fix dropstored for sparse matrices (#20516)

Author: KristofferC

base/sparse/sparsematrix.jl

@@ -2745,7 +2745,7 @@ function dropstored!(A::SparseMatrixCSC, i::Integer, j::Integer)
         # Entry A[i,j] is stored. Drop and return.
         deleteat!(A.rowval, searchk)
         deleteat!(A.nzval, searchk)
-        @simd for m in j:(A.n + 1)
+        @simd for m in (j+1):(A.n + 1)
             @inbounds A.colptr[m] -= 1
         end
     end

test/sparse/sparse.jl

@@ -1705,3 +1705,19 @@ end
 @testset "issue #14398" begin
     @test transpose(view(speye(10), 1:5, 1:5)) ≈ eye(5,5)
 end
+
+@testset "dropstored issue #20513" begin
+    x = sparse(rand(3,3))
+    Base.SparseArrays.dropstored!(x, 1, 1)
+    @test x[1, 1] == 0.0
+    @test x.colptr == [1, 3, 6, 9]
+    Base.SparseArrays.dropstored!(x, 2, 1)
+    @test x.colptr == [1, 2, 5, 8]
+    @test x[2, 1] == 0.0
+    Base.SparseArrays.dropstored!(x, 2, 2)
+    @test x.colptr == [1, 2, 4, 7]
+    @test x[2, 2] == 0.0
+    Base.SparseArrays.dropstored!(x, 2, 3)
+    @test x.colptr == [1, 2, 4, 6]
+    @test x[2, 3] == 0.0
+end