Fix \ for sparse matrices with LayoutArray right-hand sides

[dlfivefifty]

A \ B fails when A is a sparse matrix and B is a LayoutArray. Found via BlockArrays.jl, where sparse_matrix \ BlockedVector fails.

Ambiguity. For A a sparse matrix, its Adjoint or its Transpose, A \ B is ambiguous between SparseArrays' \(::AbstractSparseMatrixCSC / AdjOrTrans{sparse}, ::AbstractVecOrMat) and the \(::AbstractMatrix, ::Typ) methods from @layoutldiv. B here is a LayoutVector, a LayoutMatrix, a Diagonal{<:LayoutVector}, or a triangular wrapper of a LayoutMatrix (or of its view, adjoint or transpose).

julia> sparse([4.0 1; 1 4]) \ BlockedArray([1.0, 2], [1, 1])
ERROR: MethodError: \(::SparseMatrixCSC{Float64, Int64}, ::BlockedVector{...}) is ambiguous.

CHOLMOD. For A a Symmetric/Hermitian sparse matrix, A \ B goes through ArrayLayouts.ldiv. That ends in ldiv!(::CHOLMOD.Factor, ...), which CHOLMOD does not support. It fails whenever copy(B) is not a strided array, e.g. BlockedVector, or the MyVector/MyMatrix test types:

MethodError: no method matching ldiv!(::SparseArrays.CHOLMOD.Factor{Float64, Int64}, ::Vector{Float64})

Fix

In ArrayLayoutsSparseArraysExt, define \ for each of these sparse matrix types against each layout right-hand side type. Each method converts B to an Array and calls SparseArrays' solver, which requires strided arrays anyway. A Diagonal{<:LayoutVector} keeps its Diagonal wrapper, with only the diagonal converted.

This removes all 57 \ ambiguities between ArrayLayouts and SparseArrays that have a sparse matrix on the left. 48 ambiguities in the other direction remain: a triangular layout matrix \ a sparse vector or matrix. They're not touched here.

Tests

• Added a sparse/ldiv testset in test_layoutarray.jl covering every combination. It errors without the fix.
• The full test suite passes.
• With this branch, the 20 previously failing BlockArrays.jl combinations work, with no BlockArrays changes:
  • sparse, Adjoint, Transpose, Symmetric and Hermitian matrices
  • against BlockedVector, BlockVector, BlockedMatrix and BlockMatrix right-hand sides.

Version bumped to 1.15.1.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.57%. Comparing base (0d4c00b) to head (29765de).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #288      +/-   ##
==========================================
+ Coverage   90.33%   90.57%   +0.24%     
==========================================
  Files          12       12              
  Lines        1987     2038      +51     
==========================================
+ Hits         1795     1846      +51     
  Misses        192      192              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.