From 969d34b93b852ea0fd24d09684a3cfe430ed372a Mon Sep 17 00:00:00 2001
From: Jack Dunham <jdunham@ccqlin061.flatironinstitute.org>
Date: Mon, 19 Oct 2026 10:50:15 -0400
Subject: [PATCH 1/2] Read `scalar` of a rank-0 GPU array with `@allowscalar`

`scalar(a) = a[]` is a scalar index, which GPU array packages disallow, so reading the
result of a full contraction of `CuArray`s threw. A `GPUArraysCore` extension adds
`scalar(a::AnyGPUArray{<:Any, 0}) = @allowscalar a[]`, which copies the element to the
host; other arrays keep the generic `a[]`. Bump the version to 0.23.3.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 Project.toml                         | 5 ++++-
 ext/TensorAlgebraGPUArraysCoreExt.jl | 9 +++++++++
 test/test_scalar.jl                  | 6 +++++-
 3 files changed, 18 insertions(+), 2 deletions(-)
 create mode 100644 ext/TensorAlgebraGPUArraysCoreExt.jl

diff --git a/Project.toml b/Project.toml
index bc6956e8..76bfe1ab 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "TensorAlgebra"
 uuid = "68bd88dc-f39d-4e12-b2ca-f046b68fcc6a"
-version = "0.23.2"
+version = "0.23.3"
 authors = ["ITensor developers <support@itensor.org> and contributors"]
 
 [workspace]
@@ -15,6 +15,7 @@ StridedViews = "4db3bf67-4bd7-4b4e-b153-31dc3fb37143"
 TupleTools = "9d95972d-f1c8-5527-a6e0-b4b365fa01f6"
 
 [weakdeps]
+GPUArraysCore = "46192b85-c4d5-4398-a991-12ede77f4527"
 Mooncake = "da2b9cff-9c12-43a0-ae48-6db2b0edb7d6"
 TensorKit = "07d1fe3e-3e46-537d-9eac-e9e13d0d4cec"
 TensorKitSectors = "13a9c161-d5da-41f0-bcbd-e1a08ae0647f"
@@ -22,12 +23,14 @@ TensorOperations = "6aa20fa7-93e2-5fca-9bc0-fbd0db3c71a2"
 VectorInterface = "409d34a3-91d5-4945-b6ec-7529ddf182d8"
 
 [extensions]
+TensorAlgebraGPUArraysCoreExt = "GPUArraysCore"
 TensorAlgebraMooncakeExt = "Mooncake"
 TensorAlgebraTensorKitExt = ["TensorKit", "TensorOperations", "VectorInterface"]
 TensorAlgebraTensorKitSectorsExt = "TensorKitSectors"
 TensorAlgebraTensorOperationsExt = "TensorOperations"
 
 [compat]
+GPUArraysCore = "0.2"
 LinearAlgebra = "1.10"
 MatrixAlgebraKit = "0.6"
 Mooncake = "0.4.202, 0.5"
diff --git a/ext/TensorAlgebraGPUArraysCoreExt.jl b/ext/TensorAlgebraGPUArraysCoreExt.jl
new file mode 100644
index 00000000..076ea8ec
--- /dev/null
+++ b/ext/TensorAlgebraGPUArraysCoreExt.jl
@@ -0,0 +1,9 @@
+module TensorAlgebraGPUArraysCoreExt
+
+using GPUArraysCore: AnyGPUArray, @allowscalar
+using TensorAlgebra: TensorAlgebra
+
+# `a[]` on a GPU array is a scalar index, which GPU array packages disallow by default.
+TensorAlgebra.scalar(a::AnyGPUArray{<:Any, 0}) = @allowscalar a[]
+
+end
diff --git a/test/test_scalar.jl b/test/test_scalar.jl
index 89098c5a..90baec15 100644
--- a/test/test_scalar.jl
+++ b/test/test_scalar.jl
@@ -1,5 +1,6 @@
+using JLArrays: JLArray
 using TensorAlgebra: scalar
-using Test: @test, @testset
+using Test: @test, @test_throws, @testset
 
 @testset "scalar" begin
     @test scalar(fill(3.0)) === 3.0
@@ -7,4 +8,7 @@ using Test: @test, @testset
     a[] = 5.0
     @test scalar(a) === 5.0
     @test scalar(fill(2 + 3im)) === 2 + 3im
+    @test scalar(JLArray(fill(3.0))) === 3.0
+    @test scalar(reshape(JLArray([4.0]), ())) === 4.0
+    @test_throws ErrorException scalar(JLArray([3.0]))
 end

From 8efd57bae1191f0003633755dd668b6db20c5a27 Mon Sep 17 00:00:00 2001
From: Jack Dunham <jdunham@ccqlin061.flatironinstitute.org>
Date: Mon, 19 Oct 2026 11:00:18 -0400
Subject: [PATCH 2/2] Formatting.

---
 ext/TensorAlgebraGPUArraysCoreExt.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ext/TensorAlgebraGPUArraysCoreExt.jl b/ext/TensorAlgebraGPUArraysCoreExt.jl
index 076ea8ec..2ec60bab 100644
--- a/ext/TensorAlgebraGPUArraysCoreExt.jl
+++ b/ext/TensorAlgebraGPUArraysCoreExt.jl
@@ -1,6 +1,6 @@
 module TensorAlgebraGPUArraysCoreExt
 
-using GPUArraysCore: AnyGPUArray, @allowscalar
+using GPUArraysCore: @allowscalar, AnyGPUArray
 using TensorAlgebra: TensorAlgebra
 
 # `a[]` on a GPU array is a scalar index, which GPU array packages disallow by default.