiffejulia_hvcat_2632_inner_1wrap
       @ ~/soft/julia/julia-1.10.3/share/julia/stdlib/v1.10/SparseArrays/src/sparsevector.jl:0
     [2] macro expansion
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8839 [inlined]
     [3] enzyme_call
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8405 [inlined]
     [4] ForwardModeThunk
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8194 [inlined]
     [5] runtime_generic_fwd(activity::Type{…}, runtimeActivity::Val{…}, width::Val{…}, RT::Val{…}, f::typeof(hvcat), df::Nothing, primal_1::Tuple{…}, shadow_1_1::Nothing, primal_2::Float64, shadow_2_1::Float64, primal_3::Float64, shadow_3_1::Float64, primal_4::Float64, shadow_4_1::Float64, primal_5::Float64, shadow_5_1::Float64)
       @ Enzyme.Compiler ~/.julia/dev/Enzyme/src/rules/jitrules.jl:305
     [6] hvcat
       @ ~/soft/julia/julia-1.10.3/share/julia/stdlib/v1.10/SparseArrays/src/sparsevector.jl:1269 [inlined]
     [7] hvcat
       @ ~/soft/julia/julia-1.10.3/share/julia/stdlib/v1.10/SparseArrays/src/sparsevector.jl:0 [inlined]
--- the last 7 lines are repeated 11375 more times ---
 [79633] fwddiffejulia_hvcat_2459_inner_1wrap
       @ ~/soft/julia/julia-1.10.3/share/julia/stdlib/v1.10/SparseArrays/src/sparsevector.jl:0
 [79634] macro expansion
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8839 [inlined]
 [79635] enzyme_call
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8405 [inlined]
 [79636] ForwardModeThunk
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8194 [inlined]
 [79637] runtime_generic_fwd(activity::Type{…}, runtimeActivity::Val{…}, width::Val{…}, RT::Val{…}, f::typeof(hvcat), df::Nothing, primal_1::Tuple{…}, shadow_1_1::Nothing, primal_2::Float64, shadow_2_1::Nothing, primal_3::Float64, shadow_3_1::Float64, primal_4::Float64, shadow_4_1::Nothing, primal_5::Float64, shadow_5_1::Float64)
       @ Enzyme.Compiler ~/.julia/dev/Enzyme/src/rules/jitrules.jl:305
 [79638] hvcat
       @ ~/soft/julia/julia-1.10.3/share/julia/stdlib/v1.10/SparseArrays/src/sparsevector.jl:1269 [inlined]
 [79639] f
       @ ./REPL[2]:1 [inlined]
 [79640] fwddiffejulia_f_851wrap
       @ ./REPL[2]:0
 [79641] macro expansion
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8839 [inlined]
 [79642] enzyme_call
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8405 [inlined]
 [79643] ForwardModeThunk
       @ ~/.julia/dev/Enzyme/src/compiler.jl:8194 [inlined]
 [79644] autodiff
       @ ~/.julia/dev/Enzyme/src/Enzyme.jl:647 [inlined]
 [79645] autodiff
       @ ~/.julia/dev/Enzyme/src/Enzyme.jl:537 [inlined]
 [79646] autodiff(mode::ForwardMode{false, FFIABI, false, false}, f::typeof(f), args::Duplicated{Float64})
       @ Enzyme ~/.julia/dev/Enzyme/src/Enzyme.jl:504
Some type information was truncated. Use `show(err)` to see complete types.

Similar for reverse mode.

[jgreener64]

This also affects f(x) = rot180([x 2x; 3x 4x], 3)[1, 1] but it looks like a similar error.

[wsmoses]

Dup of #1134