Tweak order of operations to get nnz to infer as Int return type (#38302)

jmert · web-flow · commit d6a97a1c8484 · 2020-11-05T12:04:13.000-05:00
If the sparse array does not have a concrete index type, then union
splitting occurs over the possible `&lt;:Integer` types permitted by
`SparseMatrixCSC`.

It appears that union splitting over the subtraction by one includes
an `Any` branch that widens the return type of `nnz`. By instead
converting the index type to `Int` before subtracting, type inference
is able to infer that all paths give an `Int` result.
diff --git a/stdlib/SparseArrays/src/sparsematrix.jl b/stdlib/SparseArrays/src/sparsematrix.jl
@@ -108,7 +108,7 @@ julia> nnz(A)
 3
 ```
 """
-nnz(S::AbstractSparseMatrixCSC) = Int(getcolptr(S)[size(S, 2) + 1] - 1)
+nnz(S::AbstractSparseMatrixCSC) = Int(getcolptr(S)[size(S, 2) + 1]) - 1
 nnz(S::ReshapedArray{<:Any,1,<:AbstractSparseMatrixCSC}) = nnz(parent(S))
 nnz(S::UpperTriangular{<:Any,<:AbstractSparseMatrixCSC}) = nnz1(S)
 nnz(S::LowerTriangular{<:Any,<:AbstractSparseMatrixCSC}) = nnz1(S)

Original file line number	Diff line number	Diff line change
`@@ -108,7 +108,7 @@ julia> nnz(A)`
`108`	`108`	`3`
`109`	`109`	```
`110`	`110`	`"""`
`111`		`-nnz(S::AbstractSparseMatrixCSC) = Int(getcolptr(S)[size(S, 2) + 1] - 1)`
	`111`	`+nnz(S::AbstractSparseMatrixCSC) = Int(getcolptr(S)[size(S, 2) + 1]) - 1`
`112`	`112`	`nnz(S::ReshapedArray{<:Any,1,<:AbstractSparseMatrixCSC}) = nnz(parent(S))`
`113`	`113`	`nnz(S::UpperTriangular{<:Any,<:AbstractSparseMatrixCSC}) = nnz1(S)`
`114`	`114`	`nnz(S::LowerTriangular{<:Any,<:AbstractSparseMatrixCSC}) = nnz1(S)`