From 0a65c2ed7d44473b90ed1ee473c8195622c19c00 Mon Sep 17 00:00:00 2001
From: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Date: Fri, 14 Aug 2026 11:20:48 -0400
Subject: [PATCH] Avoid ambiguous reverse promotion in DiffCache

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
---
 src/PreallocationTools.jl          | 12 +++++++++---
 test/sparse_connectivity_tracer.jl | 12 ++++++++++++
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/PreallocationTools.jl b/src/PreallocationTools.jl
index aff7638..c57b11c 100644
--- a/src/PreallocationTools.jl
+++ b/src/PreallocationTools.jl
@@ -250,8 +250,14 @@ function get_tmp(dc::FixedSizeDiffCache, u::Union{Number, AbstractArray})
     return get_tmp(dc, eltype(u))
 end
 
+function _promotes_to_primal(::Type{P}, ::Type{T}) where {P, T}
+    # Query the requested type first so its custom rule can avoid an ambiguous reverse rule.
+    promoted = Base.promote_rule(T, P)
+    return promoted === Union{} ? promote_type(P, T) <: P : promoted <: P
+end
+
 function get_tmp(dc::FixedSizeDiffCache, ::Type{T}) where {T <: Number}
-    return if promote_type(eltype(dc.du), T) <: eltype(dc.du)
+    return if _promotes_to_primal(eltype(dc.du), T)
         dc.du
     else
         _typed_tmp(dc, T)
@@ -394,7 +400,7 @@ function _typed_tmp(dc, ::Type{T}) where {T}
 end
 
 function get_tmp(dc::DiffCache, u::Union{Number, AbstractArray})
-    return if promote_type(eltype(dc.du), eltype(u)) <: eltype(dc.du)
+    return if _promotes_to_primal(eltype(dc.du), eltype(u))
         dc.du
     else
         _typed_tmp(dc, eltype(u))
@@ -402,7 +408,7 @@ function get_tmp(dc::DiffCache, u::Union{Number, AbstractArray})
 end
 
 function get_tmp(dc::DiffCache, ::Type{T}) where {T <: Number}
-    return if promote_type(eltype(dc.du), T) <: eltype(dc.du)
+    return if _promotes_to_primal(eltype(dc.du), T)
         dc.du
     else
         _typed_tmp(dc, T)
diff --git a/test/sparse_connectivity_tracer.jl b/test/sparse_connectivity_tracer.jl
index 525b5cf..87cc550 100644
--- a/test/sparse_connectivity_tracer.jl
+++ b/test/sparse_connectivity_tracer.jl
@@ -2,6 +2,18 @@ module TestSparseConnectivityTracer
 
 using PreallocationTools, SparseConnectivityTracer, ForwardDiff, SparseArrays, Test
 
+@testset "BigFloat cache" begin
+    T = jacobian_eltype(BigFloat[1], TracerLocalSparsityDetector())
+    input = Vector{T}(undef, 1)
+
+    for cache in (DiffCache(BigFloat[1]), FixedSizeDiffCache(BigFloat[1], 1))
+        type_workspace = @inferred get_tmp(cache, T)
+        array_workspace = @inferred get_tmp(cache, input)
+        @test eltype(type_workspace) === T
+        @test Base.mightalias(type_workspace, array_workspace)
+    end
+end
+
 function f1(u, cache)
     c = get_tmp(cache, u)
     # This will throw if a fallback definition is used