[ViralBShah]

prod of a sparse matrix with an unstored entry returns zero without looking at the stored ones, but a stored Inf or NaN times zero is NaN, which is what the dense prod gives.

Before (main, and Julia 1.13)

julia> prod(sparse([0.0 Inf; 1.0 2.0])), prod([0.0 Inf; 1.0 2.0])
(0.0, NaN)

julia> prod(sparse([0.0 NaN; 1.0 2.0]); dims=(1, 2))
1×1 Matrix{Float64}:
 0.0

After

julia> prod(sparse([0.0 Inf; 1.0 2.0])), prod([0.0 Inf; 1.0 2.0])
(NaN, NaN)

julia> prod(sparse([0.0 NaN; 1.0 2.0]); dims=(1, 2))
1×1 Matrix{Float64}:
 NaN

The * kernel of Base._mapreduce for a SparseMatrixCSC or a column view of one short-circuits when the unstored entries multiply to zero. That is kept only where f returns an integer (Base.promote_op(f, T) <: Integer), which has no value that survives a zero. Otherwise the zero is multiplied by the product of the stored entries. That product can overflow to Inf on its own and turn the zero into a NaN that the dense fold never sees, so when the result is nonzero the stored entries are folded into the zero one at a time instead: prod(sparse([0.0 1e200; 1e200 1.0])) is still 0.0.

The cost is that prod of a float matrix with unstored entries now reads the stored entries, O(nnz) where it was O(1). For a 2000×2000 Float64 matrix with 10^6 stored entries:

	time	allocations
main	0.00006 ms	0 bytes
this PR	0.096 ms	0 bytes

Left out: prod of a sparse vector and prod(A; dims=1) or dims=2 go through other kernels and still give NaN where the stored entries of the vector or slice overflow next to a zero, as on main (prod(sparsevec([0.0, 1e200, 1e200])) is NaN, dense gives 0.0).

Split from #1022. This is a wrong result on released Julia, so it is a backport candidate.

Written by Claude Code.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.79%. Comparing base (c0f75f0) to head (676fb30).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1065      +/-   ##
==========================================
+ Coverage   94.75%   94.79%   +0.03%     
==========================================
  Files          16       16              
  Lines        9885     9885              
==========================================
+ Hits         9367     9370       +3     
+ Misses        518      515       -3     

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[ViralBShah]

Reviewed by Codex at 676fb30.

No actionable findings within the stated scope. Restricting the zero shortcut to integer mapped values allows stored non-finites to participate, and the fallback avoids an artificial NaN caused solely by overflow while reducing finite stored entries separately.