Fix #13130 such that concatenation of sparse and dense is sparse.

NEWS.md

@@ -138,6 +138,8 @@ Library improvements
 
   * The new `Base.StackTraces` module makes stack traces easier to use programmatically. ([#14469])
 
+  * Concatenating dense and sparse matrices now returns a sparse matrix ([#15172]).
+
 Deprecated or removed
 ---------------------
 
@@ -198,3 +200,4 @@ Deprecated or removed
 [#15550]: https://github.com/JuliaLang/julia/issues/15550
 [#15609]: https://github.com/JuliaLang/julia/issues/15609
 [#15763]: https://github.com/JuliaLang/julia/issues/15763
+[#15172]: https://github.com/JuliaLang/julia/issues/15172

base/array.jl

@@ -714,6 +714,14 @@ function hcat{T}(V::Vector{T}...)
     [ V[j][i]::T for i=1:length(V[1]), j=1:length(V) ]
 end
 
+hcat(A::Matrix...) = typed_hcat(promote_eltype(A...), A...)
+hcat{T}(A::Matrix{T}...) = typed_hcat(T, A...)
+
+vcat(A::Matrix...) = typed_vcat(promote_eltype(A...), A...)
+vcat{T}(A::Matrix{T}...) = typed_vcat(T, A...)
+
+hvcat(rows::Tuple{Vararg{Int}}, xs::Matrix...) = typed_hvcat(promote_eltype(xs...), rows, xs...)
+hvcat{T}(rows::Tuple{Vararg{Int}}, xs::Matrix{T}...) = typed_hvcat(T, rows, xs...)
 
 ## find ##
 

base/sparse/sparsematrix.jl

@@ -2867,7 +2867,20 @@ function hcat(X::SparseMatrixCSC...)
     SparseMatrixCSC(m, n, colptr, rowval, nzval)
 end
 
-function hvcat(rows::Tuple{Vararg{Int}}, X::SparseMatrixCSC...)
+
+# Sparse/dense concatenation
+
+function hcat(Xin::Union{Matrix, SparseMatrixCSC}...)
+    X = SparseMatrixCSC[issparse(x) ? x : sparse(x) for x in Xin]
+    hcat(X...)
+end
+
+function vcat(Xin::Union{Matrix, SparseMatrixCSC}...)
+    X = SparseMatrixCSC[issparse(x) ? x : sparse(x) for x in Xin]
+    vcat(X...)
+end
+
+function hvcat(rows::Tuple{Vararg{Int}}, X::Union{Matrix, SparseMatrixCSC}...)
     nbr = length(rows)  # number of block rows
 
     tmp_rows = Array(SparseMatrixCSC, nbr)

test/sparsedir/sparse.jl

@@ -1319,3 +1319,16 @@ let
     @test issparse(UpperTriangular(full(m))) == false
     @test issparse(LinAlg.UnitUpperTriangular(full(m))) == false
 end
+
+# dense sparse concatenation -> sparse return type
+@test issparse([sprand(10,10,.1) rand(10,10)])
+@test issparse([sprand(10,10,.1); rand(10,10)])
+@test issparse([sprand(10,10,.1) rand(10,10); rand(10,10) rand(10,10)])
+#---
+# Matrix vector cat not supported for sparse #13130
+#@test issparse([sprand(10,10,.1) rand(10)])
+#@test issparse([sprand(10,10,.1) sprand(10,.1)])
+# ---
+@test !issparse([rand(10,10)  rand(10,10)])
+@test !issparse([rand(10,10); rand(10,10)])
+@test !issparse([rand(10,10)  rand(10,10); rand(10,10) rand(10,10)])