From 938327384daec7fb982e3459f5d221008e569e35 Mon Sep 17 00:00:00 2001
From: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Date: Wed, 8 Jul 2026 09:46:40 -0400
Subject: [PATCH] Support DiffCache for nonzeroable element types

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
---
 src/PreallocationTools.jl |  6 +++++-
 test/core_dispatch.jl     | 16 ++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/src/PreallocationTools.jl b/src/PreallocationTools.jl
index 9049567..087a998 100644
--- a/src/PreallocationTools.jl
+++ b/src/PreallocationTools.jl
@@ -88,12 +88,16 @@ end
 function DiffCache(u::AbstractArray{T}, siz, chunk_sizes; warn_on_resize::Bool = true) where {T}
     x = adapt(
         ArrayInterface.parameterless_type(u),
-        zeros(T, prod(chunk_sizes .+ 1) * prod(siz))
+        _zeroed_or_uninitialized(T, prod(chunk_sizes .+ 1) * prod(siz))
     )
     xany = Any[]
     return DiffCache(u, x, xany, warn_on_resize)
 end
 
+function _zeroed_or_uninitialized(::Type{T}, dims...) where {T}
+    return hasmethod(zero, Tuple{Type{T}}) ? zeros(T, dims...) : Array{T}(undef, dims...)
+end
+
 """
 `DiffCache(u::AbstractArray, N::Int = forwarddiff_compat_chunk_size(length(u)); levels::Int = 1, warn_on_resize::Bool = true)`
 `DiffCache(u::AbstractArray; N::AbstractArray{<:Int}, warn_on_resize::Bool = true)`
diff --git a/test/core_dispatch.jl b/test/core_dispatch.jl
index ce8528b..bcc1832 100644
--- a/test/core_dispatch.jl
+++ b/test/core_dispatch.jl
@@ -33,6 +33,22 @@ end
 
 Base.zero(::Type{IsbitsPair{T}}) where {T} = IsbitsPair(zero(T), zero(T))
 
+@testset "DiffCache accepts isbits elements without zero" begin
+    u = [(; a = 1.0, b = (2.0, 3.0)), (; a = 4.0, b = (5.0, 6.0))]
+    cache = DiffCache(u, 3)
+    DualT = ForwardDiff.Dual{Nothing, Float64, 3}
+    tmp = get_tmp(cache, DualT)
+    expected_eltype = NamedTuple{(:a, :b), Tuple{DualT, Tuple{DualT, DualT}}}
+
+    @test eltype(cache.dual_du) == eltype(u)
+    @test length(cache.dual_du) == 8
+    @test size(tmp) == size(u)
+    @test eltype(tmp) == expected_eltype
+
+    tmp[1] = (; a = zero(DualT), b = (zero(DualT), zero(DualT)))
+    @test tmp[1] == (; a = zero(DualT), b = (zero(DualT), zero(DualT)))
+end
+
 #Setup Base Array tests
 chunk_size = 5
 u0 = ones(5, 5)