[ViralBShah]

SparseMatrixCSC(view(A, :, j)) returns a SparseVector, so a constructor of SparseMatrixCSC returns something that is not a SparseMatrixCSC. The same holds for SparseMatrixCSC{Tv} and for a view of a sparse vector, while SparseMatrixCSC{Tv,Ti} of the same view, and SparseMatrixCSC of a SparseVector, return an n x 1 matrix.

Before

using SparseArrays
A = sparse([1.0 0; 0 1])
for C in (SparseMatrixCSC, SparseMatrixCSC{Float64}, SparseMatrixCSC{Float64,Int})
    r = C(view(A, :, 1)); println(C, " => ", typeof(r), " ", size(r))
end

SparseMatrixCSC => SparseVector{Float64, Int64} (2,)
SparseMatrixCSC{Float64} => SparseVector{Float64, Int64} (2,)
SparseMatrixCSC{Float64, Int64} => SparseMatrixCSC{Float64, Int64} (2, 1)

After

SparseMatrixCSC => SparseMatrixCSC{Float64, Int64} (2, 1)
SparseMatrixCSC{Float64} => SparseMatrixCSC{Float64, Int64} (2, 1)
SparseMatrixCSC{Float64, Int64} => SparseMatrixCSC{Float64, Int64} (2, 1)

The wrapper constructors in sparseconvert.jl return _sparsem(A) directly when the eltype matches, and _sparsem of a 1-d view is the SparseVector that indexing the parent gives. Two methods on SubArray{<:Any,1} now pass that vector on to SparseMatrixCSC, as the typed {Tv,Ti} constructor already does. sparse(view(A, :, j)) is unchanged and still returns a SparseVector. This touches dispatch.

Potential breakage: this changes the result type of code that works today. A caller relying on SparseMatrixCSC(view(A, :, j)) being a SparseVector, for example indexing it with one index as a vector or passing it where a vector is required, now gets an n x 1 matrix. sparse(view(A, :, j)) or A[:, j] gives the vector.

Dense has no constructor from a vector: Matrix(v) and Matrix(view(M, :, 1)) throw a MethodError. A sparse vector has long converted to an n x 1 matrix, through SparseMatrixCSC(x) and convert(SparseMatrixCSC, x), so a view of one now does the same. Matching dense exactly would break those. A view of a dense vector, which dense cannot convert either, threw a TypeError from an internal invoke. It now throws the MethodError that Matrix gives:

Before

julia> SparseMatrixCSC(view([1.0, 2.0], :))
ERROR: TypeError: in invoke: argument type error, expected Tuple{AbstractMatrix}, got a value of type Tuple{SubArray{Float64, 1, Vector{Float64}, Tuple{Base.Slice{Base.OneTo{Int64}}}, true}}

After

julia> SparseMatrixCSC(view([1.0, 2.0], :))
ERROR: MethodError: no method matching SparseMatrixCSC(::SubArray{Float64, 1, Vector{Float64}, Tuple{Base.Slice{Base.OneTo{Int64}}}, true})

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.72%. Comparing base (353601e) to head (7535d57).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sparseconvert.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1113      +/-   ##
==========================================
- Coverage   94.72%   94.72%   -0.01%     
==========================================
  Files          16       16              
  Lines       10318    10323       +5     
==========================================
+ Hits         9774     9778       +4     
- Misses        544      545       +1     

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