Add informative error when threading VoA{SArray} without Polyester

When Polyester is not loaded and a user requests threaded FastBroadcast
on VectorOfArray{SArray}, throw an error explaining they need to load
Polyester.jl. Also fix tests to use Vector-of-SVector construction
(not Matrix-of-SVector) to properly exercise the SArray-specific path.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

Project.toml

@@ -7,26 +7,28 @@ authors = ["Chris Rackauckas <accounts@chrisrackauckas.com>"]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"
 ArrayInterface = "4fba245c-0d91-5ea0-9b3e-6abc04ee57a9"
 DocStringExtensions = "ffbed154-4ef7-542d-bbb7-c09d3a79fcae"
+FastBroadcast = "7034ab61-46d4-4ed7-9d0f-46aef9175898"
 GPUArraysCore = "46192b85-c4d5-4398-a991-12ede77f4527"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
+Polyester = "f517fe37-dbe3-4b94-8317-1923a5111588"
 PrecompileTools = "aea7be01-6a6a-4083-8856-8a6e6704d82a"
 RecipesBase = "3cdcf5f2-1ef4-517c-9805-6587b60abb01"
+RecursiveArrayToolsShorthandConstructors = "39fb7555-b4ad-4efd-8abe-30331df017d3"
+SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
+StaticArrays = "90137ffa-7385-5640-81b9-e52037218182"
 StaticArraysCore = "1e83bf80-4336-4d27-bf5d-d5a4f845583c"
 SymbolicIndexingInterface = "2efcf032-c050-4f8e-a9bb-153293bab1f5"
 
 [weakdeps]
 CUDA = "052768ef-5323-5732-b1bb-66c8b64840ba"
-FastBroadcast = "7034ab61-46d4-4ed7-9d0f-46aef9175898"
 ForwardDiff = "f6369f11-7733-5829-9624-2563aa707210"
 KernelAbstractions = "63c18a36-062a-441e-b654-da1e3ab1ce7c"
 Measurements = "eff96d63-e80a-5855-80a2-b1b0885c5ab7"
 MonteCarloMeasurements = "0987c9cc-fe09-11e8-30f0-b96dd679fdca"
 ReverseDiff = "37e2e3b7-166d-5795-8a7a-e32c996b4267"
-SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 StructArrays = "09ab397b-f2b6-538f-b94a-2f83cf4a842a"
 Tables = "bd369af6-aec1-5ad0-b16a-f7cc5008161c"
-Polyester = "f517fe37-dbe3-4b94-8317-1923a5111588"
 Tracker = "9f7883ad-71c0-57eb-9f7f-b5c9e6d3789c"
 Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 

ext/RecursiveArrayToolsFastBroadcastExt.jl

@@ -27,14 +27,20 @@ const AbstractVectorOfSArray = AbstractVectorOfArray{
     return dst
 end
 
-# Fallback for VectorOfArray: the generic threaded path splits along the last
-# axis via views, which does not correctly partition work for VectorOfArray.
-# Fall back to serial broadcasting. The RecursiveArrayToolsFastBroadcastPolyesterExt
-# extension provides proper Polyester-based threading when Polyester is loaded.
+# Fallback for non-SArray VectorOfArray: the generic threaded path splits along
+# the last axis via views, which does not correctly partition work for
+# VectorOfArray. Fall back to serial broadcasting.
+# For SArray VectorOfArray, throw an informative error telling the user to
+# load Polyester.jl for threaded broadcasting.
 @inline function FastBroadcast.fast_materialize!(
         ::Threaded, dst::AbstractVectorOfArray,
         bc::Broadcast.Broadcasted
     )
+    if dst isa AbstractVectorOfSArray && !RecursiveArrayTools.POLYESTER_LOADED[]
+        error("Threaded FastBroadcast on VectorOfArray{SArray} requires Polyester.jl. " *
+              "Add `using Polyester` to enable threaded broadcasting, or use " *
+              "`@.. thread=false` for serial broadcasting.")
+    end
     return FastBroadcast.fast_materialize!(Serial(), dst, bc)
 end
 

ext/RecursiveArrayToolsFastBroadcastPolyesterExt.jl

@@ -6,6 +6,9 @@ using FastBroadcast: Serial, Threaded
 using Polyester
 using StaticArraysCore
 
+# Signal to the base FastBroadcast extension that Polyester threading is available.
+RecursiveArrayTools.POLYESTER_LOADED[] = true
+
 const AbstractVectorOfSArray = AbstractVectorOfArray{
     T, N, <:AbstractVector{<:StaticArraysCore.SArray},
 } where {T, N}

src/RecursiveArrayTools.jl

@@ -195,6 +195,11 @@ module RecursiveArrayTools
 
     export ArrayPartition, AP, NamedArrayPartition
 
+    # Flag set to `true` by RecursiveArrayToolsFastBroadcastPolyesterExt when
+    # Polyester is loaded. Checked by the FastBroadcast ext to decide whether
+    # to throw an informative error on threaded VoA{SArray} operations.
+    const POLYESTER_LOADED = Ref(false)
+
     include("precompilation.jl")
 
 end # module

test/interface_tests.jl

@@ -311,15 +311,15 @@ end
         :RecursiveArrayToolsFastBroadcastPolyesterExt
     ) !== nothing
 
-    # Test basic threaded broadcast with Polyester
-    u_p = VectorOfArray(fill(SVector(2.0, 3.0), 3, 3))
+    # Test basic threaded broadcast with Polyester (Vector-of-SVector storage)
+    u_p = VectorOfArray([SVector(2.0, 3.0) for _ in 1:9])
     v_p = copy(u_p)
     @.. thread = true v_p = v_p + u_p
     @test all(x -> x == SVector(4.0, 6.0), v_p.u)
 
     # Test with larger array to exercise Polyester batching
-    u_large = VectorOfArray(fill(SVector(1.0, 1.0, 1.0), 100))
-    v_large = VectorOfArray(fill(SVector(0.0, 0.0, 0.0), 100))
+    u_large = VectorOfArray([SVector(1.0, 1.0, 1.0) for _ in 1:100])
+    v_large = VectorOfArray([SVector(0.0, 0.0, 0.0) for _ in 1:100])
     @.. thread = true v_large = u_large * 2.0
     @test all(x -> x == SVector(2.0, 2.0, 2.0), v_large.u)
 end